- #1,716
hallpa00
- 4
- 0
RM - many thanks for the advice. I will do as you suggest and report back in 12 days!
Race car suspension Class
- Automotive
- Thread starter Ranger Mike
- Start date
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- Tags
- Car Class Race Suspension
In summary,-The stock car suspension is important for understanding the complexity of a Formula Cars suspension.-When designing a (front) suspension, geometry layout is critical.-spindle choice and dimensions, kingpin and steering inclination, wheel offset, frame height, car track width, camber change curve, static roll center height and location and roll axis location are major factors.-The first critical thing to do is to establish the roll center height and lateral location. The roll center is established by fixed points and angles of the A-arms. These pivot points and angles also establish the camber gain and bump steer.-I have used Suspension Analyzer for years on Super late Model stock cars as
- #1,717
Cape15
- 11
- 2
RM- thank you for last few responses for me. I finally made it out to practice and my car actually rotates in the middle of the corner which is amazing since i have always been fighting with tight tight tight in the center of the corner. however i am slightly tight entering the corner and my RF tire is average 23 deg. more than the RR. I was thinking from all ive read through these posts that i might take out the RF spring of 325 that you recommended in my prior post and go to a 300 on the RF and go from there. Is that a good idea and would that ruin my center of corner? Also just curious as to your adjustments processes at the track or if there is a post you can direct me too that I might have missed on this thread. Thanks again!Ranger Mike said:
- #1,718
- 2,352
- 362
Yes a rt ft spring rate decrease is called for. Or add spring rate to right rear. Looks like we need to kill off some rt rear traction and cut down the PUSH.
You are very close.
you can soften the front ARB (sway bar) by lengthen the arm.
you can raise the rear roll center with panhard bar.
many options before swapping springs.
Try adding spring rubbers to right rear spring. quicker than swapping springs and better fine tuning.
these fine tune to 10#, 20# 25# a must to fine tune before spring swap
https://carolinaracingsupply.com/pr...oNfJg-sM18NQFdRVgY2GprXaapXvvnu4aAniYEALw_wcB
the rubbers below are over kill in my opinion but some racers use them.
1” spring rubber will approximately 100 lbs. ¾” is approximately 75 lbs.
https://www.speedwaymotors.com/Spee...9rFQjegyPdLVh4aa7clemUlwpqBvoKLcaAultEALw_wcB
post 1638 on track tuning. I have written about this but cant find it!
You are very close.
you can soften the front ARB (sway bar) by lengthen the arm.
you can raise the rear roll center with panhard bar.
many options before swapping springs.
Try adding spring rubbers to right rear spring. quicker than swapping springs and better fine tuning.
these fine tune to 10#, 20# 25# a must to fine tune before spring swap
https://carolinaracingsupply.com/pr...oNfJg-sM18NQFdRVgY2GprXaapXvvnu4aAniYEALw_wcB
the rubbers below are over kill in my opinion but some racers use them.
1” spring rubber will approximately 100 lbs. ¾” is approximately 75 lbs.
https://www.speedwaymotors.com/Spee...9rFQjegyPdLVh4aa7clemUlwpqBvoKLcaAultEALw_wcB
post 1638 on track tuning. I have written about this but cant find it!
Last edited:
- #1,719
- 2,352
- 362
Track Tuning Tips
See post 1419 on page 41 for driver tips
Track tuning
Two things any crew chief better have in the tool box , next to the Rolaids, are Stagger tape and a Tire Pyrometer. Preferably one with a memory that will permit you to take readings and will display three readings per tire.
Tire temperatures are the only real way to tell what is going on and it is empirical. Learn how to use the tire pyrometer and make a habit of doing it correctly. I have a nice techy one that tell you to stab the right front tire inside, middle and outside then move to the right rear tire. The cursor ques you where to take the readings. Insert the sensor needle 1/8 inch into the tire at a 45 degree angle and be consistent. You will be stabbing three places on the tire face. Inside edge ( not on the edge but an inch from the inside edge) the middle and the outside ( not the very outside..,an inch from the outside edge). When taking readings try to do it as soon as the driver comes off the track as these tires cool very quickly. Tune and test day is ideal as you can do it on the track if you are solo ones in that session. Then jack up the car and take tire stagger readings right front first. Record the tire growth and this will tell you about stagger.
The optimal tire temperatures should be in a range of 190 to 240 degrees. On a short track it is normal for the outside edge of the RF tire & the inside edge of the LF to be 5 to 10 degrees cooler. This is because of the way the tires travel down the straightaway. On a larger track with longer straights, this spread will be even further. On an oval, the RF tire will have more negative camber, thus resulting in the inside edge of the tire contacting the track more than the outside edge giving you the higher temperature. On the LF you will run with more positive camber, so just the opposite holds true. While cornering these temperatures should even out if you have the correct amounts of camber & or weight transfer. The more camber you run, the higher these spreads will be. On a small track were you spend a lot of time cornering, you'll find the spread not as high. This is because your spending more time cornering than on the straights, thus distributing the temperatures across the face of the tire more evenly. If you try to achieve even temps across the tire you may develop a push. This is telling you that you have too much positive camber. Although the tire may be flat on the track, on a straightaway, the tire will not be flat on the track while cornering.
