Tire lateral deflection: absolute and rate

[alangbaker]

I don't know if this has ever been discussed this way, but would anyone care to offer a good guesstimate of how much an FF tire (for example) deflects laterally when cornering?

Just how far does the centre of the contact patch move?

Can you estimate a lateral spring rate or is it nowhere near close to linear?

[aims20]

After typing this post out, I feel like its a long winded way of saying (about an inch) but Let me clarify what you're asking and maybe others will chime in some more.
Also, you might want to browse this older discussion... it might have some ideas: https://www.apexspeed.com/forums/sho...nd-deflections

1) How much an FF tire deflects laterally: Gut says the number is somewhere in the ballpark or less of an inch... maybe 2 for the really floopy stuff out there. If I really needed this data bad on a budget, I'd call up a good photographer to tag along at the next race weekend. If I needed an estimate right now I might carefully study some good FF photos I might find on google and count pixels in MS paint related to other parts. Crude, yes, but I don't know how desperate you need a number right now - keep in mind there are a ton of variables: Pressure, Tire camber, bank angle of corner. etc. Spring rates/Ride/CG height and weight distribution(s) on the rest of the car change the average too as the exact value is constantly oscillating. Steer angle, Track conditions.... this is where all the action is happening. For a given day/car/corner you will have to assume certain values for all of those factors or specifically define your model to exclude certain things.

2) How far does the center of the contact patch move: Keep in mind that the contact patch changes in shape when transitioning from a straight ahead to a cornering scenario. On the loaded side, the contact patch is a triangle pointing away from the centerline of the car before the steering input (assuming some camber and toe), and that triangle transitions to a different triangle when loaded on the outside of the car (typically a triangle now pointing at the car), and that shape change induces a change in the center of the contact patch too. I guess what I mean to say is that the geometric center of the contact patch is not the Center of Load of the contact patch. Those are 2 different points and they move differently. The point you are probably thinking about (which easier to envision) is the how much the center of the tread deflects from say the center of the wheel.

3) Can you estimate a lateral spring rate or is it nowhere near close to linear? If you are just starting a more elaborate model, you could start linear, knowing it will be off in other areas. But I doubt its fully linear, and again, changing camber, toe, pressures probably changes the linearity, and varies wildly depending on tire construction since there are a number of different FF tires out there. Over the years, running different pressures on different cars, sometimes the sidewall collapses (indicative of a falling rate) while higher pressures might fall in rate a lot less before going "solid"

[alangbaker]

Thanks, Andrew! Much to ponder in there.

I checked out that thread and it seems to deal exclusively with vertical deflections with a very small discussion (from DaveW) that the lateral deflection will probably have an impact on the vertical spring rate of the tire.

I wonder if Hoosier has any idea what the numbers might be. They've got data on vertical spring rates for various pressures and loads, so perhaps they might know something about lateral deflections as well.

If anyone is interested in knowing the data for vertical spring rates for FF (and for a couple of other classes), start on their "Circuit Racing" page:

https://www.hoosiertire.com/tires/circuit/

Then click on "Series Specs" and there will be links to the spring rate data beneath each series' tire table.