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uploaded 7/17/2000

A Conversation with a Driver about the Load Sensitivity of Tires

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Guy Cosmo is an open-wheel driver from New York currently racing in the Scandinavian F2000 Championship.

Guy has been extremely busy since he arrived in Europe in March. After signing with Racing Denmark, now officially known as Team Den Bla Avis, he's been adjusting to the Danish lifestyle, weight lifting, running, rollerblading, working on his apartment and testing and developing the new Mygale F2000 cars.

First message, 5/22/00.

Hello Mr. Haney,

My name is Guy Cosmo. I am a young American race driver pursuing my racing career in Europe. I have just read your article on aerodynamics. I am very interested in your books and especially your monthly newsletter. I would like to know more about the information you have available and if it's possible to contact you in the future with some technical questions. I am currently racing Formula 2000 with Team Den Bla Avis as the works driver for Mygale Race Cars. We are developing a completely new car and are experiencing a bit more difficulty with the development process than expected. I am always interested in learning as much as possible and, after reading your article, felt obligated to send you an email.

I would appreciate any information possible at your earliest convenience. Thank you for your time and I hope to hear from you soon!

Guy Cosmo

My reply, 5/23/00.


Thanks for the message. I'm glad you liked the article. That's an excerpt from Inside Racing Technology. That book and Inside Racing are both available on I can sell them to you myself but don't take credit cards.

A successful racing career is extremely difficult and I wish you all the luck. If I can help at all I will.

I don't have a lot of hands-on experience as a race engineer or designer but I have friends in the business and talk to many engineers and suppliers of racing technical components. At least I have time to think! Racers get caught up in minute details and don't ask WHY? As a result most race setups come from some voodoo beliefs inside someone's head.

I'd like to help you find out if you are aware of a couple of important concepts that few racers understand. If you'll give me short answers to a couple of questions we'll have a conversation that will tell me where you are on the learning curve. That will, of course, also tell me where your current mentors are on that curve. I guarantee you'll learn something useful.

Question: How does a change in anti-roll bar stiffness affect the balance of a car and why? What are the benefits and tradeoffs of using an anti-roll bar?

Short answers, no term papers. "I don't know" is an acceptable answer.

Thanks, Paul

Guy's reply, 5/29/00.

Hello Paul,

Sorry about the delay, I've been away at a race this past weekend. I'm not too happy with our progress so far, although I did collect points this weekend with a 5th and a 4th. Suddenly our mild oversteer problem has turned into an unbelievably evil oversteer problem, and now his friend Wheel Spin has joined in for the fun. Everything we tried to solve the problem [which in theory were correct changes] had no effect, or made the car worse. That makes me think that we're either above or below the working range of the springs and bars. Very long story, but we'll figure it out.

I'm really intrigued with your response to ask me some questions. I guess it would immediately give you an idea of where my knowledge stands and this is definitely one of the more interesting offers for help I've received. I'd like to know how much I do or don't know, so please send more my way!

Question: How does a change in anti-roll bar stiffness affect the balance of a car and why?

The stiffer the bar, the less grip in that end of the car. Why? I really don't know why.

What are the benefits and tradeoffs of using an anti-roll bar? Benefits or tradeoffs of using an anti-roll bar as opposed to...what? Not using one or changing something else in the car? I'd say a benefit is less roll! The more level the car stays during weight transfer the better. Why, I'm not really sure. You can stiffen the front bar to control most of the cars roll and reduce wheel spin, but risk losing grip in front. You can stiffen the rear to get a car to rotate but risk gaining wheel spin at the same time.

I would also imagine that during weight transfer/roll, an anti-roll bar actually helps transfer more weight to the loaded tires than if not using a roll bar. But this is why I'm not a race engineer!

Since my days of karting I spent two years working with Joe Stimola in Formula Ford and Formula Continental, where he taught me as much as he could and I learned very well the basics of car setup. I then raced for most of 1999 just engineering my car along with my father, who was right by my side as I learned from Stimola. During the latter part of last year in the United States I was working with an extremely good engineer by the name of Ian Willis. He is currently working with the Brian Stewart Indy Lights team, and runs Aim Autosport out of Canada, along with Bordin Racing. I had only spent nearly 3 months working with Ian, but immediately noticed a tremendous difference in his approach than anyone I've been in contact with, and we established a great working relationship that I see as one of my greatest experiences.

I feel that now I need to keep learning as much as possible to help myself. Please let me know what you think, and send some more questions my way. I greatly appreciate your help!

Guy Cosmo

My reply, 5/30/00.


"The stiffer the bar, the less grip in that end of the car. Why? I really don't know why."

Great answer!! Thank you!!

I've been trying to understand this stuff for many years. I got an engineering degree to try to understand it and that did help, some.

I've always read that that softer springs and shocks and lower air pressure in the tires gives more grip, but never could find an explanation of why. About six months ago I read some books on tires and found out the WHY to all this.

I can't explain it all in an email but I'm writing a book that will lay it all out. I'll just give some general basics here.

One Really Big Deal is how forces transfer among the four tire patches as the car corners, brakes, and accelerates. What makes this a Big Deal is LOAD SENSITIVITY OF TIRES. Going from no force on a tire to low force, grip goes up linearly. That means an increase in tire load gives a proportionate increase in grip. 3 pounds more force might give 2 pounds extra grip. 9 pounds of load would give 6 pounds more grip. The multiple here is obviously three. That depends on the tires.

At light loads tires behave as we were taught in high school physics. A coefficient of friction (unique for each pair of materials in contact) times the force pressing the materials together gives the amount of friction force produced. The equation is Cf x F = Ff.