The best way to read tire temperatures is to run 10 laps on a particular setup. Read the temps and don't expect to learn everything reading the temps only once. It will take a number of these sessions to sort everything out that is going on with the tires. Make sure you are not locking up the brakes or making any sudden changes in your steering outputs. These will all create erroneous tire temperatures readings.
A tire with too much NEGATIVE camber will show an excessively higher temperature at the INSIDE edges. Lean the top of the tire out to the outside.
A tire with too much POSITIVE camber will show an excessively higher temperature at the OUTSIDE edges so lean the top of the tire in.
A tire that is OVER inflated will have a higher middle temperature than the inside & outside edges.
A tire that is UNDER inflated will have a lower middle temperature than the inside & outside edges.
A car with too much toe OUT will show higher temperatures on both INSIDE edges of the front tires.
A car with too much toe IN will show higher temperatures on both OUTSIDE edges of the front tires.
A RF tire that is HOTTER by more than 10 degrees over the RR indicates a tight PUSH (understeer) condition.
A RF tire that is COLDER by more than 10 degrees over the RR indicates a loose ( oversteer) condition.
A tire with the HIGHEST average temperature is the corner of the car that you should work on first.
A tire with the LOWEST average temperature is the corner of the car that you should work on second.
A RF & LR diagonal average that is the same or higher than the front & right side average indicates too much wedge.
A RF & LR diagonal average that is more than 10 degrees lower than the front average and right side average indicates not enough wedge.
Let’s look at a few examples.
RF
I----M----O
208--202--194
Indicates too much negative camber.
RF
I----M----O
194--202--208
Indicates too much positive camber.
RF
I----M----O
204--188--197
Indicates an under inflated tire.
RF
I----M----O
204--210--197
Indicates an over inflated tire.
RF
I----M----O
204--198--194
Indicates correct camber. Overall average temp is 198.6.
RR
I----M----O
227--225--223
Overall average temp. is 225.
If the RR & RF temp above came off the same car we have a very loose racecar. The RR is 26 degrees hotter than the RF. If this RR is also the hottest tire on the car, it indicates the RR is spinning and or sliding in the corners. We need to go to a weaker RR spring to keep more weight on this tire and prevent the wheel spin. This should cool this tire & tighten up the chassis.
Caution -Temperature Averaging works when you have a fast car and need to fine tune it. If your car is a log wagon and wallowing around like a whale cure the handling first.
Panhard bar
Do not forget- The mid point between the right and left side anchor points on both panhard bar and J bar is the Roll Center. This Rear RC can be offset many inches from the vehicle centerline.
When Panhard bar has chassis mount on frame right side the Rear roll center will DROP under body roll. Opposite if chassis mount on left side.
If you raise both mount points on the Panhard bar, you are raising the Rear RC. The rear roll center will still move downward in body roll as above stated.
Tuning with sway bar (ARB)
A quick review of the ARB - sometimes also called anti-sway bars or anti-roll bars. Their purpose in life is to try to keep the car's body from "rolling" in a left turn.
When you are inside the car, you know that your body gets pulled toward the outside of the turn. The right part of the car on the outside of the turn gets pushed down toward the road and the left side part of the car on the inside of the turn rises up. In other words, the body of the car "rolls" 10 or 20 or 30 degrees toward the outside of the turn.
Too much Roll is bad. It tends to put more weight on the outside tires and less weight on the inside tires, reducing traction. The proper amount of body roll will load the right front tire and assist in improving traction through the turn. Ideally, we would like the body of the car to remain flat through a turn so that the weight stays distributed evenly on all four tires.
ARB tries to keep the car's body flat by moving force from one side of the body to another. When you go into a turn, the front suspension member of the outside of the turn gets pushed upward. The arm of the ARB gets pushed upward, and this applies torsion to the middle section. The torsion moves the arm at the other end of the rod, and this causes the suspension on the other side of the car to compress. The car's body tends to stay flat in the turn.
If you have too much ARB, you tend to lose independence between the suspension members on both sides of the car. When one wheel hits a bump, the ARB transmits the bump to the other side of the car as well, which is not what you want. The ideal is to find a setting that reduces body roll but does not hurt the independence of the front or rear springs.
Tune by adjusting the ARB Arm length. Longer arm is softer spring rate while shorter Arm increases the spring rate in front.
See post 1419 on page 41 for driver tips
Track tuning
Two things any crew chief better have in the tool box , next to the Rolaids, are Stagger tape and a Tire Pyrometer. Preferably one with a memory that will permit you to take readings and will display three readings per tire.
Tire temperatures are the only real way to tell what is going on and it is empirical. Learn how to use the tire pyrometer and make a habit of doing it correctly. I have a nice techy one that tell you to stab the right front tire inside, middle and outside then move to the right rear tire. The cursor ques you where to take the readings. Insert the sensor needle 1/8 inch into the tire at a 45 degree angle and be consistent. You will be stabbing three places on the tire face. Inside edge ( not on the edge but an inch from the inside edge) the middle and the outside ( not the very outside..,an inch from the outside edge). When taking readings try to do it as soon as the driver comes off the track as these tires cool very quickly. Tune and test day is ideal as you can do it on the track if you are solo ones in that session. Then jack up the car and take tire stagger readings right front first. Record the tire growth and this will tell you about stagger.
