Why does the steering wheel wiggle work?
11-09-2004, 11:37 AM
#31
Race Director
When I first started doing DE event I would try to keep the wheel as constant as possible. I knew that and added turing was not good. Smooth = faster.
As I go faster and faster and pushed the edge of the friction circle I had to learn to "correct" the car at times. Well now I have gotten to the point where the steering wheels is rarely fixed in a corner. It is often turn in and out. I started this as corrections and now at least "feel like" I have gotten the point where these little corrections are helping to stablize the car.
Remember that holding a car at the very limit is very much like balacing a broom with your finger. You must constantly be making adjustments and corrections to maintain the unstable equilibrium. Thus the top of the broom hardly moves, but the base and you finger are constantly making short simple movements.
As I go faster and faster and pushed the edge of the friction circle I had to learn to "correct" the car at times. Well now I have gotten to the point where the steering wheels is rarely fixed in a corner. It is often turn in and out. I started this as corrections and now at least "feel like" I have gotten the point where these little corrections are helping to stablize the car.
Remember that holding a car at the very limit is very much like balacing a broom with your finger. You must constantly be making adjustments and corrections to maintain the unstable equilibrium. Thus the top of the broom hardly moves, but the base and you finger are constantly making short simple movements.
11-09-2004, 12:04 PM
#33
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Guys, great commentary above. Permit me to add a slight clarification. Tires generate their maximum lateral grip just as they begin to slide. Perhaps this is the ideal mu-angle 924racer describes? Since no track surface is perfect, it is impossible to keep the tires exactly at the point of maximum grip - perhaps the driver has gotten the car there but a bit of dust a slight bump a change in camber will affect the perfect slip angle. These changes will require a correction with with the throttle or the steering. Since we are trying to accelerate as hard as possible as early as possible, re-establishing the slip angle means a tweak to the steering.
John - i agree with your "reactive versus proactive". I think that's what I said in my first post but you said it more eloquently.
FBB - I think John is right - perhaps the sawing gave you a better idea of the available grip and increased your confidence level which made you quicker through some turns. I think that you were calibrating your butt-o-meter whether you knew it or not.
Dnitake - yes there is hysteresis in a tire and it does apply to its slip angles. The lessons we learned in physics about friction proved that no tire could have more than one G of lateral acceleration. That was based on a perfectly smooth wheel and a perfectly smooth friction surface. That simplified model of friction would also deny that certain creatures can walk up a window glass or across the ceiling. Certain animals and insects have microscopic grippers which allow them to climb a vertical glass surface or cling to a ceiling.
Our tires behave in a similar fashion, someone called it "mechanical grip". The rubber conforms to the irregularities in the road surface a little like the tiny hairs on a Gecko's foot.
Temperature is a factor for sure, as tires get hotter they grip better because the rubber is softer and better able to conform to the surface. Beyond a certain point, the rubber loses some friction and grip is reduced quite a lot. Sadly an overheated tire does not regain its grip once it cools to a more acceptable temp range. Once it is toasted, it stays that way and the rubber is hard. Think of it being "charred" - think burst toast being less pliable/ The tire mavens will probably chuckle at this analogy but it works for me. Lastly, I am or shall I say was a sailor until about a month ago when I sold my Jeanneau 45.2 "Redline". Weekends were either 100 MPH or ten MPH
Best,
John - i agree with your "reactive versus proactive". I think that's what I said in my first post but you said it more eloquently.
FBB - I think John is right - perhaps the sawing gave you a better idea of the available grip and increased your confidence level which made you quicker through some turns. I think that you were calibrating your butt-o-meter whether you knew it or not.
Dnitake - yes there is hysteresis in a tire and it does apply to its slip angles. The lessons we learned in physics about friction proved that no tire could have more than one G of lateral acceleration. That was based on a perfectly smooth wheel and a perfectly smooth friction surface. That simplified model of friction would also deny that certain creatures can walk up a window glass or across the ceiling. Certain animals and insects have microscopic grippers which allow them to climb a vertical glass surface or cling to a ceiling.
Our tires behave in a similar fashion, someone called it "mechanical grip". The rubber conforms to the irregularities in the road surface a little like the tiny hairs on a Gecko's foot.
