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Porsche Crest Why lifting a front wheel is bad

Ok, from the discussion on wheel lift over on the technical board, I remembered something I read in Tune to Win about that.

This discussion is found on page 17 of the First edition, for those that have the book.

Basically, Smith says that as the vertical load on a tire increases, the coefficient of friction decreases. The curve is a very gentle decrease, however, so that the tire's TRACTION increases as vertical load increases, up to a point anyway. As vertical load rises, the slope of the curve of tire force gradually flattens.

"a pair of wheels with lateral load transfer between them is not capable of generating the same amount of cornerning force that the same pair of tires could if they were equally laden."

Let's use a real-world example from a 911: Corner balance numbers are: 550 RF 550 LF 795 LR 790 RR. Let's use the front wheels, and assume that the tires have a steady-state friction coefficient of 1.4.

So, equally laden, 1.4 x 550 = 770 pounds x 2 wheels = 1540 pounds. 1540 pounds divided by the 1100 pounds actually supported by the front wheels means the car can corner at 1.4g.

Now, let's assume an 80% load transfer from the inside front to the outside front. The inside front generates 20% x 550 pounds, at the full coefficient of 1.4=154 pounds and the outside generates 180% x 550 x a reduced coefficient of 1.3 = 1287 pounds. Add that to the 154 pounds generated by the other tire and you have 1441 pounds, divided by the 1100 pounds each supports, and you have a cornering force of 1.31g.

Now, assume a 100% load transfer in the same scenario. The outside tire has double the load and the inside has zero. So 200% x 550 x 1.3 = 1430 pounds / 1100 = 1.3g

Anybody have ACTUAL data on Hoosier friction coefficients?

Any thoughts on the subject?

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Old 07-25-2004, 09:57 AM
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Re: Why lifting a front wheel is bad

Quote:
Originally posted by john_cramer

"a pair of wheels with lateral load transfer between them is not capable of generating the same amount of cornerning force that the same pair of tires could if they were equally laden."
That is the key, and the reason anti-roll bars and stiffer suspensions are effective. They do not alter the weight transfer at all (only moving the CG will do that) but they more uniformly distribute the load.
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Old 07-25-2004, 11:19 AM
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John Colasante;
The problem with that logic is that stiffer sway bars or T-bars DO transfer more weight to the outside wheels. Raising the roll center will do the same. The important thing with a 911 is that with all of the weight out back, the early cars have ultimate oversteer (bigger tires in the back help). By running a stiffer front bar weight is transferred from the overworked outer rear tire to the relatively under stressed outer front tire. The grip from the inner front tire is basically sacrificed in order to get a better balanced car with a higher ultimate cornering potential.

Now admittedly, adding weight to a tire while cornering produces diminishing returns which is why if you can keep the inner tire on the ground and get grip from it, it will help that axle get more grip. But in the case of the 911, it's not the front axle's grip that is limiting the cornering potential, but the rear end's grip.

So John Cramer, you are right that from a design perspective you want to set-up or design the car with a front to rear weight balance (as well as the corresponding roll centers) such that you can keep all 4 tires on the ground and maximize their grip. In the case of the 911, the car's design is pretty well set. So in the absense of a significant redesign (by doing such things as moving the oil tank to the front, or raising the rear roll center using a Turbo rear suspension, etc. etc) you've got to try to maximize what you've got for a car. In many cases this means that the inner front wheel will be in the air in order to keep the rear end planted.

PS: BTW, if you were to redesign the suspension such that the roll centers are below ground, the result will be a car that leans into a corner rather then away from the corner. But loading up the inside tire has a number of wierd knock-on affect like jacking a car down while cornering. I think that is why I'm not aware of any cars designed with that geometry.
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Old 07-25-2004, 01:34 PM
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The problem with equation Force of Friction = (coefficient of friction) X (normal force), is that it is an approximation. It works very well in static and simple dynamic situations, but even a since tire to pavement interface is an extremely complex mix of static and dynamic friction, slip angles, heat, and tire compound issues. Attempting to account for the complexities by varying the coef of friction is one way of getting better results, but it is really missing a lot of factors.

When I was in school a professor handed out a copy of a paper on slip angles and car handling. I'm gonna have to see if I can find that. I do remember on thing from the paper. Lateral (or cornering) force in a tire increases with increasing slip angle until a certain critical slip angle. After that the force generated actually decreases, which is why you may be sliding along one moment and spinning wildly the next.
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Old 07-26-2004, 10:30 AM
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I think John Colasante is correct. The stiffer suspension only keeps the car from rolling. The weight transfer is determined by the lateral G's the car is pulling. There is slightly less weight transfered by the stiff car because when the car rolls the CG moves abit.

-Andy
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Old 07-28-2004, 07:47 PM
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John Cramer,
I too would like to see the specs on the new Hoosiers.
Your question got me looking in all my Carrol Smith books.
I learn something new everytime I reread anything he has written.
Bottom line I'll take a 1.3G with one wheel in the air.
Measured best so far is just over 1g with RS03's.
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Old 07-28-2004, 08:09 PM
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Andy;
Sure the CG moves, but so the do roll centers against which the CG is pushing. To consider one without the other is to ignore half of the system.