The optimal tire temperatures should be in a range of 190 to 240 degrees. On a short track it is normal for the outside edge of the RF tire & the inside edge of the LF to be 5 to 10 degrees cooler. This is because of the way the tires travel down the straightaway. On a larger track with longer straights, this spread will be even further. On an oval, the RF tire will have more negative camber, thus resulting in the inside edge of the tire contacting the track more than the outside edge giving you the higher temperature. On the LF you will run with more positive camber, so just the opposite holds true. While cornering these temperatures should even out if you have the correct amounts of camber & or weight transfer. The more camber you run, the higher these spreads will be. On a small track were you spend a lot of time cornering, you'll find the spread not as high. This is because your spending more time cornering than on the straights, thus distributing the temperatures across the face of the tire more evenly. If you try to achieve even temps across the tire you may develop a push. This is telling you that you have too much positive camber. Although the tire may be flat on the track, on a straightaway, the tire will not be flat on the track while cornering.
The best way to read tire temperatures is to run 10 laps on a particular setup. Read the temps and don't expect to learn everything reading the temps only once. It will take a number of these sessions to sort everything out that is going on with the tires. Make sure you are not locking up the brakes or making any sudden changes in your steering outputs. These will all create erroneous tire temperatures readings.
A tire with too much NEGATIVE camber will show an excessively higher temperature at the INSIDE edges. Lean the top of the tire out to the outside.
A tire with too much POSITIVE camber will show an excessively higher temperature at the OUTSIDE edges so lean the top of the tire in.
A tire that is OVER inflated will have a higher middle temperature than the inside & outside edges.
A tire that is UNDER inflated will have a lower middle temperature than the inside & outside edges.
A car with too much toe OUT will show higher temperatures on both INSIDE edges of the front tires.
A car with too much toe IN will show higher temperatures on both OUTSIDE edges of the front tires.
A RF tire that is HOTTER by more than 10 degrees over the RR indicates a tight PUSH (understeer) condition.
A RF tire that is COLDER by more than 10 degrees over the RR indicates a loose ( oversteer) condition.
A tire with the HIGHEST average temperature is the corner of the car that you should work on first.
A tire with the LOWEST average temperature is the corner of the car that you should work on second.
A RF & LR diagonal average that is the same or higher than the front & right side average indicates too much wedge.
A RF & LR diagonal average that is more than 10 degrees lower than the front average and right side average indicates not enough wedge.
Let’s look at a few examples.
RF
I----M----O
208--202--194
Indicates too much negative camber.
RF
I----M----O
194--202--208
Indicates too much positive camber.
RF
I----M----O
204--188--197
Indicates an under inflated tire.
RF
I----M----O
204--210--197
Indicates an over inflated tire.
RF
I----M----O
204--198--194
Indicates correct camber. Overall average temp is 198.6.
RR
I----M----O
227--225--223
Overall average temp. is 225.
If the RR & RF temp above came off the same car we have a very loose racecar. The RR is 26 degrees hotter than the RF. If this RR is also the hottest tire on the car, it indicates the RR is spinning and or sliding in the corners. We need to go to a weaker RR spring to keep more weight on this tire and prevent the wheel spin. This should cool this tire & tighten up the chassis.
Caution -Temperature Averaging works when you have a fast car and need to fine tune it. If your car is a log wagon and wallowing around like a whale cure the handling first.
Panhard bar
Do not forget- The mid point between the right and left side anchor points on both panhard bar and J bar is the Roll Center. This Rear RC can be offset many inches from the vehicle centerline.
When Panhard bar has chassis mount on frame right side the Rear roll center will DROP under body roll. Opposite if chassis mount on left side.
If you raise both mount points on the Panhard bar, you are raising the Rear RC. The rear roll center will still move downward in body roll as above stated.
Tuning with sway bar (ARB)
A quick review of the ARB - sometimes also called anti-sway bars or anti-roll bars. Their purpose in life is to try to keep the car's body from "rolling" in a left turn.
When you are inside the car, you know that your body gets pulled toward the outside of the turn. The right part of the car on the outside of the turn gets pushed down toward the road and the left side part of the car on the inside of the turn rises up. In other words, the body of the car "rolls" 10 or 20 or 30 degrees toward the outside of the turn.
Too much Roll is bad. It tends to put more weight on the outside tires and less weight on the inside tires, reducing traction. The proper amount of body roll will load the right front tire and assist in improving traction through the turn. Ideally, we would like the body of the car to remain flat through a turn so that the weight stays distributed evenly on all four tires.
ARB tries to keep the car's body flat by moving force from one side of the body to another. When you go into a turn, the front suspension member of the outside of the turn gets pushed upward. The arm of the ARB gets pushed upward, and this applies torsion to the middle section. The torsion moves the arm at the other end of the rod, and this causes the suspension on the other side of the car to compress. The car's body tends to stay flat in the turn.
