Tire/wheel selection - Rolling resistance

[MazdaMatt]

I'm in need of some wheels for my car and I'm trying to gather all the info i need to make a decision. I understand two of the basic concepts:

wider tires put more rubber on the road - important for turning
smaller diameter wheels have less rotational momentum and less weight - important for acceleration and braking

The one consideration that I havn't got figured out is rolling resistance. I imagine its important for your maximum speed more than anything, but how important is it? I think it might be important because in full race trim, my car will be under 1500 pounds with only 130hp and very low ride height on good suspension. I figure it can turn, brake and accelerate well to begin with, but may be lacking in top speed.

Anybody got a good source of info on this? or just some general insight?

[Guillermo]

Rolling resistance is a factor that does not change much with speed so it's a factor that affects fuel economy at speeds of about 100 km/hr and lower. At higher speeds rolling resistance stays the same but aero drag increases significant and is by far the most significant factor resisting the acceleration rate and top speed of the vehicle. Power/weight ratio is the most significant factor affecting acceleration at speeds below 100 km/hr. Above 120 km/hr or so the power/aero drag ratio becomes much more important then the Power/weight ratio.

Don't worry about top speed, improving cornering grip is much more important for reducing lap times.


For your car light weight and low powered FWD car the best wheels are the widest, smallest diameter and lightest wheels you can find.
For a FWD car a staggered rim and tire set up can provide faster lap times if the suspension is tuned for it.

FRONTS:
Something like a 13"x8" alloy rim with 225/45ZR13 Hoosier R6
REARS:
Since there is less weight in the rear no need for wide tires back there.
Something like 13"x6" alloy rims with 185/60ZR13 Hoosier R6.
(Note: suspension must be set up differently for a staggered tire set up).


The narrower rear rims and tires will reduce aero drag and unsprung mass as well as reduce rotating mass.

But most people find it more conveniente to have the same size rims and tires front and rear so you can move them around as they wear.

[miataboi]

I'm in need of some wheels for my car and I'm trying to gather all the info i need to make a decision. I understand two of the basic concepts:

wider tires put more rubber on the road - important for turning
smaller diameter wheels have less rotational momentum and less weight - important for acceleration and braking

The one consideration that I havn't got figured out is rolling resistance. I imagine its important for your maximum speed more than anything, but how important is it? I think it might be important because in full race trim, my car will be under 1500 pounds with only 130hp and very low ride height on good suspension. I figure it can turn, brake and accelerate well to begin with, but may be lacking in top speed.

Anybody got a good source of info on this? or just some general insight?


1500 lbs?
What is it... a super 7?

You want rubber.
You will corner better than anyone...
Unless it's Mosport... put as much rubber down as possible.

[MazdaMatt]

I'll have to see if i can fit 13's over my calipers... it has 15's on it now, but just junker steelies with street tires.

is a two-inch stagger not terribly drastic? Wouldn't i end up with a pile of entry oversteer? I guess that's not too bad of a thing unless it gets snappy.

So inconclusion, rolling resistance is not something to worry about?

no, not driving a superseven... you'll see it if I ever get it in working order :)

[Guillermo]

Like I mentioned whether the car has tons of oversteer or understeer depends on the difference between the front and rear spring rates, sway bars and the front to rear tire & rim choice.
If the suspension on the car is already set up then it's much easier to use the same size rims and tires front and rear.
If you can't use 13" rims then try for 14"x7" or 15"x8"
Usually the price, diameter and mass of the rims and tires increases with size.

[thgear]

wider tires doesnt mean more tire to the ground

the only thing that really changes is the contact patch. from a square to a rectangle.

with a light car you wont get much weight transfer, and unless you can get some serious aero mods you wont generate any serious downforce either

so really you will have a narrow but wide contact patch, which is good but only if the rest of the cars suspension is designed to take advantage of it

ie, it has to be really stiff, otherwise you will be tilting on the wide tire during cornering, wasting all that valuble contact patch.

[Sasha]

biggest tires you can fit

you'll be the faster than running tiny tires that will just overheat

[malcolm]

biggest tires you can fit

you'll be the faster than running tiny tires that will just overheat

i agree. unless you have a 500 lb car, you'll pretty much want as much tire as you can fit under it.

[kmorris]

i agree. unless you have a 500 lb car, you'll pretty much want as much tire as you can fit under it.

If the car really is 1500 pounds, oversizing the tires is definitely possible. On my 1000 pound car, the tires I use are marginally too large; if the weather is not hot, I have trouble getting them up to temperature, and lose lots of grip as a result. (I could use a softer compound if it was available, but then I would need to spend mucho $$ for tires). So it is entirely possible to have too much tire. Probably the best idea would be to look for a type of car with a similar performance envelope and see what tires work for them. Based on HP and weight, the car could have similar performance to a Formula Ford, so tires that work for them (outside of spec tire series of course) could probably work for you. You haven't really mentioned what the car is, or what kind of competition you're doing, which would help to direct what general type of tire you're looking for.

