Physics of an automobile, suspension, and weight transfer?

[wimms]

Originally posted by Mr. Robin Parsons
wimms, the energy transferred to the shocks is lost, why, thereafter, you seem to decide that I somehow need to have the reasons for the "use of shocks" explained to me, well you've lost me as to why you are doing this. (not entirely, I can/could 'speculate', easily)
Heck wimms, I had training from people in the automotive industry, batteries, shocks, tires, exhaust systems, braking systems, ignition systems, transmissions, electrical systems, etc. etc.

Robin, I do assume that you know what you are talking about. Can't you realize that your position isn't easily acceptable? If you want others to understand, please try to explain it in more detail why it is that you are right. I posted this shocks function to express my point, not to teach you anything "new". And point is: energy that goes into springs already IS taken away from forward force. Springs compress only when vertical force is present. And shocks really transfer that vertical energy to the ground, they resist vertical compression. The only function of suspension is to doze vertical energy in controlled manner. The only energy loss is in the friction. Do you agree here? I'm not arguing that spring energy is recovered for forward motion. I'm only arguing that it already wasn't available for forward motion.

If you are still missing the point, try practicallity, goes like this, get a rear wheel drive car, automatic tranny, open the hood so you can see the motor from inside the car, get into the car, start the car, place your foot solidly on the brake-and hold down firmly, shift car into drive, place your other foot on the throttle, press gently to slightly accelerate the engine, note the lifting of he engine by the torque that is generated.

Chassis is not lifting from that. If you lock clutch, engine would stall. Engine is not lifting, it is twisting around gearbox and some of supports under the hood. At best, its jammed against the chassis. Its not same thing.

Again, please understand that locking wheels changes scenario completely. Then, energy has only one place to go - lifting chassis. When wheels are free, energy goes into accelerating chassis, not lifting it. Although the effect you support is probably there, it is so small that its irrelevant. At 5G accelerations, inertial effects dominate.

BTW the "Boss pin" is the pin that connects the connecting rod to the piston, and it is subjected to tons of force inside a very normal car engine. Just because you seemed to think that such forces would be unbearable by the components of cars, they aren't they are, kinda normal.

Fine. You got me here. I assumed we talk about tons of torque, like nm. But to compare things, we need to bring them to common metric. Let's approach from torque of engine. http://www.amatoracing.com/carspecs.shtml
It offers 6,250 lbft or 8500nm of torque. To give 50tons on rods, cam radius should be about 17cm. Sure, forces at pins are huge, maybe even larger (17cm sounds kinda excessive).

After some thought it seems that dragsters don't have any tranny. Rear diff ratio is 3.2, and wheel radius is 0.5m, basically adds ratio of 2:1. Notice that these ratios apply to increase torque at wheels. This translates into 8500x3.2x2=54000nm of torque at tire patches. a=F/m, and /975kg offers 5.7G linear acceleration.

To lift chassis, the only torque we have is 8500nm. If wheels are locked, then whole chassis has to accelerate at some rate about rear axle. We need to account for engine working rpm range, let's assume max torque is achieved at 2000rpm, 210 radians/sec. If engine can't make it, it'll either stall or fail. Something has to give. With free wheels, they'd spinup.
From standstill to max torque its angular acceleration of 210rad/sec² at least. Now find how much torque is needed to give such angular acceleration to the whole chassis.

We've found that angular inertia of chassis is about 19 tons. To give such inertia angular acceleration of 210rad/ss, we'd need torque 3,990,000nm! http://hyperphysics.phy-astr.gsu.edu/hbase/n2r.html And that's without accounting for need to overcome gravity.

At 8500nm, angular acceleration of chassis about rear end is at best 0.45rad/ss, and that's when all energy goes there. No engine can work at such low rpm. It either stalls, or engine block blows up. When wheels are free, most of energy goes to linear acceleration, thus even less is avail to lift chassis. Even if you do not agree with 19tons of chassis angular inertia, by using plain 975kg and 1meter distance of COG from axle, needed torque is 204,750nm, which is much more than available.

