22 replies to this topic

[zzyzx]

Let's say you stiffen up the back end of a car so that when it corners more weight is transfered to the outside rear tire. Is it correct to say that although the outside rear tire will now produce more grip because it has more weight on it, the back end of the car will slide more easily because the extra grip that is produced is not proportional to the amount of centrifugal force that is now acting on that end of the car due to the extra weight?

If that is true, why is it that when you transfer weight onto the front tires while trailbraking, or onto the rear tires while stepping on the gas while cornering, allows the tires to develop more grip? In other words, if weight transfer is always bad, then shouldn't, for example, the car understeer more when you trailbrake?

[maxay1]

Hmmm, I'm an idiot, but I'll give it a go...and the people who actually know what they're talking about can correct me.

There is load transfer, obviously, but relatively little weight transfer, as the CG probably doesn't move a great deal (discussing a fairly stiffly sprung vehicle here). As such, I don't think that there is much additional inertial (centrifugal) force for the tires to react.

The overall grip level at the rear would be reduced somewhat, I would think, due to load sensitivity of the coefficient of friction of the tires; an unevenly loaded tire pair has less grip than a more evenly loaded one. Not sure if this effect is due to tire heating or what, hopefully DaveW, Ben, Fat Boy, and RDV will contribute. They actually know what they're doing.

Not sure that I agree that we can say that load transfer is always bad, though I see your point. The tire becomes less efficient as load is placed on it, but it does produce more grip. I suspect that CG location is very important here, in all 3 axes. I suspect trail-braking a Porsche 911 is a very different thing than doing so in a recent BMW M3, or V8-engined Audi...and different still from doing so in a formula car with wings.

Bowing out now.

[zzyzx]

Thanks for your reply!

Okay, so load transfer won't affect the center of gravity.

If you stuck a 200 lb. engine block in the trunk of a rear-wheel-drive car, you'd get better grip from the rear tires, but when you corner, the extra grip that those tires produce can't overcome the extra forces acting through the center of gravity due to the increased weight. Is that correct?

Say that you did not add weight to a car but just changed the load transfer, say, by stiffening up the back end with a thick sway bar. I understand that overall the grip would be reduced because if all 4 tires were more evenly loaded, then they would be more efficient at generating grip. But since the back tires have extra load on them, shouldn't they have more grip than the front tires? In that case the car should not oversteer (as we know will happen when we stiffen up the back end of the car) but should understeer. What am I getting wrong here?

[maxay1]

Just a real quick reply here, in particular to the last portion of your post (again, with the caveat that I am an idiot): I don't think that increasing stiffness in roll via a thick roll bar is equivalent to increasing load, as you indicate, rather I think of it as increasing the load disparity between the rear tires.
This differs significantly from your earlier idea of throwing a motor in the trunk of a rear wheel drive car. Now those rear tires have to react more inertial force before they produce centripetal acceleration, because of the aft CG location.

I hope this helps a little, and I have to apologize to you, because I'm using your question to test my understanding as well.

Wil

[Greg Locock]

I suggest you do a search on this site. Bear in mind that people throw sacks of cement in the backs of trucks to calm the back end down.

[cheapracer]

1/ Let's say you stiffen up the back end of a car so that when it corners more weight is transfered to the outside rear tire.

2/ If that is true, why is it that when you transfer weight onto the front tires while trailbraking,

KISS...

1/ Because your taking weight from the inside rear tyre (and for most cars adding to the outside front but not taking from the inside front).

2/ Because your adding weight to both tyres.

[zzyzx]

KISS...

1/ Because your taking weight from the inside rear tyre (and for most cars adding to the outside front but not taking from the inside front).

2/ Because your adding weight to both tyres.

Hmmm...I thought that if you stiffen up one end of the car, you are adding weight (load transfer during cornering) to that end of the car and taking it away from the the other end. So, if say you stiffened up the back end, you would get more load transfer going to the back of the car and away from the front when you are cornering. So I was wrong about that?

By the way, I did a search on this site under "load transfer," but I wasn't able to find anything that would answer my questions. Can anyone suggest a link? Thanks.

