Loss of traction=time for suspension stuff?

[gopggo]

Hi guys, this is probably a dumb question so laugh now...then answer!

I got a downpipe and JB4, and am on stage 2 with a highly noticeable difference in power. When accelerating I'm noticing a larger transfer of weight to the back wheels than I was ever expecting and of course a loss of some traction in the front.

I'm probably due for an alignment, but my question is this: I'm concerned for when I'm on the track, is this the point that people start messing with springs, coilovers or suspension stuff? If so what should I start looking at..?

Other possibly relevant info: 19" stock wheels, newer non-OEM PSS's on all 4 tires, and I've tracked 3 days in less than 7000 miles

[Woodengun]

Bigger tires. Stickier tires.

Depends on track. Drag : softer suspension to help transfer more weight to rears. But not so soft that the fronts lift.

Road track: compromise between acceleration grip and corning/braking grip. Generally give up acceleration grip...not often you accelerate from a stop.

[CanAutM3]

I am not sure where to start, so much misunderstanding in this thread header post . Maybe you should read-up some literature on vehicle dynamics and then ask specific questions if there are elements for which you need clarification.

[gopggo]

Originally Posted by Woodengun

Bigger tires. Stickier tires.

Depends on track. Drag : softer suspension to help transfer more weight to rears. But not so soft that the fronts lift.

Road track: compromise between acceleration grip and corning/braking grip. Generally give up acceleration grip...not often you accelerate from a stop.

Thanks. This is helpful, so many posts and searching leads to a world of bad opinion. So if I needed more track and less drag, seems logical to go for tires but possibly stiffen up suspension to not transfer too much (which is what my issue is).

[gopggo]

Originally Posted by CanAutM3

I am not sure where to start, so much misunderstanding in this thread header post . Maybe you should read-up some literature on vehicle dynamics and then ask specific questions if there are elements for which you need clarification.

I've read too much stuff...that's the problem. Reading through all the posts you get every wrong opinion, plus a million guys that say this was already covered in another post. Needed to get practical and sorry for not providing you with enough info to be helpful.

I want to track, and have too much weight transfer to rear tires on acceleration. That means no drags and more curves, so it looks like I'll focus on tires and maybe then the suspension stuff.

Feel free to help and steer me (no pun intended) in the right direction.

[CanAutM3]

Originally Posted by gopggo

I've read too much stuff...that's the problem. Reading through all the posts you get every wrong opinion, plus a million guys that say this was already covered in another post. Needed to get practical and sorry for not providing you with enough info to be helpful.

I want to track, and have too much weight transfer to rear tires on acceleration. That means no drags and more curves, so it looks like I'll focus on tires and maybe then the suspension stuff.

Feel free to help and steer me (no pun intended) in the right direction.

There are not tons of ways you can reduce weight transfer from a chassis perspective. Lowering the center of gravity is one way of doing it. Side-to-side weight transfer can be reduced by widening the track and fore-aft weight transfer can be reduced by lengthening the wheelbase (the latter is not really practical on an existing chassis).

I'd say, the easiest way to control weight transfer is with you right foot. Less acceleration means less weight transfer. Further, you mostly need front-end grip while turning-in, where you should be decelerating, so the "extra power" does not have an impact. At track out, simply modulate the throttle to keep the balance in check.

The F8X chassis is very capable out of the box and is in no dire need of suspension tweaks shy of camber plates to optimize tire wear. Based on the questions you ask here, I seriously doubt your driving skill level is able to exploit the full potential of the stock chassis. I'd recommend focusing on the driver mod (track training and seat time) .

[gopggo]

Originally Posted by CanAutM3

There are not tons of ways you can reduce weight transfer from a chassis perspective. Lowering the center of gravity is one way of doing it. Side-to-side weight transfer can be reduced by widening the track and fore-aft weight transfer can be reduced by lengthening the wheelbase (the latter is not really practical on an existing chassis).

I'd say, the easiest way to control weight transfer is with you right foot. Less acceleration means less weight transfer. Further, you mostly need front-end grip while turning-in, where you should be decelerating, so the "extra power" does not have an impact. At track out, simply modulate the throttle to keep the balance in check.

The F8X chassis is very capable out of the box and is in no dire need of suspension tweaks shy of camber plates to optimize tire wear. Based on the questions you ask here, I seriously doubt your driving skill level is able to exploit the full potential of the stock chassis. I'd recommend focusing on the driver mod (track training and seat time) .

Perfect response! Thanks man. But how do you know I don't have the skills

Will definitely play around with modulating the throttle, I'm just really surprised at the jump in power on the stage 2 jb4 once the turbos are kicking in. So if I'm not having a good time with the acceleration transfer after a sharp turn (will be in Sebring in 2 weeks) then maybe I'll just move it back to stage 1 and keep having fun.

Thanks again

[CanAutM3]

Originally Posted by Woodengun

Depends on track. Drag : softer suspension to help transfer more weight to rears. But not so soft that the fronts lift.