If the distance between the roll centers and the CG's at each end of the car small, the car will transfer more weight to the outside wheels and roll less. If the distance is larger (thus making a longer lever arm) the roll will be greater but the weight transfer less. Having the roll center below ground can make the load transfer actually be to the inside wheel.

Then keep in mind the comparative length of the lever arms (distance between the roll centers and CG at that end of the car) front to back also makes a difference in where the weight gets transfered to.
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Last edited by jluetjen; 07-29-2004 at 03:00 AM..
Old 07-29-2004, 02:56 AM
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John, very good follow-up. I certainly take your point that everything in racing is a compromise.

Can you explain how the ARB transfers weight at the OTHER end of the car? I've read your discussion in the archives and I still don't quite grasp it.

Has anyone actually plotted the suspension pickup points of the 911 to determine, statically anyway, the location of the roll center? What is the calculation for CG, I have found a spreadsheet that does it but want to code one myself. (I know how to figure it in an airplane. . .)
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Old 07-29-2004, 06:05 AM
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There's a method that involves corner weighting the car, then weighing it again after putting the front (or rear) scales up on a raised platform of some kind. You see what the weight differences are. You also try with both left wheels (or right) up in the air by a given amount. There's some trig that you can then do that will give you an approximation of the CG.

Hopefully someone smarter than I am (at ten til 8 in the AM, pre-caffeine!) can figure it out from that, or will remember the exact procedure.

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Old 07-29-2004, 06:50 AM
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Quote:
Can you explain how the ARB transfers weight at the OTHER end of the car? I've read your discussion in the archives and I still don't quite grasp it.
Hmmm.....



Excuse the artwork. Imagine a 20 lb frame suspended on some springs on top of scales. For the sake of discussion, the frame is infinitely stiff and can not slide off of the springs.

1) The 20 lb frame is supported on 4 springs. The scales under the springs would read 5 lbs in a static situation.

2) Now apply a 20 lb force laterally against the center of the frame to simulate a 1G turn. The result would be that the scales under the springs on the far side would read 9 lbs each while the scales on the near side would read 1 lb each. Think of it as being an additional 8 lbs on the far side and 8 fewer lbs on the near side across the axis of the lateral force.Still the total would still be 20 lbs.

3) Now add a very stiff sway bar across one end. This sway bar would transfer much of the frame's weight directly down on left-far-side scale bypassing the spring. Conversely, it would pull up on the left-near-side spring and thus would register a negative value on that scale. Because the sway bar is resisting the twisting force on the left side, the springs on the right side have essentially no deflection (remember, the frame is infinitely stiff and the left side of the frame is propped up by the sway bar) and so by cooincidence wind up at about 5 lbs each again.

Does this help?

Quote:
Has anyone actually plotted the suspension pickup points of the 911 to determine, statically anyway, the location of the roll center?
Doing a couple of real rough measurements and visual estimating, my guess is that the front roll center is about 4 inches above the ground (on my car at it's current ride height), and 7.7 inches above ground in the back. My really rough estimate is that the CG is about 21 inches high. Now keep in mind that all of these will change depending on the height of the car. This is especially true in the front where the roll centers can move around a lot, even below ground level with fairly small changes in the ride height depending on the ange of the front A-arms.

Can anyone confirm or contridict my estimates?
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Old 07-29-2004, 01:18 PM
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NOW I get it. That was a very well-done explanation.

Thinking about it like that, the ARB has a couple advantages:

1) By limiting the amount that the roll center moves, you limit potential camber change, resulting in higher coefficient of friction; and

2) By controlling weight transfer, you load the left REAR tire more, which diminishes its coefficient of friction, but increases total traction, and

you reduce the load transfer to the left FRONT, improving its coefficient of friction, AND you gain traction from the RIGHT FRONT as well.

I need to build an excel model of the coefficient of friction fall-off to really experiment with stuff, but I get the idea now.

By the way, you are showing your history as an FWD racer by lifiting the RIGHT REAR tire off the ground!
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Old 07-30-2004, 07:47 AM
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If you are interested in understanding suspension I highly suggest:
Race Car Vehicle Dynamics by William F. Milliken and Douglas L. Milliken
It is by far the best reference on cars I have ever read. I think it cost about $100, but worth every penny.
The Carrol Smith books are good for simple understanding of racecar basics, along with ways to improve suspensions, but does not provide real indepth analysis or theory (but they are great for an introduction and probably should be the first book someone should read on suspensions).
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Old 07-30-2004, 08:25 AM
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John; Actually when I was referring to the "front", I meant the side of the model that was closest to the reader. As I wrote it, I was thinking of a 911 travelling from right to left, so the sway bar was put on the "front". In fact the model as drawn is really directionless.

