Race car suspension Class

[SSguy]

I was wondering if someone could explain something to me. If you run a big coil car soft spring setup, It is roughly 600's acroos the front and 200 lr and a 400 rr. Why is it that a coil over bbss set up would be 200 across the front 175 lr and 400 rr, "all spring rates are rough rates". Why are the front springs so much lighter than regular 5.5 coils yet in the back they are almost identical ?
Thanks for the help !

 

[Ranger Mike]

read BBSS page 19#362,pg 20 #381,pg 29 #568 pg 56 # 118..motion rate is different when you go from A-Arms to Coil overs and this means wheel rate changes,
see pg 2 #19,pg 30 # 589,

 

[SSguy]

Thank Mike ! Great group here with a lot of knowledge!

 

[Ranger Mike]

SSguy
thanks for the kind words..i had some time so here is the skinny on the motion rate.
stock Chevy A-arm is
16.5 inch inside frame mount point to BJ and 9 inch from inside frame mount point to center of spring pocket
assume you have a 800 lbs. spring
wheel rate = 9 / 16.5 = .54.54 x .54 x 800 =
.29 x 800 = 233 lbs. spring

coil overs typically used on Sportsman/ late models in place of stock Chevy A-arm have 17" inch inside frame mount point to BJ and 13 inch from inside frame mount point to center of spring pocket
13/17 = .76
you have to add in the mount angle of the coil over. rule of thumb is if it is mounted 0 to 8 degrees, the cosine of the angle will be .99 so dontl worry about it.
Typically you mount at 15 degrees. Cosine of 15 degree is 0.966
.996 squared is 0.992

assume you have a 800 lbs. spring
wheel rate = .76 x .76 x .992 x 800 lbs. spring =
.57 x .992 x 800 = 458 # spring
so to get the same wheel rate we used on the A-Arm set up we work it backwards
233 / ( .57 x .992) = 233 / .56 = 416 # spring

lets plug in the new spring to the above coil over formula
.57 x .992 x 416 = 235

savvy?
rm

 

[SSguy]

Thank you for the detailed explanation ! I am looking into running a "softer" set up in a metric monte, this simple math makes things easier to understand. Thanks again !

 

[Vintageracer13]

Ranger Mike can you direct me to what pages discuss roll center distance front vs rear? I need to change my rear frame panhard mount since installing my qc and so I want to know if I should match the measurement left to right to what I set the front at

 

[Ranger Mike]

rc distribution #229, pg 12, #244 pg 13, #251 pg 12,
#691 pg 35

 

[Vintageracer13]

Thank you sir

 

[Dirtracer8]

I have had some very good nights of reading here. I really appreciate the knowledge that has been put in the eyes of racers here. It's kind of a unspoken voodoo around here.
I do have a few questions before I go and hack up my front end.

My static rc is 4.035 high and 9.649 right
And
Dynamic it moves to 3.144 high and 15.517right.

My car is nosing hard on right front.
And does not seem to be using left rear at all.

The way I am understanding things the farther right the roll center is the higher % of left front weigh that is transferred to the right front.
So, this has to be a lot of my issue.

This car was fast, top three every week.
I feel like I got to tone down the roll and get it back on left rear to even out traction across the rear. I literally raced this car three full nights on a new left rear and the tits were still on it.

Please assure me I'm going to do the right thing.

Yes, I have a program, yes I understand what changes need to be made to get it to start 4 high and 3 right and move to 3.75 high and 4.25 right.

What I need is your guys opinion that it should help w my issue. As I said before Thai car was fast ... I don't want to spend 500$ and hours fabbing just to go slower this season.

 

[Ranger Mike]

you are currently racing a 3 legged milk stool..25% of the total tire contact is not working for you. By cutting back on the RC migration so far to the right you will keep the car from lifting the LR so far off the track.

Also , how much rear roll steer have you cranked into the rear end?

The set up you have today is not working though it may seem fast. i suspect he driver has a huge amount of credit for this success.. Imagine getting into a dialed in set up and driving??

 

[Dirtracer8]

On the strings The race car staticly has 1/4 under steer.
It rolls to even or straight up. I tightened it up by putting shorter right rear lower arm in so I could drive it in harder with more drive off corner.