Temperature is a factor for sure, as tires get hotter they grip better because the rubber is softer and better able to conform to the surface. Beyond a certain point, the rubber loses some friction and grip is reduced quite a lot. Sadly an overheated tire does not regain its grip once it cools to a more acceptable temp range. Once it is toasted, it stays that way and the rubber is hard. Think of it being "charred" - think burst toast being less pliable/ The tire mavens will probably chuckle at this analogy but it works for me. Lastly, I am or shall I say was a sailor until about a month ago when I sold my Jeanneau 45.2 "Redline". Weekends were either 100 MPH or ten MPH
Best,
11-09-2004, 01:46 PM
#34
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Couple of things just to clarify...
There is no slip angle in braking, however there is something called a slip ratio (or percent slip). Usually at somewhere around 10% slip the tire has max friction coefficient in braking and acceleration. Additionally, the dropoff in the slip curve after the max is generally broader than the slip angle curve...meaning it is easier relative to turning at the limit, to be around the limit while braking or accelerating and not totally losing it.
EDITED
There is no slip angle in braking, however there is something called a slip ratio (or percent slip). Usually at somewhere around 10% slip the tire has max friction coefficient in braking and acceleration. Additionally, the dropoff in the slip curve after the max is generally broader than the slip angle curve...meaning it is easier relative to turning at the limit, to be around the limit while braking or accelerating and not totally losing it.
EDITED
Last edited by FormulaOne10; 11-09-2004 at 02:08 PM.
11-09-2004, 03:14 PM
#35
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F110 - same applies to acceleration as in braking. applying power increases the traction -up to a point.
11-09-2004, 03:27 PM
#36
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I agree Bob. I was making the point that braking/accel and turning are not totally equal in reguards to traction loss. Traction falloff from too much slip angle (turning) is steeper than traction falloff from too much slip ratio (braking/accel). Carroll Smith does a good job of representing this in "Tune to Win." I think I phrased it rather poorly in my first post.
11-09-2004, 03:52 PM
#37
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F110 - agree. I have CS's books on my shelf too! Should be required reading for all
11-09-2004, 05:00 PM
#38
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I've just turned in my resignation so I can peruse this board on a full time basis. I'd like to work out the terms of my employment here. Bonus structure, residuals, IPO options... Please get back to me as I'll be holding my breath...
Redline man, did you have a problem w/ my slip angle. Slip angle is the optimal angle to do a perfect parallel park job on the infield, right? Great explanations BTW... Whoever started this thread I'm sure got more than their money's worth...
OT - Bob Rouleau, a Jeanneau 45... I spent time this year on a friend's boat here in SF Bay Big Boat Series. A boat called Acabar. It's less a boat and more of a love shack if you know what I mean...
I've gone through one too many heat cycles on my front tires. Too bad as there is plenty of tread left for a few more sessions. They car now pushes like crazy which makes getting the car to rotate an interesting throttle practice point. Do tires come pre-"charred" from Tire Rack?
Redline man, did you have a problem w/ my slip angle. Slip angle is the optimal angle to do a perfect parallel park job on the infield, right? Great explanations BTW... Whoever started this thread I'm sure got more than their money's worth...
OT - Bob Rouleau, a Jeanneau 45... I spent time this year on a friend's boat here in SF Bay Big Boat Series. A boat called Acabar. It's less a boat and more of a love shack if you know what I mean...
I've gone through one too many heat cycles on my front tires. Too bad as there is plenty of tread left for a few more sessions. They car now pushes like crazy which makes getting the car to rotate an interesting throttle practice point. Do tires come pre-"charred" from Tire Rack?
11-09-2004, 06:17 PM
#39
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Dan - as far as I know, Tire rack does not toast the tires before shipping. Guys like us manage to do it all by ourselves. One of the last events I attended (a private track day) featured a bunch of GT3's. They called us collectively a "GT-12" as there were four cars going at it inches apart. We all agreed later that it was the best racing we "never had". All of us toasted our tires that day much to the amusement of the local Pirelli dealer. We also learned that by drafting the entire group took a second or more off the lap times - faster than we could go on our own. Those Nascar guys are on to something We all had plenty of tread left mind you but the usually soft tires had the consistency of hockey pucks (something we know a lot about up here) and some blisters the factory didn't put there originally.
We all had plenty of tread left mind you but the usually soft tires had the consistency of hockey pucks (something we know a lot about up here) and some blisters the factory didn't put there originally.
11-09-2004, 09:53 PM
#41
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CC - it's not a sin for us to agree you know. Now,please start a controversial thread - it's getting dull around here