If you have too much ARB, you tend to lose independence between the suspension members on both sides of the car. When one wheel hits a bump, the ARB transmits the bump to the other side of the car as well, which is not what you want. The ideal is to find a setting that reduces body roll but does not hurt the independence of the front or rear springs.
Tune by adjusting the ARB Arm length. Longer arm is softer spring rate while shorter Arm increases the spring rate in front.
Attachments
Last edited:
- #1,720
- 2,352
- 362
Mid turn Handling and corner exit cures
Much has been written in this class about Roll center location and offset and the need for proper location. I have been sent many messages from racers thanking me for this information and now the car is pivoting, rotating and corner entry is much better. This post will deal with Mid corner and corner exit problems.
Mid Turn handling problems usually come down to Stagger and Cross weight.
We must use Stagger so the tires will not shed going through the turn. Put a Styrofoam coffee cup on its side and roll it. The top or bog end rolls different than the small diameter bottom. On circle racetracks in USA we turn left so the left rear tire is smaller diameter then the right rear tire so assist the turning process. There is no way around this and all racers in your race must use stagger. You can, however, use a few tricks to assist you. We took an old refrigerator apart and used the vacuum pump to pump out the compressed air in our tires. We filled the tire with Nitrogen. Compressed air has moisture, and this turns to steam and expands the tire when heated and your right front tire will heat up. Nitrogen expands at a much lower rate and holds stagger longer. One side benefit is the nitrogen molecules and fatter and do not bleed through the tire side wall like air does. When the tire does expand it throws weight to the left rear tire. It adds Wedge or cross weight.
minimum stagger you need is:
D + .5 (TW) x CL \ D - .5 (TW)
where D is the track diameter in feet
TW = rear track width of the car in feet ( divide inch measurement by 12)
CL = the left rear tire circumference
example- race track has 200 ft. diameter
car has 64 inch track (64 / 12 = 5.33 inch)
left rear is 86 inch circumference.
run the numbers and you get 88.33 inch right rear circumference or MINIMUM 2.33 inch stagger. If you run a locked differential or spool we add 10 percent to minimum stagger. If the track banking is 0 to 10 degrees, add another ten percent. If banking is 15 degrees up, take off ten percent from min. stagger.
Wedge or cross weight
Checking for Wedge – Crazy! I spent an hour googling and no photos of this. Word picture time. When the race car enters the turn the left rear spring goes into droop because the spring is extending to its max travel and the right rear spring is compressed. The left side droop means the left rear tire has minimum traction. We use wedge (cross weight) to increase tire load on the left rear tire. This increases tire bite off the corner. We attained this with the jack screws we have on the car. We add cross weight or Wedge to force the left rear spring into the track when cornering.
We measure Wedge with a ½” socket placed on the floor jack and center it under the rear end housing to lift both rear tires. The right rear should lift up an inch, 2 inch or 3 inch as the track demands, just as the left rear tire shows daylight under the contact patch.
In the turn, the body will roll over and the wedge drives the left rear to wedge the tires to keep in contact. The % cross weight on street stock is usually 1 to 2 % total weight, Late models with slicks run 2% typically.
Rule of thumb - every inch of wedge is 1% Cross weight. When the wedge goes past the normal amount (and inch more than what the normal amount is) then make a left rear spring change. Then reduce the wedge to under the normal amount and adjust at the track as required. Cross weight is adjustable because the local track will change due to temperature, amount of rain washing off the oil and rubber, etc. You need to have a way to dial the chassis in for new conditions and not use it up for basic chassis setup.
Much has been written in this class about Roll center location and offset and the need for proper location. I have been sent many messages from racers thanking me for this information and now the car is pivoting, rotating and corner entry is much better. This post will deal with Mid corner and corner exit problems.
Mid Turn handling problems usually come down to Stagger and Cross weight.
We must use Stagger so the tires will not shed going through the turn. Put a Styrofoam coffee cup on its side and roll it. The top or bog end rolls different than the small diameter bottom. On circle racetracks in USA we turn left so the left rear tire is smaller diameter then the right rear tire so assist the turning process. There is no way around this and all racers in your race must use stagger. You can, however, use a few tricks to assist you. We took an old refrigerator apart and used the vacuum pump to pump out the compressed air in our tires. We filled the tire with Nitrogen. Compressed air has moisture, and this turns to steam and expands the tire when heated and your right front tire will heat up. Nitrogen expands at a much lower rate and holds stagger longer. One side benefit is the nitrogen molecules and fatter and do not bleed through the tire side wall like air does. When the tire does expand it throws weight to the left rear tire. It adds Wedge or cross weight.
minimum stagger you need is:
D + .5 (TW) x CL \ D - .5 (TW)
where D is the track diameter in feet
TW = rear track width of the car in feet ( divide inch measurement by 12)
CL = the left rear tire circumference
example- race track has 200 ft. diameter
car has 64 inch track (64 / 12 = 5.33 inch)
left rear is 86 inch circumference.
run the numbers and you get 88.33 inch right rear circumference or MINIMUM 2.33 inch stagger. If you run a locked differential or spool we add 10 percent to minimum stagger. If the track banking is 0 to 10 degrees, add another ten percent. If banking is 15 degrees up, take off ten percent from min. stagger.
