Tire slip ratio

JamesShort · **Posted:** Tue May 31, 2011 11:24 am

I've been reading some books lately on tire engineering etc and I understand what tire slip ratio is from a purely conceptual standpoint, but I really is eluding me what is happening from a practical standpoint. I know that under drive torque (acceleration) the front of the contact patch is compressed, enters the patch, sticks to the road and then relaxes towards the rear of the contact patch as the normal force gets to the point that the max static friction between the tire/road is eclipsed and then it slips and gets back into the uncompressed state.

Does the slip ratio basically say that since the contact patch 'circumference' is always shorter than what it would be if there was no applied drive torque because of this compression, that the amount of circumferential distance traversed by a dot on the circumference of the tires is more than a free rolling tire and hence the 'slip' percentage?

It's just hard to visualize that more than 100% of the circumference of the tire (translational distance) actually touches road in one revolution of the wheel under drive/brake torque.

shawnwhipple · **Joined:** Wed Aug 20, 2003 6:28 pm **Posts:** 603

From what I remember about slip ratio. Slipping is always occuring at some ratio whenever a load is applied to the tire which is in contact with the road. So a high spot of rubber on the tire rotates into contact and then slips/grips, slips/grips continually until it rotates out of contact again.

Slip ratio is the difference between the distance the tire travels versus the distance the car moves forwards. Or think about it this way: Slip ratio is the difference between the powered and non-powered wheels on a car if you are looking at acceleration.

I hope that helps.

Shawn

ps. I'm not an engineer but I did stay at a Holiday Inn Express last night.

steve remchak · **Posted:** Fri Jun 03, 2011 10:54 pm

you really need some track time dude............... slip right up to the wall at Charlotte. it'll put hair on ........ well something

Graham Jagger · **Joined:** Fri Aug 29, 2003 12:53 am **Posts:** 1718

steve remchak wrote:

you really need some track time dude............... slip right up to the wall at Charlotte. it'll put hair on ........ well something

... but is that slip ratio or just hitting the flap spot on the tire? :lol:

steve remchak · **Posted:** Sat Jun 04, 2011 9:01 pm

Graham Jagger wrote:

steve remchak wrote:

you really need some track time dude............... slip right up to the wall at Charlotte. it'll put hair on ........ well something

... but is that slip ratio or just hitting the flap spot on the tire? :lol:

round tires are for wussies.

Cash Davidson · **Posted:** Tue Jun 07, 2011 8:25 pm

It might be helpful to look at percent slip from a different perspective. Alot of guys do the 1/4 mile thing and have figured out that the most important thing for low ETs is the launch and hooking up. Obviously, easing into the throttle doesn't yeild fast ETs, nor does smoking the tires off the line. Maximum grip occurs at 10% slip, ie when the tires are rotating 10% faster than they would if they were rolling freely. For a given tire, the fastest theoretical 1/4 mile time would occur if you could maintain 10% slip (max grip) over the entire 1/4 mile, right? Unfortunately, you would need a Gazzilion hp. Most cars can only maintain that 10% slip for a short distance. Thats why you look at 60ft times. Assuming you weren't caught sleeping at the line, the 60ft time is a measure of how well the tires are hooking up. Beyond 60ft, the engine is responsible for the speed of the car and the tires are just along for the ride. So . . . if your engine were able to keep the tires at 10% slip for the entire 1/4mi, any increase in hp would not yeild faster times. Any additional power would cause the wheels to spin and slow the car.

Fortunately, the rubber molecules in your tires are dumb, they don't know if you are braking or accelerating. Max grip during braking also occurs at 10% slip (wheel rotating 10% SLOWER than if it were rolling free - this is the whole idea behind ABS). If your current brakes can maintain 10% slip for the entire braking distance, any additional braking would cause the wheels to skid and increase the braking distance. If you want to decrease the braking distance you would have to use wider tires, stickier tires or increase down force.

Make sense?

JamesShort · **Posted:** Tue Jun 07, 2011 9:23 pm

Sort of. With drag racing, people actually spin the tires at launch which is over 100% slip ratio (per SAE formula).

Literature I have says 15% is peak, but anyway.

I guess where I'm still lost is how/why under acceleration the effective radius of the tire is basically larger and therefore traverses more distance than the pure rolling motion of the same wheel assembly if no tractive force was present. And vice versa for decel (ie the effective radius is smaller). My understanding is that the whole contact patch is never full slipping till you hit 100% slip ratio so, for instance, at 15% slip ratio part of the contact patch is full 'stuck' to the ground and the trailing part of the contact patch is 'slipping'. Since part of it is 'stuck' to the ground, how can the outer radius of the tire spin faster or slower than the angular velocity would dictate for a given effective radius.

Kevin Allen · **Joined:** Thu Aug 28, 2003 6:42 pm **Posts:** 3479

Chuck Branscomb · **Posted:** Sun Jun 19, 2011 1:15 pm

Here are a couple of links that facilitate thinking about this subject. The first one is an excerpt from Paul Haney's book on tires. It is a good book to read if you really want to get into the nitty-gritty of tire traction and all, but it is a bit light in some areas where he is weak on his own understanding of a few things as I recall (it's been 5-6 years since I've read it). If you get a copy, go to his web site and print off the errata sheets before you get too deep in studying some of the diagrams. Amazingly, this whole subject of tires is still a difficult and somewhat "not fully understood" field of engineering for the most part.

http://insideracingtechnology.com/tirebkexerpt2.htm

Also, here is a web forum discussion with some good (mixed with some useless) info and ways to visualize the stretching nature of the tire, etc; he asks the same basic question you did James I think:

http://www.pistonheads.com/gassing/topic.asp?h=0&t=576203&mid=0&nmt=tire+Slip+Ratio

Aaron McCurdy · **Joined:** Fri Mar 25, 2011 10:17 pm **Posts:** 21

I think it's best illustrated here: http://www.youtube.com/watch?v=XttOe4rerLo

Yeah it's back to drag racing again, but you can nearly watch the contact patch you described moving along the racing surface.

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Tire slip ratio

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