I know that not using the left rear is killing me.

 

[Ranger Mike]

look at page 60 post # 1200 on rear roll oversteer. This is why the leaders run a lot of it. I have seen quote a few dirt track races and when the left rear wheel moves way forward in the corner, you know the car is running huge roll oversteer...3 to 4 inch change in wheelbase.
but you need all 4 tires working togeather

 

[Dirtracer8]

This car is a metric chassis w stock rear links and ford 9".

We are only allowed 3 holes on bottom of rear end for adjustment. W 3 inches from axle tube being furthest measurement .

That huge amount of rear steer is used in mods and latmodels. And yes, as a former driver of a mod, it was a huge factor in going fast.

 

[Ranger Mike]

this may help. I put all the more frequent topics on Microsoft Xcel spreadsheet for easy indexing.

Subject , post # and Page

note that info on street stock may also be in Metric subjects as well but should help sift thru the 60 plus pages. never thought it would get this big!
rm

 

[Dirtracer8]

Thanks Ranger Mike, that might help guys find the stuff they are seeking.
I know I'm going to be reading some..

 

[drobbie]

this may help. I put all the more frequent topics on Microsoft Xcel spreadsheet for easy indexing.

Subject , post # and Page

note that info on street stock may also be in Metric subjects as well but should help sift thru the 60 plus pages. never thought it would get this big!
rm

Sweet!

 

[Ranger Mike]

i have been neglecting the Street stock boys on here of late. I am working on how to fix the front RC problem ..legally..not easy..but is relatively cheap.
stay tuned

 

[Ranger Mike]

Ok Street Stockers, I am not an expert on this series having never raced it. The closest was Hobby Stock class where we had a whole lot of latitude in car modification. Raced a 64 Ford 390 and was a blast when I was 15.The Street Stock class is very limited on modifications you can make LEGALLY. Most rules cover Midsize Metric cars made by GM from 1978 to 1987. Monte Carlo, Chevelle, Camaro, Firebird, Nova, Malibu, Cutlass, Grand Prix, Regal. The Full size Metric cars are Cadillac Impala, Caprice. Can be a good choice but so heavy not worth it on the crummy spec tires we have to use. We will concentrate on Monte Carlo and the Camaro.To win we need a very good edge and the front roll center is the number one thing we can change to give us that edge. We have to relocate it and make it work to plant the hard spec tire so the car will turn.

A subset of this is the camber build for optimum tire contact patch. Once we rework the front suspension to get these two we are on the way to max handling Street Stocker. These modifications are labor intensive but pretty cheap compared to dollars spent elsewhere.You need chassis software. Buy it. Map the front end and find out what you have with Roll Center location. Do not pass GO and collect the Win $$ if you are not doing this.

When you plug in your stock chassis numbers in the software program you will to find the present Roll Center.

Next you need to start gaming the software by changing BJ heights and locations.

Two things we need to know. Lower A-Arm lengths and spindle height. It is assumed the rules let you use aftermarket ball joints and aftermarket bushings in the A-Arms. It let's you use custom Upper A-Arms as long a location is stock. Let us assume you have the talent to ream out taper holes for bigger tie rod and ball joints and i assume you can modify the drag link to get proper bump steer. If you do not know how, learn to or be happy with being a back packer. Racers are innovative mechanics, not part replacers!GM short light metric spindles that came on all midsize GM cars from 78-87 and 2wd S-10 trucks are 7 ½” tall and these spindles have 4 3/4 bolt pattern and 11” brake rotor.

All of the short spindles dimensions are same relative to ball joint location and tie rod centers. Pay attention to the differences in outer bearings depending on what year and model. Tie rod taper may not be the same so ream it.All of the Full size Metric cars have the 5 on 5” bolt pattern and 12 inch rotor. Came on heavy duty applications like limos, station wagons, and mid-70s full size cars like Cadillac Seville , Fleetwood, Buick Riviera, Oldsmobile, Pontiacs Chevrolet, Buicks.

These are way too heavy sprung weight and not worth the effort.The Chevy Impala spindles are the " hot set up " on a mid-size metric chassis. Big Chevy Impala and Caprice 1980 to 1990 full size metric spindle is 8 3/4” tall used 11 diameter brake rotor on 5 x 4 ¾” bolt center.