Wedge or cross weight
Checking for Wedge – Crazy! I spent an hour googling and no photos of this. Word picture time. When the race car enters the turn the left rear spring goes into droop because the spring is extending to its max travel and the right rear spring is compressed. The left side droop means the left rear tire has minimum traction. We use wedge (cross weight) to increase tire load on the left rear tire. This increases tire bite off the corner. We attained this with the jack screws we have on the car. We add cross weight or Wedge to force the left rear spring into the track when cornering.
We measure Wedge with a ½” socket placed on the floor jack and center it under the rear end housing to lift both rear tires. The right rear should lift up an inch, 2 inch or 3 inch as the track demands, just as the left rear tire shows daylight under the contact patch.
In the turn, the body will roll over and the wedge drives the left rear to wedge the tires to keep in contact. The % cross weight on street stock is usually 1 to 2 % total weight, Late models with slicks run 2% typically.
Rule of thumb - every inch of wedge is 1% Cross weight. When the wedge goes past the normal amount (and inch more than what the normal amount is) then make a left rear spring change. Then reduce the wedge to under the normal amount and adjust at the track as required. Cross weight is adjustable because the local track will change due to temperature, amount of rain washing off the oil and rubber, etc. You need to have a way to dial the chassis in for new conditions and not use it up for basic chassis setup.
Attachments
- #1,721
- 2,352
- 362
You really have two Race cars - Corner Exit Fix
History - Remember back in the muscle car day. The 426-cube engine would torque up the left front. Many motor mount broke as the rubber mount would separate. We had to put a chain on the left side of the motor and chain it to the chassis. Engine torque would twist the car lifting the left side up and add load to the right rear tire. The car would swerve left when you dropped the clutch at 5000 to 7000 rpm. Look on a country road Saturday morning for the rubber tire tracks showing this after the hot rodders had fun the night before. Tire marks going to the left!
Why? Because the Right rear tire was loaded, the rear left side tire was unloaded, car shot to the unloaded side, instant push under acceleration. Factory engineers realized this and the big cubic inch cars had 7 leaf right rear springs and 6 leaf left rear springs to counter this. Otherwise, you had a built-in push under acceleration to the left.
Here are typical recommended cures for phase three corner exit problems –
Loose on exit – decrease tire stagger
Increase cross weight
Soften right rear spring
Check for right rear tire over inflation.
Check brake bias
Change rear roll center
The problem with these solutions is you will impact the corner entry handling. It took a huge amount of time to get the right front tire to perform and a major chassis change like a spring change or Roll Center change just screws up the entry!
No one has addressed the elephant in the room.
You really have two race cars! You have a round track car great for phase one and two. You have a drag car at corner exit! I say again, you have a 600 HP, 3000 lbs. drag car with a locked rear end and two different size tires with a huge difference in rear tire weight, side to side trying to race down the straights. We need to hook up these rear tires WITHOUT CHANGING THE CORNER ENTRY SET UP.
Discussion - Factory late model chassis builders are great craftsman and build quality late model race cars. A great value but all in all generic as they did not know what track, or the track rules or conditions the car would race. It saved a huge amount of time for the small race teams. They had one great platform so you could customize for your track and rules! Here is the problem.
Usually, the car was sold with 4 race wheels to accommodate the track rules regarding tire size. You had to race 10-inch tires max, so you got 4 ten-inch-wide tires and appropriate wheel. Makes sense No one is a mind reader. Again, the rules for cars would vary all over the places. Different tracks had different rules on engine offset, % left side weight. maximum track width etc. goes on and on. You got a generic, well built, safe race car that out of the box was pretty darn good.
The car probably had 1 inch engine offset to the left as this was universal. The factory car was set up with at most 52% left side weight. This is still the norm, I think. The majority of racers today are running these older chassis unless you have cubic money and buy the latest, greatest, new chassis. The big problem is your race team decides on where to put the weight and what wheels to use.
Wheels - factory chassis guys ship the car with standard wheels on all four corners. Normally the wheel has 4 inch offset measuring from the back of the wheel to the wheel backing plate. Saves money and is a good baseline. If you take the 4-inch offset wheel off the car and put 3 inch offset on the right side of the car and 5 inch offset on the left side of the car you gain left side weight. You can gain 2% more left side weight doing this. Now you are at 54% left side weight. Now you add a 1-inch spacer between the hub and the backing plate so you can get up to 56% left side weight. Some even go radical using 6 inch offset wheels. Next you find out the track has a 58% maximum left side weight rule, so you move weight around to do this. Now you are at the legal 58% left side weight.
The problem
The location of the top link on any factory chassis is about a 52% left side weight set up on the standard 4 inch offset wheels and the tire track width. A very logical and safe starting point. Unless you ordered it with a specific requirement to change these settings.
The problem is the 3rd link is mounted at the original 52% weight location and should now be at the 58% location and it is not.
Why is top link mount so critical? The top link is the rear end link that pulls the race car. This top link directs all rear end force to the chassis. The rear tires grip the track and try to pull the rear end to the rear. On the bottom, the trailing arms push the car to the front and try to climb up under the car.