Folks often confuse weight transfer and suspension movement (roll/squat/dive). Weight transfer is mostly dependent on the geometry of the vehicle (height of the center of gravity, track and wheelbase) and the forces applied to the vehicle (longitudinal/lateral acceleration). Suspension movement is a resultant of that weight transfer.

There can be some weight transfer induced by the Cg moving about the roll/pitch centers, but that amount is usually quite negligible.

Changing the suspension stiffness does not alter the amount of weight transfer by much, but does alter how the suspension reacts to the weight transfer. For example stiffer springs mean less movement for a given weight transfer.

[Woodengun]

Would be interesting to see what weight difference there is on each wheel when you raise one side of the car 0.5, 1, 2, 3 inches higher than the other side.

[CanAutM3]

Originally Posted by Woodengun

Would be interesting to see what weight difference there is on each wheel when you raise one side of the car 0.5, 1, 2, 3 inches higher than the other side.

...and that would demonstrate what ?

[Woodengun]

I will argue that body roll makes a difference.

Body roll changes corner balancing by shifting more weight to one side or front/rear of a car. Hence my inquiry about how much weight shifted with "body roll."

If it doesn't make a difference why corner balance for a road track race car? Why is a front heavy car undesirable?

[CanAutM3]

Originally Posted by Woodengun

I will argue that body roll makes a difference.

I am not saying that body roll does not transfer any weight. If you read my OP, I said that roll transfers a negligible amount of weight compared to cornering forces.

Originally Posted by Woodengun

Body roll changes corner balancing by shifting more weight to one side or front/rear of a car.

Do the math/physics with a free body diagram. Assume a 3600lb car with a 62" wide track, a Cg height of 27" and a roll centre 13" off the ground. Doing the math for a total 2" tilt (1" compression on one side and 1" extension on the other) and the Cg lateral movement about the roll centre will yield ~25lb of weight transfer from one side to the other. Do the math for 0.85g of cornering force, and you'll find ~1330lb of weight transfer from one side to the other. Quite a difference.

Originally Posted by Woodengun

Hence my inquiry about how much weight shifted with "body roll."

The analogy of lifting one side of the car is not representative of body roll. When you lift one side of the car, the car pivots around the point where the wheels contacts the ground. When a car rolls, it pivots around the roll centre defined by the suspension geometry. For a given tilt, the displacement of the Cg will not be the same in both instances.

Originally Posted by Woodengun

If it doesn't make a difference why corner balance for a road track race car?

Corner balancing is about having the two diagonal static weights equal to ensure the car has an equivalent handling behaviour when turning in both directions, left or right.

Originally Posted by Woodengun

Why is a front heavy car undesirable?

A front heavy car is undesirable because the farther the Cg is from the rear, the less nimble the car becomes (less eager to change direction). When a car turns, it pivots around the centre point between the two rear wheels (assuming same front and rear slip angles). The farther the Cg is from that pivot point, the greater the polar moment of inertia and hence a greater force needed from the front tires to rotate the car, which also makes it understeer more. That is why rear-mid engine cars are so nimble. And contrary to what BMW would like us to believe, a 50-50 weight distribution is far from ideal. For the reason aforementioned, rear weight bias is preferable.

[shockin330i]

Someone had to go and question an engineer.....

[CanAutM3]

Originally Posted by shockin330i

Someone had to go and question an engineer.....

[gopggo]

Originally Posted by CanAutM3

I am not saying that body roll does not transfer any weight. If you read my OP, I said that roll transfers a negligible amount of weight compared to cornering forces.

Do the math/physics with a free body diagram. Assume a 3600lb car with a 62" wide track, a Cg height of 27" and a roll centre 13" off the ground. Doing the math for a total 2" tilt (1" compression on one side and 1" extension on the other) and the Cg lateral movement about the roll centre will yield ~25lb of weight transfer from one side to the other. Do the math for 0.85g of cornering force, and you'll find ~1330lb of weight transfer from one side to the other. Quite a difference.

The analogy of lifting one side of the car is not representative of body roll. When you lift one side of the car, the car pivots around the point where the wheels contacts the ground. When a car rolls, it pivots around the roll centre defined by the suspension geometry. For a given tilt, the displacement of the Cg will not be the same in both instances.

Corner balancing is about having the two diagonal static weights equal to ensure the car has an equivalent handling behaviour when turning in both directions, left or right.

A front heavy car is undesirable because the farther the Cg is from the rear, the less nimble the car becomes (less eager to change direction). When a car turns, it pivots around the centre point between the two rear wheels (assuming same front and rear slip angles). The farther the Cg is from that pivot point, the greater the polar moment of inertia and hence a greater force needed from the front tires to rotate the car, which also makes it understeer more. That is why rear-mid engine cars are so nimble. And contrary to what BMW would like us to believe, a 50-50 weight distribution is far from ideal. For the reason aforementioned, rear weight bias is preferable.

