Choosing Rim Size

[Knight]

Hi

We are a first year team from India. We are confused whether to use 10" or 13" rims. plz can someone help us out.
http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Knight]

Hi

We are a first year team from India. We are confused whether to use 10" or 13" rims. plz can someone help us out.
http://fsae.com/groupee_common/emoticons/icon_smile.gif

[D J Yates]

Main argument for and against 10" rims:

10" rims are lighter and so reduce unsprung mass and overall mass.

10" rims are much harder to package than 13" rims and will result in compromises in the design of the suspension.


There is no right or wrong answer. It depends on what your overall goals are. If you are planning a car like delft's 04 car (super light weight), then 10" rims will provide a significant advantage. However, most first year cars are typically overweight due to inexperience (2004 was our first entry, we brought a 300kg car), so the weight saving advantage of 10" rims would be small in compraison to the packaging compromises.


A few other issues i've become aware of whilst reading these forums, regarding 10" rims:

During competition, light weight cars often have trouble generating enough heat in their tyres when using 13" rims.

There are more tyres available for 13" than 10".

10" rims are cheaper.


Hope i could help. Good luck with your design.

[Buckingham]

I am curious as to why you claim that FSAE teams "often have trouble generating enough heat in their tyres when using 13" rims."

The Goodyear 20.0x6.5-13 is actually 1/2 lb lighter than the 18.0x6.5-10. One would think that the tire of lower mass would heat up faster???

Of course there are many other factors contributing to tire temperature, such as aerodynamic cooling, scrub, pressure, solar radiation, etc. I was just curious as to which factor(s) you used to draw your conclusion.

[Charlie]

How much do you think the Goodyear 13" weighs? Because it's not wha Goodyear says..

[D J Yates]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by dhaidinger:
I am curious as to why you claim that FSAE teams "often have trouble generating enough heat in their tyres when using 13" rims." <HR></BLOCKQUOTE>

It's just something that i'd seen mentioned a couple of times. I've seen no explanation as to why, but expect it's because of the following:

Over a given distance, a smaller tyre will see more revolutions than a larger tyre. So a point on the surface of the smaller tire will be loaded and have heat generated more frequently than the larger tyre. Assuming that the both tires have a similar construction and rubber, the rate of heat dissapation would be the same. Overall, this would cause the smaller tire to heat up quicker.

I doubt tire mass or any of the other things you mentioned have any significant effect in comparisen.

[TG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by D J Yates:
Over a given distance, a smaller tyre will see more revolutions than a larger tyre. </div></BLOCKQUOTE>

You have to look at the outside diameter of the tires. You'll find that the tires for 10" rims are around 19.5" OD and the tires for 13" rims are usually between 20-20.5"... not a big difference.

While we're on the topic, I e-mailed Lola as to why they had put 18" wheels on the A1GP cars. One of their engineers replied that it was for aesthetic reasons and given the choice, they would have gone with 13". He said go with the tires with the best mu, which he thought would be 13" tires.

[syoung]

A1GP cars will be hitting 180-200 mph so I wouldn't take that one as proof for FSAE applications! It's probably not wrong to say go for the best mu (within reason), but I'd expect very similar values between the 10" and 13" tyres from Goodyear, and likewise from Hoosier, etc. I suppose we'll see when it comes to the FSAE tyre tests!

Our 10" rims are fitted with 18" OD tyres, so there's a fairly significant difference in circumference there compared to the 20" OD 13s.

Pat Symonds (Renault F1 Technical Director) gave a talk at Warwick a couple of months ago. He mentioned that one of the things the Technical Working Group (which he sits on, helping to decide future technical regulations and the direction of F1) considered was to go for big 18-20" wheels. The reason was purely aesthetic, to match the trend in the (automotive industry (especially aftermarket) for bigger wheels to be seen as cooler.

[D J Yates]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by TG:
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by D J Yates:
Over a given distance, a smaller tyre will see more revolutions than a larger tyre. <HR></BLOCKQUOTE>

You have to look at the outside diameter of the tires. You'll find that the tires for 10" rims are around 19.5" OD and the tires for 13" rims are usually between 20-20.5"... not a big difference. <HR></BLOCKQUOTE>

Your quite right, which is why i refered to tire (or would have said tire if i could spell) size not rim size. I should have made it much clearer though.

I hope Pat Symonds wasn't serious about bigger wheels because they're cool. As if racing cars aren't cool enough allready.

[rjwoods77]

We are running 10" rims with 18"od tires. Weight is a big consideration. About 2lbs per tire less and the rims are about 1-2lbs less depending on type of rim(good ones). So you are looking at anywhere from 10 to 20 lbs you lose of your car. There are more of a bitch to package on but you can get just as good geometry with a 10" than you can get with a 13". Something else to consider is the weight you might lose from smaller center to center dimensions on your upright. We are using 8 and it is the max you really can go, which is about 2-3 inches less than a common fsae 13" wheel upright. Weight and rotational inertia was the big factor for us and I am being conservative with those numbers as well.

[jonno]

the lecont 10" wheel we intend to run with at formula student 2005 has a 16" o.d, significantly smaller than most of the competition

[Eddie Martin]

Knight,

For a first year team your biggest problem will be getting the car finished on time and then getting enough testing in before the competition.
It's probably a bit easier for a first year team to use 13's because of packaging and availability but it may be easier to get 10's in India, i don't know.

In terms of which tyre is best i'm sure the upcoming tyre testing will give a much better picture. Real testing of the tyres is the only way to do it, whether on a rig or on the track. Any change in the compound or construction of the tyre will make it behave differently.
The amount of long. and lat. grip that the tyre can produce is the most important factor by far.

I don't know about the other teams running 13's but we don't have trouble getting them up to temperature, far from it.

[Larsen?]

We recently (03) switched from 10's to 13's. One significant factor for us was side-wall deflection: it's hard to find low-profile tires for 10's.

[Charlie]

I don't buy the 13s are better than 18s arguement. The only cars that don't run giant wheels with rubber band tires are the ones that rules don't allow. All sports cars run huge wheels. Yes partly because some are heavy and require giant brakes. But that cannot be the biggest reason for a big wheel, if it was detremental to handling they wouldn't use it. (F1 cars stop just fine http://fsae.com/groupee_common/emoticons/icon_wink.gif ). Plus the LMP cars aren't insanely heavy.

It's easier to make a tire with a larger sidewall stiffness slope with a short sidewall. Firestone when they came to Champ car in 1994 tried to introduce 17" tires, CART said they looked goofy and made rules to disaalow them. I don't think Firestone had brakes in mind either.

Of course this doesn't mean 13" wheels are better, the tires can and do vary a great deal. But all else being equal a shorter sidewall should be better and that usually will come with a 13" wheel in this series.

[Greg 08]

I wanted to resurrect this one just to get some different suppliers names. I have seen the numerous opinions about what szie wheel is better, but I checked out kodiaks prices and nearly had to change my pants. Anyone purchase wheels, shells and or centers from anyone other than keizer, kodiak and douglas? Thanks for the help.

Greg Ehlert
Michigan Tech

[murpia]

I would be very interested if someone could post some total weight data for various tyre and rim combinations?

Something fairly comparable e.g. similar width and o.d. so maybe low profile 13" vs. std profile 10"?

Thanks, Ian

[kwancho]

Weight's cool, but what you really want to know is moment of inertia. http://fsae.com/groupee_common/emoticons/icon_wink.gif