But at some point the relationship between a new increment of force down on the tire and the next increment of grip produced changes. The next 10 pounds of load on the tire produces less grip than the last 10 pounds did, maybe only 8 pounds. This is the load sensitivity of tires.

That's the reason a racecar goes faster through a corner when its center of gravity is lowered. A lower CG transfers less weight (force) from the inside to the outside tires. Force transfer from inside to outside results in the outside tires gaining less grip than the inside tires give up. So less overall grip. An anti-roll bar does the same thing as you said in your email. It provides some roll stiffness so softer springs can be used but the price is force transfer from the inside tire to the outside tire and less grip at that end, as you also said.

If you have trouble visualizing what a sway bar does look at your car and imagine the chassis rolling. The sway bar resists that movement by pressing down on the outside tire and picking up the inside tire. But I think you already understand that.

This is only part of the story but it's a start. Does this make sense?

One of the big benefits a driver could get from knowing this stuff is he could interview an engineer and find out his level of knowledge. If it was me I wouldn't hire a guy who couldn't explain to me in plain English how a sway bar works.


His reply. 6/7/00.

Hello Paul!

I have to say that your answer was one of the most interesting and clarifying I've ever come across! All of that makes complete sense to me, and I feel like the covers are being removed from my eyes!

I had the privilege of speaking with current Team Green engineer Tony Cicale early last year. In case you didn't know [which I'm sure you do] Mr. Cicale is considered one of the world's best race engineers. He mentioned during our conversation that he had always run all of his cars as low as possible, and was actually known for splitting a car in half at one time, running a bit too low. Although he did not get into specific physical and technical theories [being that the conversation was based around the "direction" of my career], he hinted slightly at the same theory you just proposed to me.

Another question I then have is if this theory is still applicable in wet conditions. Maybe more weight transfer is needed for the rain for more grip, or maybe even less weight transfer for the rain, given the lack of grip in the track - and to not over load the tires...?

Then in the dry, I would assume the load sensitivity of the tires can be altered by tire pressure, but then tire temperature must be taken into consideration to not over heat the tires.[?]

I could probably sit and think about this forever, and come up with a few thousand questions for you. I really appreciate your taking the time to explain all of this to me, I feel this is going to be a big help in my learning curve. I would really love it if you could ask me some more questions, this is really exciting!

Write back when you have a chance, and thanks again!

Guy Cosmo

My reply, 6/8/00.


Thanks for the message. I'm glad you see the significance of the load sensitivity of tires. I'm not surprised that Tony is familiar with that. He is one of the best driver coaches around. And a great guy.

All of your questions are relevant and show you get the picture. But you need the complete explanation to see how it all fits together. I'm working on how to put all that together. I make it hard on myself. When I write an explanation I don't want there to be any room for questions. That means I have to understand it all myself and figure out how to present the information so everyone else understands it.

Here's a partial explanation:

Tires generate grip by surface friction, mechanical keying, and tearing and abrasion. When the surface is wet the surface friction goes away leaving the mechanical keying to generate the grip. That's why wet tires are so soft. But the mechanical keying is very load sensitive, it gains less grip at higher loads. Soft springs and shocks result in lower tire loads. That's why wet setups are soft A smooth driver also produces lower tire loads.

Most drivers need to feel the car take a set for confidence. But a very responsive, quick-setting car generates high tire loads. That's great if the tires are hard and need heat. A driver with the confidence to drive hard while the car is still moving is getting more grip from the tires than a guy who has to feel it take a set right now.

Does that make sense?


Guy's reply, 6/12/00.


I think all of that makes complete sense. This is really interesting. If you don't mind, I'd love for you to ask me some more questions. Since we've begun this series of emails you've really got my brain working, and I'm excited to see what else I do or don't know.

As for my racing, this weekend has been much better for us. For some unknown reason the cars are working well, and I finished 2nd in yesterday's race, along with setting fastest race lap...! We've done nothing to the cars, so I'm really baffled as to why they're working well, especially since last week they were horrible - but at a different track. I race again tomorrow, starting from outside pole. Hopefully in my next email I'll tell you I won. Time will tell!


My reply, 6/12/00.


Glad you've been stimulated by the discussion. I'd like to know if thinking about tires differently has affected your driving style. Is that possible?

Your not knowing what's different about your cars between feeling bad and good is not unique in racing. I interviewed Claude Rouelle, an engineer who's been giving vehicle dynamics seminars for a few years. In this interview, which appeared in my newsletter and Racecar Engineering magazine, Claude laments the widespread ignorance in racing and says, "It's almost as bad to qualify on the pole and not know why as to fail to qualify and not know why."


His reply, 6/15/00.

Hello Paul!

I can't say that what I've learned about a tire's load sensitivity has changed my driving style YET, but it certainly has changed the way I think about setup and what goes on while I'm driving. Unfortunately it's been very rare lately when my car is balanced and working properly, but at a point this past weekend the car was actually working very well. During this time was the first in a long time that I was able to concentrate on exactly HOW I was driving. The car was so balanced and comfortable for me to drive that I was able to think about being as smooth as possible and scrubbing as little speed as possible. Which led me to start thinking about how much weight transfer I created in the car, all coming from the tire load sensitivity theory you just explained to me.

I certainly believe that over time I get behind the wheel I'll always be thinking of weight transfer and load sensitivity. I used to thrive on an extremely stiff car that was very snappy. Over the years I have learned that a softer car is more forgiving and more adaptable to changing conditions, and what you've now taught me about tire load sensitivity will have an effect on how I drive, how I feel what the car is doing and how I approach set up.

Talk to you soon!

Guy Cosmo

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