[gordmac]

The maximum friction coefficient of a tyre reduces as the vertical load on it increases, this means that the total grip of a pair of tyres reduces as the vertical load difference between them increases (constant everything else). Assuming no aero the total vertical load is constant and for a given latac the weight transfer is constant so yes if you transfer more at one end you transfer less at the other. When you brake/accelerate you get fore/aft weight transfer which affects the tyre vertical loads, if you are cornering as well the tyre has to cope with longnitudinal and lateral loads.
The best way to get your head round it is to take a hypothetical car, calculate tyre vertical loads and from those tyre friction coefficients to see what relative grip you have at each end. For a single seater tyre take mu as 2.2 @ 100kg varying linearly to 1.7 @ 300kg and assume linear/lateral grip is a circle.

[MatsNorway]

I suggest you do a search on this site. Bear in mind that people throw sacks of cement in the backs of trucks to calm the back end down.

vinter conditions and racy setup are not comparable.

We had some weight in our cargo car. it also had a diff lock thing.

the weight made it harder for the diff to lock both wheels and therefore preventing slides in ordinary traffic speed. Must have something to do about inner forces in diff and carry loads keeping it down, kinda.

But the weight was primarily there to get up the hill next to our house. (isn't it a better word for hill?)

[McGuire]

If you stuck a 200 lb. engine block in the trunk of a rear-wheel-drive car, you'd get better grip from the rear tires, but when you corner, the extra grip that those tires produce can't overcome the extra forces acting through the center of gravity due to the increased weight. Is that correct?

Depends entirely on the balance to start with. Assuming ideal weight and load distribution resulting in ideal balance, adding weight to the rear will induce oversteer. However, with less than ideal balance, as in a vehicle that is sufficiently nose-heavy, adding weight to the rear will improve balance and latacc. The total load on the tires is increased but its distribution among the four tires is improved. Thus the confounded pickup truck, where handling is improved by throwing some sandbags in the bed. Of course, subtracting weight from the front would be better -- load distribution is improved while the total load on the tires is decreased at the same time. The truck owner will say that unladen, his truck is "too light in the rear" when in fact, in terms of load and weight distribution and cornering capability, it is too heavy in the front -- but he has no practical way to fix that. Balance is a relative property.

vinter conditions and racy setup are not comparable.

Same thing, really. Load distribution at the limits of grip.

[cheapracer]

Hmmm...I thought that if you stiffen up one end of the car, you are adding weight (load transfer during cornering) to that end of the car and taking it away from the the other end. So, if say you stiffened up the back end, you would get more load transfer going to the back of the car and away from the front when you are cornering. So I was wrong about that?

Keeping it simple you are wrong (though in the automotive world theres always an example that goes against the mainflow).


Turning a left hand corner;

If the car is balanced the weight will fall equally to the right side.

Stiffen the rear and the weight wants to fall but the resistance from the rear forces the weight diagonally to the still soft front - eventually the inside rear tyre will come off the ground as the front falls.

..and vice versa if you stiffen the front.

Handling is a bit like golf, you study and practice it for many years just to learn how to keep it as simple and as basic as possible.

[CSquared]

Haven't read the whole thing yet, but this article looks good. The grip-vs-vertical-load graphs I found helpful.
Load Transfer

[MatsNorway]

Same thing, really. Load distribution at the limits of grip.

No its not. The reason people put weight in the back (at least why we did it) is because the forces working in the diff becomes smaller compared to the rest when you add weight.

The forces in the diff that wanted to lock the L/R wheels was lesser compared to the grip when you had the weight.

You kinda softens the diff compared to the weight.


second thoughts says its all relative to the other variables tho, and that's the important thing.

trowing cement in the trunk says that the initial setup might be bad.

Kinda offtopic to trailbraking...

[Fat Boy]

If you stiffen the rear of the car, then you get more weight transfer. Why? Because stiffening the rear means you've used bigger springs and/or anti-roll bars. Bigger springs and bars have more steel and weigh more. If the springs weigh more, then you must have transferred weight to the rear.

[desmo]

If you stiffen the rear of the car, then you get more weight transfer. Why? Because stiffening the rear means you've used bigger springs and/or anti-roll bars. Bigger springs and bars have more steel and weigh more. If the springs weigh more, then you must have transferred weight to the rear.