We are discussing left turn race car where you want a left side weight bias. When in a turn you want the left side weight rolling over to the right side to assist the car with better traction (tire grip). Correct location of the 3rd link mounting is a real help so the tires are pulling on the third link equally otherwise when you step on the gas, one tire will bite more than the other and shoot you toward the outside of the track or the inside.
If the 3rd link is offset to the left more than the right side, the left will have more load and the right rear tire will have less load. Exactly opposite of the muscle car scenario above. What really happens is more force is added to the side of the car the link is biased to. In this case we have more force added to the left rear tire, so we create an on gas, corner exit LOOSE condition.
The 3rd Link Location (Static)
Mark each rear tire centerline. Measure the distance between them. This is the track width. Let's say 66 inches so the mid-point is located at 33 inches. Now measure the right-side tire centerline to the middle of the top link. Let's say it is 34 ¼ inches. 34 / 66 = 52% left side weight. If you are scaling at 58% left side weight we multiply the total track width time the % so 66" x .58 = 38 ¼ inches. The top link mount should be at 38 ¼ " so, move it left 4" then the push goes away.
See attached photo a forum member, RaceMan12 posted on page 46 post 1602 this post. You can make this happen.
Look at photo below. Notice the original top link mount was offset about 1 ½ inch to the left of the quick-change housing centerline. The racer relocated it about 4 inches more to the left. See post # 1576 page 46.
This is a huge step to solving the traction problem and it will only get you close.
The problem is that this is a static fix. Look at the cross-weight illustration below. We have left rear tire weight of 1027 # and right rear tire weight of 560# when we scale the car. We use the Static measurement of % left side weight to locate and center the 3rd link to provide 50-50% traction.
This remount does not take into account the dynamics of corner exit. Look at # 43 car lifting the front end under acceleration. It is also twisting and throwing left side weight over to the right side. We have to dial this additional left and rear weight into the equation.
Let us say we have 100 lbs. weight coming of the left front tire and dumping on the right rear tire under acceleration. The car is twisting and unloading 25 lbs. from the left rear tire and it goes to the right rear tire
Now we have a dynamic left side % weight of
Lft ft rt ft
677 747
Lft rear rt rear
1002 685
We now have 54% left side weight under acceleration.
How to properly measure the dynamic weight transfer?
Take the car in full race trim to a straight section of road parking lot, airfield.
Set the damper (shock absorber stops) and make a full power acceleration for 50 feet. Do not brake, coast to a stop, let the chassis settle. Read the shock travels on all four corners.
Next step is to scale the car and floor jack the chassis in front and on left side to duplicate the shock travel you recorded.
You can read the weight scale on each wheel to find a truer dynamic left side weight % and then calculate 3rd link position for a much better 50-50% rear traction.
Fine Tune Traction - When relocation the 3rd link mounts both at chassis and on the rear end, make it adjustable so the link can be shifted from left to right. I recommend a few 1/4 “thick spacers and many Spacers. ½ “spacers. This way you can dial in the rear traction.
Think about this, racers. Where are races won on the round track?
On the straights!
In road courses we win by out braking the other racers. We have 9 left hand and 8 right hand turns and only two to three straight we can out gun the other guys but rules are so tight the HP is very close. So many races we won by sucking in the younger drivers to go way deep into the turn that they overload and go super squirrelly and have to back off because they drove over their head! See ya! haha
In round track we gun the other cars on corner exit. (Australian and euro racers, this means out accelerating the other car on corner exit. I have many Aussies and Euro readers and THANK YOU much for reading!) But we are so concerned with pushing going in and loose off, we never concentrate on how to win. If we can gain advantage by hooking up BETTER than the other racer on corner exit, we can gain an advantage and accelerate past him to get a half nose or whole car advantage going into the next turn. But we spend NO TIME working on this. Crazy!
Spend a Saturday at the parking lot doing nothing but launching the car. Get car in race trim with an old set of tires. Make sure the stagger is right. Work on hook up of both rear tires so you have straight Gas on acceleration when you mash the go pedal. Adjust until you go straight and leave equal tire marks on the pavement. (You do not need to leave tire tracks though it would help). The point is to launch the car and adjust so it goes straight when you gas it on corner exit. Hand off the wheel when you get it right.
If it wants to drift to the left, move the 3rd link a little to the right until it runs straight. Use the tire pyrometer so both tires are getting the same heat (load) on launch. This is very hard to do because you need to compensate for the smaller diameter left rear tire.
Always overlooked. Take time to tune the CARB. Accelerator pump shot, jetting, read the spark plugs. Launch the car, kill it and coast to a stop. You want to read the spark plugs. Tune for best throttle response. You are tuning for the straights that are the majority of the race track. You do not want a bog off the corner. You are throttle off with carb butterfly's (throttle plates) closed.
So you are in fact, in a drag race when you exit.
