Scrub

[silente]

Hi!

First of all hope everyone had an happy Christmas!

My question today is about scrub (Track variation during suspension movement, tipically heave).

I have read from different sources that it is good practice to keep it as low as possible. The reason for that should be that, in a corner, this movement of the wheel could keep grip away from the tyres because actually some of the cornering force is used by the tyre without a real scope, only because of track variation. Another reason i read to avoid track to change in heave is tyre wear.

I would like to have your opinion about that and about how much does scrub can have an influence on tyre wear and on tyre temperatures.
Could it be helpful to have some scrub in order to help tyres to warm up quickly? And how much could it affect tyre grip in a corner on a not perfectly smooth surface?

Thanks!

[Jersey Tom]

I haven't tested it, but I don't see why "scrub" would have a significant or dramatic effect on heat and wear. I put the term in quotes because I think it's a bit misleading in its description.

What is it? Lateral movement of the wheel with suspension travel. On a stationary car we might think of that as literally "scrubbing" the tire against the pavement. On a MOVING car, with rolling tires, you're introducing a bit of lateral velocity which means you'll give rise to a small amount of side slip (aka slip angle).

I could see that being bad for handling in general in that the car will probably want to "wander" over bumps and undulations, but whatever effect that has on the tire duty cycle I see as being very small compared to everything else.

[hardingfv32]

So in your opinion this is not a common option in F1 for getting heat into a tire that is operating a little below spec? Trying to understand what options are available for increasing tire temps.

Brian

[Tim.Wright]

I'd guess it would have a measurable but not large effect on tyre temperatures. Though the only way to tell would be a back to back test or simulation. A carefully setup simulation would be nice because you could measure the heating power input to the tyre from a lot of different sources e.g;
Power from lateral slip = Lateral force times lateral contact patch velocity
Power from longitudinal slip = long force times long. contact patch velocity
Power from vertical forces = vertical load times times radius velocity

Anyway, I doubt lateral "scrub" is used as a tuning thing because it is too hard to change independently. It is a geometric characteristic which is tightly related to camber change, roll centres and anti-geometries. Changing the scrub without changing these is not really possible.

So in short, Id say, yes it has some effect, but no you can't really tune with it.

Tim

[Jersey Tom]

So in your opinion this is not a common option in F1 for getting heat into a tire that is operating a little below spec?

That would be my opinion, yes. It's also my opinion that there's no such thing as a "spec" operating temp for a tire.

I'd also agree with Tim's general assessment that it MAY do something but it will be little. Maybe you get "extra heat" from the driver having to dance around with the car wandering over every bump and undulation. Maybe you lose heat because the car drives like ass and your lap times are slower.

Want more heat in a tire? Drive faster. Or take air out. Or put a more appropriate tread compound on.

[Dynofiend]

I am surprised that none of the replies here picked up on the changes that Ferrari made to their rear suspension uprights around July in the 2011 season. Ferrari introduced higher pickup points on the upright where it meets the top wishbone. This will have the effect of inducing greater scrub. Interesting too that this modification was made around the time that Ferrari were realising that their car was failing to warm its tyres effectively.

[raymondu999]

As memory serves wasn't it the A-arm the they messed with? As the rear of the car "squatted," by virtue of inertia on traction or downforce pushing the car down, the A-arm was shorter and so caused the camber to increase.

[DaveW]

Power from vertical forces = vertical load times times radius velocity.

Nitpicking, perhaps, but I'm not sure I agree with that statement, Tim. If the tyre had been built from pure spring elements, then displacement energy stored would be returned with no losses, hence no power would be consumed.

I suspect your statement is rather similar to the assumption that the graph of peak load against peak velocity for a damper describes its damping coefficient. It doesn't...

[Jersey Tom]

Power from vertical forces = vertical load times times radius velocity.

Nitpicking, perhaps, but I'm not sure I agree with that statement, Tim. If the tyre had been built from pure spring elements, then displacement energy stored would be returned with no losses, hence no power would be consumed.

Roughly proportional to, rather than equal to then. More deflection or at a higher rate -> more energy loss.

[Belatti]

Just my opinion, I would minimize scrub and find tyre temp with another method.

[timbo]

I remember it was said that Ferrari altered rear geometry on F150 mid-season to get more temperature via scrub.
After a while they abandoned that setup although it was used for Silverstone race.

[DaveW]

What is it? Lateral movement of the wheel with suspension travel. On a stationary car we might think of that as literally "scrubbing" the tire against the pavement. On a MOVING car, with rolling tires, you're introducing a bit of lateral velocity which means you'll give rise to a small amount of side slip (aka slip angle).

I think that the slip angle introduced by scrub is a function of tyre relaxation length (hence frequency & speed).

This has a number of consequences, including non-linear coupling of sprung mass roll, yaw & lateral modes. This a bit difficult to explain simply, but a vehicle with high roll inertia can be driven into self-sustained instability (with a combination of sprung mass roll, lateral & yaw displacement) at the limit, when the rapid non-linear changes in tyre loads cease to be "relieved" by tyre relaxation.