Options I found are Ford Granada, Mustang II , Ford Pinto spindle is 7. 625” height. These are E mod options not recommended for heavy Street Stocker.Let’s look at a 4th generation 1981- 88 Street Stock 1986 Monte Carlo.

Stock chassis suspension mounting points front and rear, stock lower A-arms. After market upper A-Arms.

We add aftermarket longer ball joints in lower A-arms to raise the spindle .75" and we add longer upper ball joints to add up to an inch more height to the upper tube A-arm. This raised the front Roll Center to about 3 to 3.5 Inch we wanted but only moves the RC to the right about 1.5 inch. We need at least 3 inch offset to plant the right front tire.

The other factor was our camber build curve was pretty poor and bump steer needs fixing. The car is drivable and a lot better than the way it was but not as good as we can make it with these lame rules.Lets look at a project car in Steve Smiths books “Street Stock Chassis Technology”. Stock 1978 Camaro has front Roll Center of 0.487” above the pavement. Bad negative camber gain 2.9° in 4 inches of bump and neg. 1.17° in 4 inch rebound (droop).

Bump steer needed work too. After lowering the car 1 inch and adding 1” longer ball joints to stock Right side upper and lower A-Arms the RC was 2.75” height and better camber change. Next the right front lower BJ was moved out ¼” and the left lower BJ was moved in ¼”. Lowering the right front upper A-arm inner pivot points .5 inch each and lowering the right front tie rod end down 1 inch got us front RC of 3.375” height and right front camber gain is 1.47° per inch. They added a taller AFCO lower ball joint on left front lower A-arm to put camber gain in ball park, We still have bump steer to fix and need rework drag link and the front Roll Center location is left about 3 inches. A lot better but not as good as we can make it with a little more work.Goal – move Roll Center to 3 inch height and 3 inch offset to the right (paved track) 4 inch offset to right on dirt.Looking at the stock A-arms on GM metric cars we have 14.25” length from the frame mount to the ball joint center, this is for 1978 and up GM metric cars like Monte Carlo,

1978-1987 Metric Impala lower control arms are 16” length , 15.31” wide and these work on 73 to 77 Chevelle, 70 to 81 Camaro and 75 to 79 Nova.

1967 – 69 Camaro, 1968 – 1974 Nova lower A-arm is 16” length but 13.25” wide.The Nova lower A-Arm conversion on a Metric car will move the lower ball joint out 1.75 to 2 “ and forward 1.5" to 2" to correct camber gain. You will make the front end wider by about 3 inches. You need a full custom upper A-Arm with adjustable upper A-Arm screw in ball joint or similar. It is possible to retrofit the Nova 16” center to center lower control arm to a Metric frame using proper control arm bushings. You will need to modify and change several things on your car to use these. These control arms use 5" OD springs, longer tie rod ends (which get very close to the control arm. Use custom fab Upper A arm as the typical metric upper control arms will not work. The stock Metric ball joint cannot be used with this control arm so reaming of the spindle for different upper and lower ball joints will be necessary as well. Installing these control arms on a Metric car is a complete front suspension redesign and certainly not a bolt on swap. Be prepared to move the lower ball joints toward the frame or away from the frame before you weld in the screw in ball joint thread ring.

Finally, you need to use Impala spindles to compete the camber gain and end up with the 3 inch roll center offset. The 1 inch taller spindles mean you can run level lower A-arms and angle the upper A-arms for the proper RC offset and still have good camber build.Tips for the metric rear end. Figure out how to put some rear roll over steer in the car. You want the wheelbase to shrink on the left side and grow on the right side when the body rolls in the turn. Start with left lower trail arms a little uphill on the low left side and right side level. Soften the bushings in the trail arms so the left side moves forward and the right side moves to the rear. Rubber bushings can be drilled so they collapse in the direction you want. The rules may say you can use metal bushings here but the obvious advantage means you use rubber bushings. Put in 5 to 8 ° pinon angle down.I might have missed something in the research but this is as good as I can do without building one in the garage.

RM

 

[Ognib]

Well written & gave me more to think about.

 

[Ranger Mike]

its winter. time to go back to school!