Now ask yourself this question? Has the local hot dog racer done this? Or is he buying the latest expensive whizzy bit or speed trick for more horsepower he cannot use really use because he is fighting a corner exit loose condition? He just figures this is how it is and has to live with it. His answer, more power. Is he going to spend a day working on this aspect of winning? I remind you Racers, Caroll Shelby quote number one - https://www.shelbystore.com/carroll-shelby-quotes-s/2499.htm
Do you not think your brother drag racers spend hours getting the car to go straight when the green light pops? Drag races are won and lost at the starting line. Ok top end matters but lets be real.
When you are down to launching the car, you are very close to best your car can be.
What will this cost you? a few gallons of fuel, old tires, and time! And a Parking lot????
One final thought - do drag car tune with rear roll center? NO! This is for working out phase one and two problems. Do not forget you have two cars, not one.
Yes, this takes time but if you can tune the rear traction without screwing up the corner entry, it got my vote!
History - Remember back in the muscle car day. The 426-cube engine would torque up the left front. Many motor mount broke as the rubber mount would separate. We had to put a chain on the left side of the motor and chain it to the chassis. Engine torque would twist the car lifting the left side up and add load to the right rear tire. The car would swerve left when you dropped the clutch at 5000 to 7000 rpm. Look on a country road Saturday morning for the rubber tire tracks showing this after the hot rodders had fun the night before. Tire marks going to the left!
Why? Because the Right rear tire was loaded, the rear left side tire was unloaded, car shot to the unloaded side, instant push under acceleration. Factory engineers realized this and the big cubic inch cars had 7 leaf right rear springs and 6 leaf left rear springs to counter this. Otherwise, you had a built-in push under acceleration to the left.
Here are typical recommended cures for phase three corner exit problems –
Loose on exit – decrease tire stagger
Increase cross weight
Soften right rear spring
Check for right rear tire over inflation.
Check brake bias
Change rear roll center
The problem with these solutions is you will impact the corner entry handling. It took a huge amount of time to get the right front tire to perform and a major chassis change like a spring change or Roll Center change just screws up the entry!
No one has addressed the elephant in the room.
You really have two race cars! You have a round track car great for phase one and two. You have a drag car at corner exit! I say again, you have a 600 HP, 3000 lbs. drag car with a locked rear end and two different size tires with a huge difference in rear tire weight, side to side trying to race down the straights. We need to hook up these rear tires WITHOUT CHANGING THE CORNER ENTRY SET UP.
Discussion - Factory late model chassis builders are great craftsman and build quality late model race cars. A great value but all in all generic as they did not know what track, or the track rules or conditions the car would race. It saved a huge amount of time for the small race teams. They had one great platform so you could customize for your track and rules! Here is the problem.
Usually, the car was sold with 4 race wheels to accommodate the track rules regarding tire size. You had to race 10-inch tires max, so you got 4 ten-inch-wide tires and appropriate wheel. Makes sense No one is a mind reader. Again, the rules for cars would vary all over the places. Different tracks had different rules on engine offset, % left side weight. maximum track width etc. goes on and on. You got a generic, well built, safe race car that out of the box was pretty darn good.
The car probably had 1 inch engine offset to the left as this was universal. The factory car was set up with at most 52% left side weight. This is still the norm, I think. The majority of racers today are running these older chassis unless you have cubic money and buy the latest, greatest, new chassis. The big problem is your race team decides on where to put the weight and what wheels to use.
Wheels - factory chassis guys ship the car with standard wheels on all four corners. Normally the wheel has 4 inch offset measuring from the back of the wheel to the wheel backing plate. Saves money and is a good baseline. If you take the 4-inch offset wheel off the car and put 3 inch offset on the right side of the car and 5 inch offset on the left side of the car you gain left side weight. You can gain 2% more left side weight doing this. Now you are at 54% left side weight. Now you add a 1-inch spacer between the hub and the backing plate so you can get up to 56% left side weight. Some even go radical using 6 inch offset wheels. Next you find out the track has a 58% maximum left side weight rule, so you move weight around to do this. Now you are at the legal 58% left side weight.
The problem
The location of the top link on any factory chassis is about a 52% left side weight set up on the standard 4 inch offset wheels and the tire track width. A very logical and safe starting point. Unless you ordered it with a specific requirement to change these settings.
The problem is the 3rd link is mounted at the original 52% weight location and should now be at the 58% location and it is not.
Why is top link mount so critical? The top link is the rear end link that pulls the race car. This top link directs all rear end force to the chassis. The rear tires grip the track and try to pull the rear end to the rear. On the bottom, the trailing arms push the car to the front and try to climb up under the car.
We are discussing left turn race car where you want a left side weight bias. When in a turn you want the left side weight rolling over to the right side to assist the car with better traction (tire grip). Correct location of the 3rd link mounting is a real help so the tires are pulling on the third link equally otherwise when you step on the gas, one tire will bite more than the other and shoot you toward the outside of the track or the inside.
If the 3rd link is offset to the left more than the right side, the left will have more load and the right rear tire will have less load. Exactly opposite of the muscle car scenario above. What really happens is more force is added to the side of the car the link is biased to. In this case we have more force added to the left rear tire, so we create an on gas, corner exit LOOSE condition.