[zzyzx]

Thanks, guys! It all makes sense to me now.

[tristancliffe]

Easy explanation:

Due to the way rubber works, there is a phenomenon called load sensitivity. In short, it means that two equally loaded tyres produce more grip than when one tyre takes the majority of the load. To maximise grip you want the least amount of load transfer (all other things being equal - which they never are of course. Geometry, response, handling, ride height, aerodynamics etc etc etc mean that you might trade nearly 100% load transfer at one end of the car for better overall performance. See how stiff the front end of the McLaren was last year).

[gruntguru]

Easy explanation:

Due to the way rubber works, there is a phenomenon called load sensitivity. In short, it means that two equally loaded tyres produce more grip than when one tyre takes the majority of the load. To maximise grip you want the least amount of load transfer (all other things being equal - which they never are of course. Geometry, response, handling, ride height, aerodynamics etc etc etc mean that you might trade nearly 100% load transfer at one end of the car for better overall performance. See how stiff the front end of the McLaren was last year).

100% transfer at one end is quite common. When you see a race car lifting a wheel you are certainly seeing 100% transfer at that end. Usually the non-driving wheels because this minimises transfer at the driving end and maximises longtiudinal acceleration when the power is applied.

[tristancliffe]

100% transfer at one end is quite common. When you see a race car lifting a wheel you are certainly seeing 100% transfer at that end. Usually the non-driving wheels because this minimises transfer at the driving end and maximises longtiudinal acceleration when the power is applied.

My point exactly!

[MatsNorway]

Just wanna inform about some things in the strange world of RC i am about to begin racing in.

There seems to be so that a small 1/10 scale rc car on rubber tires get more grip beeing all lose and wobbly at the back. (more grip at the back)

It also slides easier if the diff in the back is lose.

And its common to race with a spool (no diff) in the front.

I find all this very strange.

Edited by MatsNorway, 22 March 2010 - 23:07.

[CSquared]

Due to the way rubber works, there is a phenomenon called load sensitivity. In short, it means that two equally loaded tyres produce more grip than when one tyre takes the majority of the load. To maximise grip you want the least amount of load transfer

The graphs in the article I linked to helped me understand, or at least visualize, this better than I had before.
Load Transfer by Wm. C. Mitchell

[McGuire]

I suggest you do a search on this site. Bear in mind that people throw sacks of cement in the backs of trucks to calm the back end down.

No its not. The reason people put weight in the back (at least why we did it) is because the forces working in the diff becomes smaller compared to the rest when you add weight.

Most users add weight to the rear of their otherwise unladen trucks to improve the cornering -- which it will. This fact should not baffle or annoy anyone, nor is the purpose to irritate people but simply to illustrate vehicle load and balance. A truck chassis is designed for reasonably neutral handling with hundreds, maybe thousands of pounds loaded over the rear axle. With no cargo load its handling properties will not be the same -- the CG has migrated forward; the balance has changed. As every truck driver knows, here the rear tires do not have greater grip with a lighter load. They have less grip: On wet pavement or at excessive speed the rear end can step around in a heartbeat. Oversteer in the official SAE definition: rear slip angle > front. Adding ballast will correct that. The total load carried by all the tires is increased, but the load distribution is improved.

Around here we have a simple sort of steady-state model that says adding weight to an axle must tend to unstick that end. True, if the grip balance is ideal to start with. But if not, not necessarily. And we also have a presumption that a race car is like our model, certainly not like our truck in the above; but again, not necessarily. Example: when the driver reports that the race car handles better on full tanks. According to our model that can't happen, but on race tracks it happens every Sunday.

Balance is a relative property. As Gordmac first noted here and has been repeated: any uneven distribution of load between two tires will decrease the combined load capacity of the pair. The reason for this is simple: the two tires are headed in different directions on their asymmetrical load/grip curves. This holds not only for left/right axle pairs but front/rear and diagonally. Race car, dump truck, farm wagon. If it has one or more tires on each corner, that's how it works.

[cheapracer]

. Example: when the driver reports that the race car handles better on full tanks. According to our model that can't happen, but on race tracks it happens every Sunday.

people note he said "handles better" not "goes faster".

Edited by cheapracer, 24 March 2010 - 15:45.