See? Key points I make:
- torque at tire patch is 3.2x2=6.4 times bigger than at ring gear
- angular acceleration needed accordingly 6.4 times higher at ring
- linear inertia of chassis is 19 times less than angular inertia about rear axle
- engine has not enough torque to flip chassis over at its working rpm range
- engine has enough torque to produce linear acceleration of over 5Gs.
- tires spin up and work as slipping clutch
- energy spent on acceleration is not available for angular lift of chassis about rear axle
- COG height and acceleration force is enough to account for front lift

 

[Mr. Robin Parsons]

Clearly wimms the problems here are, you don't read what I am writing, or, it is a comprehension problem, or, you are simply ignoring the facts and fishing me (ends HERE!)

I stated a car with an automatic tranny, you come back with popping the clutch stalls out the motor! (Please tell me how you pop any of the three clutches on an automatic transmission, cause I have never seen them popped, burnt yes, popped NO!)

A bit like the spring energy problem, lost in the translation.

So wimms, 1 (one) horsepower equals 33,000 lbs/ft/minute, or 550 ft/lbs per second. The rails engine has 6000 HP, so 3,300,000 ft/lbs per sec, or 1650 TONS/ft/sec, or 165 tons/ft/tenth of a second.

Is that enough to lift the car? (YES!)

P.S. You seem to wish to use the "5 G" figure for the cars acceleration, it does not develop that 5 G immediately, first tenth of a second, but it does lift it's wheels

And please wimms you said;

Originally posted by wimms
It offers 6,250 lbft or 8500nm of torque. To give 50tons on rods, cam radius should be about 17cm. Sure, forces at pins are huge, maybe even larger (17cm sounds kinda excessive).

Sooo, PLeeease Explain to everyone here how cam radius is related to wrist pin pressure!

(Don't know much about engines do you?)

 

[Mr. Robin Parsons]

P.S. wimms, I had mentioned that the pressure on the "boss/wrist" pins was 10 (ten) tons, and I had linked a site that told of 5 (five) tons in a race car.

Given F=ma, and the simplicity that a race cars pistons are as "shaved/trimmed" as much as is possible, it is very possible that the normal car engines pistons weigh in at twice the weight of the race cars pistons, hence the forces upon them, inertial forces would be twice that of the race cars engine, hence 10 (ten) tons.

So I am probably not off all that much. My source for that was a scientist, on television, to the best of my recollection.

 

[russ_watters]

Originally posted by krab
That isn't true. Here's a thought experiment which should make things clearer. Let's say the car with the soft suspension (that the original poster brought up remember? we weren't talking about dragsters) has its rear tires glued to the road. With a system of rods or some such, imagine I have a mounting point for a cable at the exact COG of the car. Pull back on the cable. What happens? The car begins to tilt. Pull the cable about 1 inch back and the rear springs compress maybe about 4 inches, the front springs extend a similar amount. Let go and the COG moves forward as the car levels. In fact, it Accelerates forward.

Yes. Again, torque. It magically converts vertical forces into horizontal ones.

For my next trick, I'll make something go up by pushing down on it!

 

[Mr. Robin Parsons]

Originally posted by Krab
Let go and the COG moves forward as the car levels. In fact, it Accelerates forward.

You mean because of inertial forces, right?

 

[wimms]

Originally posted by Mr. Robin Parsons
Clearly wimms the problems here are, you don't read what I am writing, or, it is a comprehension problem, or, you are simply ignoring the facts and fishing me

Funny, I thought this about you. What I don't comprehend is why all disagreements in this site must develop into unpleasant confrontation.

I stated a car with an automatic tranny, you come back with popping the clutch stalls out the motor!

chill. We talked about torque, not tranny. Effect you implied would have to manifest with any tranny. And of course I meant manual tranny when I said clutch.

So wimms, 1 (one) horsepower equals 33,000 lbs/ft/minute, or 550 ft/lbs per second. The rails engine has 6000 HP, so 3,300,000 ft/lbs per sec, or 1650 TONS/ft/sec, or 165 tons/ft/tenth of a second.