 

[Mike4]

Ranger Mike I’d like your opinion... 2400 lb outlaw late model. Over the course of a couple years still with success and wins I’ve always fought the same issue. Can be throttle tight through the corners. Have great forward bite though. Here’s my question. If I am happy with forward bite and entry would I actually leave the rear of the car alone? Being it’s doing exactly what I want it to do. I need to keep working on the front end to be able to cut and rotate without removing any of the bite the rear has? If this is a possibility would I be focusing more so on lowering roll center? Tire temps are optimal. Double adjustable shocks and I feel like I’m pretty knowleadge in that department as well as Corey and Advanced.. Starting to think it’s a slight geometry change I’m after. Front suspension roll center is centered 2.25 above ground. I forgot to mention we turn left and right on tight tracks!

Thank you

 

[Ranger Mike]

am out of town til friday..are you running dirt or paved track?
good front RC height but is it centered?
pls restate your probelm. When you get on the throttle it pushes at mid turn and out??

 

[Mike4]

am out of town til friday..are you running dirt or paved track?
good front RC height but is it centered?
pls restate your probelm. When you get on the throttle it pushes at mid turn and out??

Yes sir. These are outlaw late models with leaf spring rear suspension. We run a pull bar as well as 9010 shock. We run them on tight 8 tracks. I’m asking for your opinion as far as if we don’t want to give up any rear grip by stiffening the back to make it turn. Would lowering the front roll center a smidge more maybe help the front of the car be able to cut while still maintaining the level of rear grip? It’s alwahs a slight throttle push through the middle of the corners. As well as when your rounding off the corners picking the throttle back up. I was thinking either lower front roll center a hair or maybe soften front sway bar to allow front of car to get some roll in it?

Thank you

 

[Mike4]

Yes sir. These are outlaw late models with leaf spring rear suspension. We run a pull bar as well as 9010 shock. We run them on tight 8 tracks. I’m asking for your opinion as far as if we don’t want to give up any rear grip by stiffening the back to make it turn. Would lowering the front roll center a smidge more maybe help the front of the car be able to cut while still maintaining the level of rear grip? It’s alwahs a slight throttle push through the middle of the corners. As well as when your rounding off the corners picking the throttle back up. I was thinking either lower front roll center a hair or maybe soften front sway bar to allow front of car to get some roll in it?

Thank you

I’m sorry I forgot to mention it is pavement!

 

[Ranger Mike]

since you are turning left and right, you have to keep the car pretty neutral and not bias anyone side. you are correct, a lower ft rc will cause more roll and down load the tire more...make sure to check camber build

 

[Mike4]

since you are turning left and right, you have to keep the car pretty neutral and not bias anyone side. you are correct, a lower ft rc will cause more roll and down load the tire more...make sure to check camber build

Thank you sir. I know I have been slowly removing camber gain with raising inner upper a arm points as well as making uppers longer. I’ve been neglecting mapping out the current front end. I will get that done and send you the results. Maybe I’ve got some improvements somewhere I’m not catching! Thank you

 

[mikey531]

Ranger Mike thoughts on stagger We are running a pavement tour type modified. We run a 100 laps on 1/4 and 1/3 oval If I reduce the rear stagger by 1/4 inch instead of 3" it would 2 3/4" and increase cross will that help save the right rear tire for the end?

 

[Ranger Mike]

We need stagger when running a locked differential to minimize the tire scrubbing o n the rear tires. Too little and we are loose, too much and we push.

We minimize the damage by using Nitrogen in place of compressed air. The main reason is the molecules are bigger and will not bleed thru the side walls, the moisture is way less than compressed air that typically has a lot of water content. Remember what happens when you drain the air compressor!

Compressed air will cause more tire growth when heat is applied as the tire warms. Moisture becomes steam and we have more growth than that of Nitrogen. We used to use a vacuum pump form and old refrigerator to suck out all t he air on a late model tire before replacing it with Nitrogen.

