Tire size selection

[Ben_tire]

Hello,

i have a few question on tires.

What is the reason of wider rear tires on a few race cars?

In my opinion:

When the weight distribution is 50/50 the car can use equal tire wides because of the yaw moment.

1. Wider tire can normally generate more FY, when the weight is more on the rear the tire has to create more Fy to get neutral behaviour in high speed turns? Because of the Moments and Forces around the CG and Yaw moment? The geometric slip angles are no more equal (due to bicycle model ) right?
2. Wider tires are better for acceleration
3. With Aerodynamic and more downforce the center od pressure is mostly behind the CG so the point 1 comes to play?

But what is the prof. way to select the tire size based on tiredata?

It would be glad to discuss this topic with you.

Thank you

[Callum]

Hello,

There are quite a few threads on this forum regarding the theme of tyre width. Here is a start for you http://www.f1technical.net/forum/viewto ... tyre+width

Enjoy.

[Ben_tire]

thank you

[Jersey Tom]

What is the reason of wider rear tires on a few race cars?

Race cars have these in them.

But what is the prof. way to select the tire size based on tiredata?

Don't bank on having tire data - often you won't. And even if you do, it's easy for the data (or model) to be junk. Track testing is still a great thing. Probably rare these days to even have the ability to have a wide choice in what type or size tires you get to use anyway.

But if you do have all the tire data you want and it's all great - well, it's no different than anything else in your car. An analogy might be how much downforce, or rear downforce do you want? Or what roll distribution? Everything is balanced out against everything else to try to get as much potential out of the car as possible.

[mrluke]

The simple answer here is to cope with the power (or "jerk") to the rear wheels. If the cars were 4wd the need for a larger rear tire would diminish (but not disappear as you still want >50% power to rear wheels)

[Greg Locock]

Ah, but mrluke, I'd want the front tires bigger than the rears for braking. So if you want the rears bigger than the fronts, and I want the fronts bigger than the rears, how does one decide?

[Jersey Tom]

Ah, but mrluke, I'd want the front tires bigger than the rears for braking. So if you want the rears bigger than the fronts, and I want the fronts bigger than the rears, how does one decide?

Coin flip is the only answer.

[Tim.Wright]

As Greg alluded to, you would ideally have a different priority of fromt and rear grip at different points on the track. Since you can only have one set of tyres on the car, you need to accept that they wont be ideally sized for every condition around a lap. So you need to find the best compromise. Since going forward and yaw stability are 2 of the highest priorities of a racecar, the rear axle (of a RWD) are usually bigger.

[DaveW]

But what is the prof. way to select the tire size based on tiredata?

That is a very interesting question, & I am reasonably certain that there is no simple answer.

JT provided a good contribution, particularly the last sentence:

But if you do have all the tire data you want and it's all great - well, it's no different than anything else in your car. An analogy might be how much downforce, or rear downforce do you want? Or what roll distribution? Everything is balanced out against everything else to try to get as much potential out of the car as possible.

I think that tyre size is just one of a range of properties that tyre designers consider (if they have that freedom - often tyre size is regulated). My observation would be that those properties would include construction and compound (but I am sure that JT, for example, could write a book on each of those).

Once the tyres are available, then it is up to aerodymicists and vehicle dynamicists to try and use the tyres properly, bearing in mind that each will have a (usually different) view on what is required. The ultimate cost function is the time required to complete a race distance and that, in itself, requires comprise (e.g. Lotus vs Mercedes during the early part of the last F1 season).

A couple of examples, observed from a rig test perspective.

The old LMP rules allowed different rear tyre sizes for open cockpit & closed cockpit layouts. On team tried to transfer a well proven mechanical set-up from an open to a closed layout vehicle. It didn't work. Or at least it did, but only after rematching ballast, spring and damper settings to the different tyres - probably other changes were also necessary.

In another case (still LMP), one designer decided (for aero. reasons, I believe) to fit rear tyres to both axles. On a rig, it proved to be very difficult to achieve a sensible set-up. I had the opportunity to "see" the vehicle again after testing. It was interesting (to me) to observe that the front tyre properties had changed - which again required changes to the suspension set-up. The vehicle had a fairly successful season, I recall.

Hence my comment that there is no simple answer...

[Ben_tire]

Thank you all for your comments, there is a lot to think about.

[Lycoming]

In another case (still LMP), one designer decided (for aero. reasons, I believe) to fit rear tyres to both axles. On a rig, it proved to be very difficult to achieve a sensible set-up. I had the opportunity to "see" the vehicle again after testing. It was interesting (to me) to observe that the front tyre properties had changed - which again required changes to the suspension set-up. The vehicle had a fairly successful season, I recall.

This sounds to me suspiciously like the HPD ARX-02, though the commonly cited reason for what they did there was mechanical rather than aerodynamic....