Quote:
1) By limiting the amount that the roll center moves, you limit potential camber change, resulting in higher coefficient of friction;
I generally wouldn't think of it as limiting the roll center movement so much as just resisting the rolling motion. If you car's camber curves result in sub-optimal camber in roll, a stiffer sway bar can help, but then so can stiffer springs. I don't think that a sway bar will change the movement of the roll center much, even in the front of the 911 since a sway bar doesn't affect the suspension in bump.

Quote:
2) By controlling weight transfer, you load the left REAR tire more, which diminishes its coefficient of friction, but increases total traction, and
The coefficent of friction does not necessarily decrease by increasing the load of a tire. It actually looks somewhat like a HP curve. So increasing load to a point increases traction, but then you'll go over the top and traction will decrease. Most car/tire combinations are designed such that they operate pretty close to that peak. But there are some (SR's for example) which are so light on their tires that increasing the load on a corner can actually increase traction until the camber falls off. But that tends to be the exception, not the rule.

The rule in general is that by stiffening up the front sway bar, you will increase traction at the back -- and vice versa. The important idea is that by adding a stiffer front sway bar, you are transferring weight in roll to the end of the car which has the (stiffer) sway bar. This un-loading of the opposite end of the car can pull the outside tire back from downward side of the traction curve.

BTW, another excellent source on suspension dynamics (and much cheaper too) is Alan Staniforth's "Competition Car Suspension" and "Race and Rally Car Source Book", both of which are available from Amazon.

Quote:
By the way, you are showing your history as an FWD racer by lifiting the RIGHT REAR tire off the ground!
BTW; While I used to race a FWD car, when did I say anything recently about lifting the Right Rear tire off of the ground???

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Old 07-30-2004, 11:22 AM
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With the inside rear at -2, that would be off the ground!

Here's what I thought you meant:
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Old 07-30-2004, 11:55 AM
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OHHH. That just goes to show that the chassis dynamics of a Front-Engined - FWD car are very similar to those of a Rear-Engined - RWD car, just in reverse. Like I said, I envisioned the frame travelling from right to left, you saw it going from left to right.

Now I gotcha!
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Old 07-30-2004, 12:00 PM
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LIFTING A FRONT WHEEL IS NOT BAD!
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Old 07-30-2004, 01:44 PM
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You have to admit, lifting a rear wheel going around a turn sure looks worse! It's like walking my dog!

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Old 07-31-2004, 04:49 PM
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John,

Can you diagram how a car could have a roll center below the ground? When I say that the weight transfer is the same between the stiff car and the soft car I make the assumption that the car doesn't roll at all. I realize that this is not strictly correct, however the amount of weight transfer due to the car rolling is much smaller than the effect of weight transfer due to pure lateral force. The largest factor in weight transfer by far is the distance of the CG above the ground and the g force of the horizontal acceleration.

The point of my argument is that a stiff car will corner better only by controling camber better and by the fact that you can lower a stiff car more than a soft one thereby shortening the distance of the CG above the ground.

-Andy
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Old 08-01-2004, 09:26 PM
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Come to think of it...if you have the roll center below the ground I think you would have the CG of the car move to the outside of the turn more than if you have the roll center above the CG. If you constructed a car with the roll center co-located with the CG then the weight transfer would be all due to lateral acceleration. You could never have a car transfer weight to the inside wheel on a corner unless the CG was underground (a difficult engineering problem).

-Andy
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Old 08-01-2004, 09:32 PM
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Andy;
Here's an example that I drew up based on the front suspension on an "aggressively" lowered 911. It would also apply to the front suspensions of any other car which uses McPherson Struts. Note that it is not so much the height of the CG above the contact patch that matters so much as the height of the CG above the RC and the relationship of the RC to the contact patches.



While you're right that it's not exactly rolling onto the inside tire (a poor choice of words on my part), note that with the RC below the ground level that the torque being applied to it can cause car to "jack-down" on it's suspension. This is the exact opposite of the situation that the early Spitfire was experiencing in the picture that I posted above. So rather then trying to jack up and roll over the tire's contact patch like in the Spitfire picture, an overly lowered 911 could try to jack down and roll under the tire's contact patch. It sounds strange, but it is not unheard of.

I just thought of a simple model to test this. Torque up on a wheel lugnut that is below the horizontal centerline of the wheel using a torque wrench or ratchet where you are applying torque just above the centerline of the wheel. Which way does the wheel turn??? Very the lever length. If the the wrench's lever is too long, it will spin the wheel the same way that the wrench is pushing, but I think if the distance from where you are applying force onto the wrench's handle to the wheel center is less then then distance from the wheel center to the lug nut, the wheel will spin in the same direction but the point on the wrench where the force is applied will pass under the wheel's center.

Note that the wheel's centerline will represent the tire's contact patch, the lug nut the roll center and you're hand on the wrench handle the inertial forces applied to the CG.

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Last edited by jluetjen; 08-02-2004 at 06:29 AM..
Old 08-02-2004, 05:56 AM
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