how does weight transfer actually work?

[NowyszRacing6]

This sounds like a question that a google search could quickly answer, but I've been looking for awhile and I'm having trouble getting a conclusive answer. My question is, at a given lateral G force in steady state lateral cornering, what determines the amount of weight transfer and therefore the load on each wheel of a car (with independent suspension)? I have found sources that say "weight transfer is only a function of CG height, track width, and vehicle mass". Others say that it is affected by roll stiffness distribution (meaning things like springs, roll centers, and arbs change the weight transfer). I know the second explanation is true for transient conditions, but is it also true for steady state? It would be great to finally get a definite answer on this, thanks

[trinidefender]

This sounds like a question that a google search could quickly answer, but I've been looking for awhile and I'm having trouble getting a conclusive answer. My question is, at a given lateral G force in steady state lateral cornering, what determines the amount of weight transfer and therefore the load on each wheel of a car (with independent suspension)? I have found sources that say "weight transfer is only a function of CG height, track width, and vehicle mass". Others say that it is affected by roll stiffness distribution (meaning things like springs, roll centers, and arbs change the weight transfer). I know the second explanation is true for transient conditions, but is it also true for steady state? It would be great to finally get a definite answer on this, thanks

I am no suspension/vehicle dynamics expert and can easily be wrong here but my thinking goes that in a steady state lateral corner, of which I think doesn't exist except in theory for F1, there is no weight transfer. Don't get me wrong, I don't mean to says at a corner doesn't involve weight transfer. What I mean is that say you turn into a corner there is a weight transfer, you then continue to drive around the corner at a constant velocity and turning rate. The weight transfer has already taken place and during the constant turn nothing changes. Then as you straighten up, a different weight transfer takes place until you are going in a straight line at a constant velocity.

That is my understanding on it when it comes to weight transfer during transient conditions as in my mind that is the only time that a weight transfer will take place.

I am probably talking out of my ass and/or not explaining myself properly but let's wait for the real suspension/vehicle dynamics guys to explain it all.

[Tommy Cookers]

..... at a given lateral G force in steady state lateral cornering, what determines the amount of weight transfer and therefore the load on each wheel of a car (with independent suspension)?
....."weight transfer is only a function of CG height, track width, and vehicle mass".
Others say that it is affected by roll stiffness distribution (meaning things like springs, roll centers, and arbs change the weight transfer). I know the second explanation is true for transient conditions, but is it also true for steady state ?

it is the front :rear distribution of the weight transfer that determines so-called balance
ARB changes adjust that front:rear distribution and so (if done correctly) produce a balance
(usually other things eg spring rates are not changed for this purpose, but would also affect the balance if they were changed))

steady state cornering of the kind that interests us requires a rather large tractive thrust ('power') to maintain the steady state
against the mechanical drag from the tyres operated in the maximal lateral load regime (and the car aerodynamic drag)

[Ciro Pabón]

Well, first, what people in this forum calls weight transfer is a function of acceleration, mass, COG height and track width. However, to answer your question the way I like to answer:

Yes.

(I mean, in steady state the weight transfer depends on rolling stiffness, center of roll and tyre spring constants).

Simple: when you create weight transfer, there are movements in the suspension.

Outer springs will compress and the inner ones will expand.

The contact patch will displace.

Therefore, the COG will move (albeit slightly, true).


In a typical car this movement of COG can be of the order of a couple of centimeters.

Eppur si muove


Now, in really potent cars (I'm talking about dragster here, not puny F1 cars, 7.000 hp, not 700) the lateral weight transfer has to include an even more potent influence: the torque of the engine.

Function of COG? Hardly


So, yes, people, even in steady state there is additional things happening (it's like marriage, I guess).

[piast9]

My question is, at a given lateral G force in steady state lateral cornering, what determines the amount of weight transfer and therefore the load on each wheel of a car (with independent suspension)?

What do you mean by the weight transfer? Side to side weight transfer? I am no car suspension specialist but I think it's just simple physics. At the first approximation of the side-to-side weight transfer depends on the speed, COG and track width. Imagine yourself standing in the bus that corners flat with face turned to the front and your legs moved apart. You can feel that weight transfer in your feet.

Now if I think right... If you start to roll from side to side from the cornering forces then your COG may shift adding to the mass tranfer. If the COG is higher than the roll centre then the mass transfer will be higher because COG will shift outside during the cornering. I don't know how big influence of that phenomenon is present in the actual cars.

[Ciro Pabón]

Yes, piast9, I believe the question is about lateral weight transfer, although the principles also apply to longitudinal transfer.

NowyszRacing6 states clearly "at a given lateral G force in steady state lateral cornering, what determines the amount of weight transfer"?

Now, piast9, your additional question about the magnitude of the effect is easy to answer.