[JoeFromPA]

You guys DO realize that weight transfer in and of itself can be a GOOD thing, right?

The balance of force pressing DOWN on the contact patch vs. the force trying to skid the contact patch laterally is relevant. When you reduce weight transfer, you may be reducing contact patch traction in some instances.

I'm not getting into an engineering argument here (I'd lose), but please bear in mind it is possible to reduce body motions and achieve inferior traction in some situations. The best example I can think of is increasing sway bar stiffness alone.

[CanAutM3]

Originally Posted by JoeFromPA

You guys DO realize that weight transfer in and of itself can be a GOOD thing, right?

The balance of force pressing DOWN on the contact patch vs. the force trying to skid the contact patch laterally is relevant. When you reduce weight transfer, you may be reducing contact patch traction in some instances.

I'm not getting into an engineering argument here (I'd lose), but please bear in mind it is possible to reduce body motions and achieve inferior traction in some situations. The best example I can think of is increasing sway bar stiffness alone.

A few pointers:

In most circumstances, weight transfer is not a good thing. Because of the nature of how tires grip the road, the grip they can provide does not increase linearly with the normal load. Transferring weight from one tire to another usually results in less total available grip. See picture below for an illustration.

You are however correct in saying that in some instances it can be beneficial to transfer weight. Such circumstance occur when one axle does not need more grip while the other axle can benefit from more grip. Acceleration on a RWD vehicle is one example where transferring weight to the rear wheels is actually desired. Or at the turn-in point going into a corner where more grip is needed at the front to initiate the rotation. I however cannot think of an instance where side-to-side weight transfer is beneficial though.

Again here, it is important to understand that "body motions" have relatively little impact on weight transfer. As I posted in detail previously, it is actually mostly the other way around, where it is weight transfer that causes body motion (more precisely, it is the loads on the vehicle).

Usually, a more compliant suspension will yield more total grip by allowing the tire to stay in better contact with the road. However, a compliant suspension increases the delay before the car takes a set and starts to grip, yielding more sluggish handling (less nimble), but can also help making the handling more predictable (less snappy). Tuning a suspension is about finding just the right balance and compromise between nimbleness and total grip. That sweet spot varies greatly depending on the road surface (bumpy vs smooth).

Stiffer roll bars are good to improve the response of the suspension (make the car more nimble) on a smooth surface, but can make the vehicle skittish (less total grip) over bumps when cornering. They however have very little impact on how much total weight is transferred side-to-side, but can be used to fine tune how much of the side-to-side weight transfer is shared between the front and rear axles.

[JoeFromPA]

Originally Posted by CanAutM3

A few pointers:

In most circumstances, weight transfer is not a good thing. Because of the nature of how tires grip the road, the grip they can provide does not increase linearly with the normal load. Transferring weight from one tire to another usually results in less total available grip. See picture below for an illustration.

You are however correct in saying that in some instances it can be beneficial to transfer weight. Such circumstance occur when one axle does not need more grip while the other axle can benefit from more grip. Acceleration on a RWD vehicle is one example where transferring weight to the rear wheels is actually desired. Or at the turn-in point going into a corner where more grip is needed at the front to initiate the rotation. I however cannot think of an instance where side-to-side weight transfer is beneficial though.

Again here, it is important to understand that "body motions" have relatively little impact on weight transfer. As I posted in detail previously, it is actually mostly the other way around, where it is weight transfer that causes body motion (more precisely, it is the loads on the vehicle).

Usually, a more compliant suspension will yield more total grip by allowing the tire to stay in better contact with the road. However, a compliant suspension increases the delay before the car takes a set and starts to grip, yielding more sluggish handling (less nimble), but also making the handling more predictable (less snappy). Tuning a suspension is about finding just the right balance and compromise between nimbleness and total grip. That sweet spot varies greatly depending on the road surface (bumpy vs smooth).

Stiffer roll bars are good to improve the response of the suspension (make the car more nimble) on a smooth surface, but can make the vehicle skittish (less total grip) over bumps when cornering. They however have very little impact on how much weight is transferred.

Thank you sir! I have some prior experience with this in Subaru's which have (for their class) very low centers of gravity and which Subaru usually then allows higher degrees of suspension travel/compliance - thus they have lot of body motion but turn in very good performance numbers for their size/weight/class. Drivers often want to add sway bars to reduce side to side body motions, but since Subarus are a solid vehicle off-road (not smooth road) or in non-track environments this can actually decrease available traction and ability for the car to maintain good vehicle contact patches.

Your points are excellent and my only point here is that people often lower their cars, stiffen their suspensions, or put on wider tires without understanding all of the dynamics. I certainly don't understand them all. The person who best educated me was a guy who ran a ~600whp LS engine in an e39 chassis rolling on 225 wide tires and helped me understand that contact patch for straight line traction is not all about static tire width.