When new have a lot of stagger we have a static weight situation where the left rear tire is not in contact with the pavement compared to the right front tire. If we were to scale the car, the left rear would show less weight.Cross weight – wedge, diagonal weight , all the same. Total the right front weight and the left rear weight and divide by the race cars total weight. Shoot for 55% on light Emods, 57% wedge on Sprotsman. On street stock 51 to 52%. Mini stocks – 52% wedge.More wedge keeps the rear end tight on turn entry and improves traction off. We are loading the left rear tire.

Running 58 to 60% wedge will make a three legged race car. Quick on a short 20 lap race but bad set up for 50 lapper. Wedge helps on turn entry and exit but may cause mid turn push. So you need more stagger. More wedge, more the stagger

 

[Ranger Mike]

2018 Indy 500 -rules change took out a lot of down force, lot of late lap spins, Chevy and Honda HP really getting close to each other, lot of drivers sawing the steering wheel late in the race stints, wild and wooley racing

 

[Shorttrack2018]

Hey ranger mike.

I have a grand American pro late model.

Springs lf-135 rf-175 lr-175rr-500
Wedge 60.5% 1 1/4 bar - 2 turns
Locked rear end - 3 1/2” stagger
57%left 2830 total
Track 3/8 flat track, tight corners
I’ve been struggling with this car being tight in the center for a while and I believe it’s due to wedge. I believe these cars turn well and was thinking of increasing lr spring rate #250 and reducing wedge to 57% keeping everything else the same.

Thoughts?

Thanks

 

[Ranger Mike]

in airport on way to germany next week
you run a 500# rr spring ??
normally 55 to 57% cross weight is ideal
running 30 lapper with 60% to 58% cross makes for a 3 legged race car. quick the 1st 1/3 of race but fades final 3rd due to too much heat on rf and lr tires
more cross weight means more stagger required.
wedge helps bite on turn entry and exit, stagger helps in the middle
try to get 58% left side
how is rear roll steer?

 

[Shorttrack2018]

That’s awesome, Have a safe trip. Thanks for the quick reply.

Yes, I’ve run as low as a #400 and as high as #650 with a #250 lr.

Rear roll centre is fairly high Ls-12” rs- 10.5”

With a 57% cross do you recommend I drop RR spring rate and still add LR spring rate?

 

[Garage Engineer]

Hi Ranger Mike,
Here's a question that I've always wanted answered and maybe you can help

Does the motion ration of the suspension have any impact on the compression rating of the shock for that respective corner or does it only impact the spring rate.

To be a bit more specific:
If we are only racing on Ovals ( pavement ) and we have suspension arms on the right side of the car the are 12" or even 15" longer then the left side arms and we place the same Compression rated shocks IE: #4 compression on both the RF and LF will the compression be softer on the right (vs) the left?

I've asked this question to a few people and as always you get mixed reviews, shoot I even argue with myself on what the right answer would be - HAHA

I anxiously await to hear your response on this one

Thanks again, I appreciate all the help and support you provide us all through this forum

 

[Ranger Mike]

short trac - i still have hard time understanding why you run so high a RR spring? are you trying to replicate the Big BAR SOFT Spring setup?
how did you arrive at this set up?

Garage eng .. will take some writing but should get back later today

btw - forgot to say welcome and thank you for the nice words!
rm

 

[Ranger Mike]

Spring rate is when you compress spring one inch and read the total pounds it takes. 200 # per inch.

Wheel rate is spring rate adjusted by the mechanical advantage of the lever, in this case, the bottom A-arm length. It is the motion rate of the linkage squared.

More specifically stated – Motion rate is the inner pivot point of A-Arm to the spring mounting point divided by the distance from the inner pivot point to the lower ball joint on the A-arm.

Next we calculate Wheel rate but we have to figure spring mount angle, cosines and square up motion rates and see post 17 on page 2. Then we go into Wheel Load rate and square instant centers. Lovely! As with any lever, the longer the leaver, the more mechanical advantage we have. Think tetter totter and the fat kid next door sitting in the seat and you being the fat beer drinking 200 # slob scooted closer to the fulcrum or middle point of the tetter totter. Thus a small kid equals the fat slob thru mechanical advantage. You have a much shorter lever than the fat kid.
Longer levers and A-arms mean less effort required to compress a spring over a set distance. Same with a damper or shock absorber. So given the same spring and shock absorber, but with two lengths A-arms, the longer one will be “softer”.