Have it occurred to you, that moving 1 lbs 3M feet in one second and moving 1650 tons 1 feet has a difference?
Engines generate torque. Horsepower is product of torque and rpm:
Horsepower = Torque x 2 pi x rpm / 33000 which simplifies to:
Horsepower = Torque x rpm / 5252
Torque = Horsepower x 5252 / rpm
for 6000hp @8200rpm this is 3842 lbs.ft of torque, meaning 3842 lbs of force at arm length of 1 ft, at tangential velocity of 858ft/sec, which is achieved only at the end of drag run with chassis speed of over 200 mph. Sure, if you had gearing ratio of 858:1, you'd have 858x3842 = 3M lbs x 1ft /sec, but your gearing ratio is 3.2:1 !
So gimme a break with this 1650 TONS of lifting force. This is not heavyweight lifter crane, its a vehicle. Max HP is developed only when vehicle is speeding. At low speed you have nearly no HP, and only torque.

If you want launch forces, talk about torque. I specially searched for you and found a spec with 6,250 lbft of torque, which is more than is produced at 8200rpm, normal for any engine.
I spent time to find the calculations needed. You obviously didn't read anything. You see 6000HP -> boom - 1650 tons of lifting force.

P.S. You seem to wish to use the "5 G" figure for the cars acceleration, it does not develop that 5 G immediately, first tenth of a second, but it does lift it's wheels

Interesting. What is the supporting reaction force? Surely not inertia of car to acceleration, right? Some magical 'sticking' to the ground. Wheels spinning, producing 50000nm of traction torque, and for the first 10th of a second vehicle instead of accelerating, opts to spend some fun time on lifting its fronts instead.
In first 1/10 seconds, 5Gs produces 25cm displacement. Calculate. 5Gs all the way! I can agree that 5Gs aren't achieved immediately, because of engine reving up and tires warming up, but I can't see how lifting can be independant from acceleration.

You're arguing wrong thing. So far angular moment of inertia of rear wheels haven't been discussed, but spinning up rear wheels takes considerable amount of energy. That might be your first 10th of a second you are seeking for.

Sooo, PLeeease Explain to everyone here how cam radius is related to wrist pin pressure!
(Don't know much about engines do you?)

English is not my language. I meant crank. If you'd not mean disrespect you'd not try to make fun of it. Or, is it that I'm having personality comprehension problem here?

 

[VeeP]

first post :D

hey yall


just to clear something up (and help some of you understand this particular idea).

Softening the rear suspension (to a limit) on a RWD car in a drag launch situation is there to help provide the rear subframe receive as much of the cars weight as possible (giving the nose up tendency).
As the nose lifts, it removes wieght from the front wheels (which serve no purpose other than to steer the vehicle) to the rear wheels. More wieght over the rear wheels means that the tire contact patch grows (and u still have the same amount of pressure per square unit) - which means that your tire can generate more traction, which means that you can apply more force to the ground without the tire lighting up - which means you can accelerate harder. . . .

However - this does NOT apply to certain types of cars (specifically dedicated drag cars) - particularly rail cars - why?
Becuase in rail cars, most of the weight is already over the rear tires. Same goes for funny cars, a lot of the weight already sits over the back wheels.
Oh, rail cars don't even have suspension actually. . . its just the wheels directly bolted to the chassis (obviously through very low friction means) - the tire provides most of the suspension. . . .

FWD cars are a completely different story, maybe if someone specifically requests it i will explain.


oh. . . btw, i hope i can positively contribute to these forums and learn much from yall too :D

 

[Mr. Robin Parsons]

Originally posted by MRP
Clearly wimms the problems here are, you don't read what I am writing, or, it is a comprehension problem, or, you are simply ignoring the facts and fishing me (ends HERE!)

wimms, did you read this?? (above and below)

Originally posted by wimms
So gimme a break with this 1650 TONS of lifting force. This is not heavyweight lifter crane, its a vehicle. Max HP is developed only when vehicle is speeding. At low speed you have nearly no HP, and only torque.

Where did I call it lifting force, I simply stated force, aside from that, listen to the car's engine at the start line, revvvvvving up, well past 2000 rpm, as it is engine speed that counts, NOT the cars speed.

Originally posted by wimms
English is not my language. I meant crank. If you'd not mean disrespect you'd not try to make fun of it. Or, is it that I'm having personality comprehension problem here?

Should have told me it wasn't your first language from the start, it would have helped. No. I am not making fun of you, simply pointing out, from your posting the appearance YOU created of NOT knowing what you are talking about. Sorry if it caused you any negative emotive, it was NOT my intention.

Once again,

Originally posted by MRP(ends HERE!)