The 3rd Link Location (Static)
Mark each rear tire centerline. Measure the distance between them. This is the track width. Let's say 66 inches so the mid-point is located at 33 inches. Now measure the right-side tire centerline to the middle of the top link. Let's say it is 34 ¼ inches. 34 / 66 = 52% left side weight. If you are scaling at 58% left side weight we multiply the total track width time the % so 66" x .58 = 38 ¼ inches. The top link mount should be at 38 ¼ " so, move it left 4" then the push goes away.
See attached photo a forum member, RaceMan12 posted on page 46 post 1602 this post. You can make this happen.
Look at photo below. Notice the original top link mount was offset about 1 ½ inch to the left of the quick-change housing centerline. The racer relocated it about 4 inches more to the left. See post # 1576 page 46.
This is a huge step to solving the traction problem and it will only get you close.
The problem is that this is a static fix. Look at the cross-weight illustration below. We have left rear tire weight of 1027 # and right rear tire weight of 560# when we scale the car. We use the Static measurement of % left side weight to locate and center the 3rd link to provide 50-50% traction.
This remount does not take into account the dynamics of corner exit. Look at # 43 car lifting the front end under acceleration. It is also twisting and throwing left side weight over to the right side. We have to dial this additional left and rear weight into the equation.
Let us say we have 100 lbs. weight coming of the left front tire and dumping on the right rear tire under acceleration. The car is twisting and unloading 25 lbs. from the left rear tire and it goes to the right rear tire
Now we have a dynamic left side % weight of
Lft ft rt ft
677 747
Lft rear rt rear
1002 685
We now have 54% left side weight under acceleration.
How to properly measure the dynamic weight transfer?
Take the car in full race trim to a straight section of road parking lot, airfield.
Set the damper (shock absorber stops) and make a full power acceleration for 50 feet. Do not brake, coast to a stop, let the chassis settle. Read the shock travels on all four corners.
Next step is to scale the car and floor jack the chassis in front and on left side to duplicate the shock travel you recorded.
You can read the weight scale on each wheel to find a truer dynamic left side weight % and then calculate 3rd link position for a much better 50-50% rear traction.
Fine Tune Traction - When relocation the 3rd link mounts both at chassis and on the rear end, make it adjustable so the link can be shifted from left to right. I recommend a few 1/4 “thick spacers and many Spacers. ½ “spacers. This way you can dial in the rear traction.
Think about this, racers. Where are races won on the round track?
On the straights!
In road courses we win by out braking the other racers. We have 9 left hand and 8 right hand turns and only two to three straight we can out gun the other guys but rules are so tight the HP is very close. So many races we won by sucking in the younger drivers to go way deep into the turn that they overload and go super squirrelly and have to back off because they drove over their head! See ya! haha
In round track we gun the other cars on corner exit. (Australian and euro racers, this means out accelerating the other car on corner exit. I have many Aussies and Euro readers and THANK YOU much for reading!) But we are so concerned with pushing going in and loose off, we never concentrate on how to win. If we can gain advantage by hooking up BETTER than the other racer on corner exit, we can gain an advantage and accelerate past him to get a half nose or whole car advantage going into the next turn. But we spend NO TIME working on this. Crazy!
Spend a Saturday at the parking lot doing nothing but launching the car. Get car in race trim with an old set of tires. Make sure the stagger is right. Work on hook up of both rear tires so you have straight Gas on acceleration when you mash the go pedal. Adjust until you go straight and leave equal tire marks on the pavement. (You do not need to leave tire tracks though it would help). The point is to launch the car and adjust so it goes straight when you gas it on corner exit. Hand off the wheel when you get it right.
If it wants to drift to the left, move the 3rd link a little to the right until it runs straight. Use the tire pyrometer so both tires are getting the same heat (load) on launch. This is very hard to do because you need to compensate for the smaller diameter left rear tire.
Always overlooked. Take time to tune the CARB. Accelerator pump shot, jetting, read the spark plugs. Launch the car, kill it and coast to a stop. You want to read the spark plugs. Tune for best throttle response. You are tuning for the straights that are the majority of the race track. You do not want a bog off the corner. You are throttle off with carb butterfly's (throttle plates) closed.
So you are in fact, in a drag race when you exit.
Now ask yourself this question? Has the local hot dog racer done this? Or is he buying the latest expensive whizzy bit or speed trick for more horsepower he cannot use really use because he is fighting a corner exit loose condition? He just figures this is how it is and has to live with it. His answer, more power. Is he going to spend a day working on this aspect of winning? I remind you Racers, Caroll Shelby quote number one - https://www.shelbystore.com/carroll-shelby-quotes-s/2499.htm
There's never enough horsepower... just not enough traction."
Do you not think your brother drag racers spend hours getting the car to go straight when the green light pops? Drag races are won and lost at the starting line. Ok top end matters but lets be real.
When you are down to launching the car, you are very close to best your car can be.
What will this cost you? a few gallons of fuel, old tires, and time! And a Parking lot????
One final thought - do drag car tune with rear roll center? NO! This is for working out phase one and two problems. Do not forget you have two cars, not one.
Yes, this takes time but if you can tune the rear traction without screwing up the corner entry, it got my vote!
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