You might lose around 5% of grip to lateral weight transfer (given around 30% of weight transfer). That is, you lose around 1/20th of your grip to "regular" weight transfer.

You might lose around 0.1% of grip to COG displacement (given one or two inches of COG displacement). That is, "advanced" weight transfer.

In a 90 seconds typical F1 lap your talking about one tenth (probably less, because F1 cars have a very stiff suspension, very low COGs and, of course, roll bars or their equivalent, precisely to minimize the displacement of COG).

However, I'd say less than one tenth it's enough for a victory...

[NowyszRacing6]

Just to clarify, this is for lateral steady state conditions, neglecting CG movement (since this usually is minimal for a typical formula car) and drive torque. Assume the car is not accelerating forward or braking at all. So can the car be treated as a rigid block with a given CG and you apply a lateral force to it, or do you have to account for springs/roll centers/etc? If it is the 2nd one, an explanation of why would be really helpful.

[Ciro Pabón]

If there is no movement of CG then it's "a block", as you say.

[Jersey Tom]

So can the car be treated as a rigid block with a given CG and you apply a lateral force to it, or do you have to account for springs/roll centers/etc? If it is the 2nd one, an explanation of why would be really helpful.

For the total amount of load transfer, you treat it as a block. But how much is transferred on the front tires vs. the rears, that's where the springs and all come into play. Both are important (the total amount and the front or rear proportioning).

[NowyszRacing6]

For the total amount of load transfer, you treat it as a block. But how much is transferred on the front tires vs. the rears, that's where the springs and all come into play. Both are important (the total amount and the front or rear proportioning).

Could you explain that a little more? if the car is treated as a rigid block (like in a statics problem), then the front and rear track widths determine how much reaction force is needed on each tire to make the sum of moments=0 for a lateral force. That was how i understood it until i heard about the other explanation i mentioned. If this first way is true, then how can the springs etc also affect it at the same time?

[Greg Locock]

The total weight transfer (excluding the lateral motion of the CG which is trivial in any normal vehicle that is not carrying bulk fluids) is governed by cgz/track, but the proportion taken by each axle depends on the roll stiffness distribution and roll centre heights.

[Jersey Tom]

For the total amount of load transfer, you treat it as a block. But how much is transferred on the front tires vs. the rears, that's where the springs and all come into play. Both are important (the total amount and the front or rear proportioning).

Could you explain that a little more? if the car is treated as a rigid block (like in a statics problem), then the front and rear track widths determine how much reaction force is needed on each tire to make the sum of moments=0 for a lateral force. That was how i understood it until i heard about the other explanation i mentioned. If this first way is true, then how can the springs etc also affect it at the same time?

No you're correct. What Greg and I are getting at is that to go from the totally rigid assumption to real world, the springs and bars just change the distribution of that load transfer, front to rear.

[NowyszRacing6]

I've been thinking about it some more and I think i understand it....so technically it would be wrong to say weight transfer only comes from cg height, track widths, gforce, and overall weight. It is a function of all the other things, which track widths and cg height are part of. I can picture it as if the chassis is just a beam with a lever sticking straight up to the CG height, and there are front and rear solid axles attached to it by coiled springs (so if the chassis rolls from a force on the lever, the spring will twist the middle of the axle to push one end down and the other end up). a stiffer spring (aka roll stiffness?) would add resistance, so the end of the axle will push down harder, meaning more weight transfer. if the front has a softer spring than the rear, the rear would then have to take more of the weight transfer to make the moment sum around the central beam be 0. Does this sound like a good model? I wish i could draw it to be more clear...It is similar to something i read earlier, so i think i'm on the right track.

[Greg Locock]

try

https://iversity.org/my/courses/vehicle ... nits/31157

you are probably too late to do the whole course but that chapter is directly relevant.

[Ciro Pabón]

I've been thinking about it some more and I think i understand it....so technically it would be wrong to say weight transfer only comes from cg height, track widths, gforce, and overall weight. It is a function of all the other things, which track widths and cg height are part of. I can picture it as if the chassis is just a beam with a lever sticking straight up to the CG height, and there are front and rear solid axles attached to it by coiled springs (so if the chassis rolls from a force on the lever, the spring will twist the middle of the axle to push one end down and the other end up). a stiffer spring (aka roll stiffness?) would add resistance, so the end of the axle will push down harder, meaning more weight transfer. if the front has a softer spring than the rear, the rear would then have to take more of the weight transfer to make the moment sum around the central beam be 0. Does this sound like a good model? I wish i could draw it to be more clear...It is similar to something i read earlier, so i think i'm on the right track.

No, I believe you're wrong. There is no "resistance" added by the springs. At all.

All that happens is that when the springs give way (and they do, no matter how stiff they are, they are not infinitely hard), then the body moves and the CG moves. Then the balance changes, thus the reactions on each wheel. End of story.