[MightyMouse]

i agree. unless you have a 500 lb car, you'll pretty much want as much tire as you can fit under it.

Just don't go too big. The wider tire also weighs a lot more, and yes, does have more drag. Going from a 225 down to a 205 on my Civic dropped 5 lbs from each rotating assembly and dropped a full second off my laptimes at Mosport, even with the reduced cornering speeds, mainly because my top speed down the back straight went up a significant amount. Even with 180 WHP turning the wheels, the 225's were just too much rubber to power up the back straight.

With 1500 lbs and only ~130 WHP, you probably don't want to go over a 205.

[malcolm]

ahh, didn't notice the 130whp. haha. yeah, depending on downforce, I would say between 205 and 225 for Mosport, depening on tire construction. Something like an 8" wide slick from Hoosier or Goodyear would likely work quite well (unless you wanted a radial, from Dunlop or something). :)

[MazdaMatt]

So now i'm getting conflicting opinions... Is the 'no bigger than 205' opinion just based on Mosport? I love that place and I want to go fast there, and of course 205 is cheaper and lighter...

I'll be driving a SEVERELY weight reduced honda Del Sol. There will be no downforce addons unless i can mould a fancy bumper or something. it has the 93 Si engine, so its 130flywheel hp. I will be racing solosprints only.

As much as I love mosport, I want to try the other tracks this year. If they have shorter straights, i guess i'm better with a wider tire? Should I just get some 13 or 14 by 8's, wrap it in 225 up front, 205 in the rear and call it a day?

Most likely, i'll be running RA1's... oh crap, do they make those in 13's and 14's?

[Sasha]

id still go with bigger tires and overdrive the car and destroy the tires cole trickle styles

its so much more fun than taking care of puny tires

and if its fwd set it up super loose so youre sliding everywhere just to keep the rear tires hot

mmmm sliding

[MightyMouse]

So now i'm getting conflicting opinions... Is the 'no bigger than 205' opinion just based on Mosport? I love that place and I want to go fast there, and of course 205 is cheaper and lighter...

I'll be driving a SEVERELY weight reduced honda Del Sol. There will be no downforce addons unless i can mould a fancy bumper or something. it has the 93 Si engine, so its 130flywheel hp. I will be racing solosprints only.

As much as I love mosport, I want to try the other tracks this year. If they have shorter straights, i guess i'm better with a wider tire? Should I just get some 13 or 14 by 8's, wrap it in 225 up front, 205 in the rear and call it a day?

Most likely, i'll be running RA1's... oh crap, do they make those in 13's and 14's?

A Del Slo around 1300 lbs? This I gotta see. My "SEVERELY" weight reduced 92 Civic is still just a shade over 2000 lbs, and that is a lighter chassis than the Del Slo....

[miataboi]

So now i'm getting conflicting opinions... Is the 'no bigger than 205' opinion just based on Mosport? I love that place and I want to go fast there, and of course 205 is cheaper and lighter...

I'll be driving a SEVERELY weight reduced honda Del Sol. There will be no downforce addons unless i can mould a fancy bumper or something. it has the 93 Si engine, so its 130flywheel hp. I will be racing solosprints only.

As much as I love mosport, I want to try the other tracks this year. If they have shorter straights, i guess i'm better with a wider tire? Should I just get some 13 or 14 by 8's, wrap it in 225 up front, 205 in the rear and call it a day?

Most likely, i'll be running RA1's... oh crap, do they make those in 13's and 14's?


K - that's different altogether.
IF you're dead-set on the RA-1... you only have these sizes to choose from:
http://www.toyo.com/docs/tires/product_spec.asp?lpid=18968&name=Proxes%20RA-1™&category=sport

But there's more to tire selection than just width, rotational inertia and rolling resistance.

In your case - you have to worry about fitment in the wheel wells... and overall diameter and it's affect on gearing.

In reality - the only 2 sizes I'd run for solosprint would be the 225/50R14 and the 225/45/15

I'd suggest a set of 15x7.5 rims.. and some 225/45/15 RA-1's.
That way you can also run 205's and 225/50's should the need arise. 205's would be a great starting point... and learn to drive the car with these... and they are plentyful in this size.

For solosprint... rolling resistance isn't as much of a factor... (altho it is on faster tracks and VERY sticky compounds) especially if you've already determined a compound that you're going to use. Gearing and fitment will be more important...

Either way... just get out there and start learning to drive a 1300 lb. FWD car!

[MazdaMatt]

A Del Slo around 1300 lbs? This I gotta see. My "SEVERELY" weight reduced 92 Civic is still just a shade over 2000 lbs, and that is a lighter chassis than the Del Slo....

Does your civic have any steel in front of the front suspension? or any steel behind the rear suspension? This thing should be a hell of a ride... I can't wait to get it on the track. (don't worry, i plan on adding some metal... can't go driving around with an exposed engine)

Thank you all for your comments and guidance in wheel selection. Having seen the toyo chart, I will probably shoot for their 225/50/14's, but if i score a good deal on some 15 inch rims i'd step up to the 225 15s

[MazdaMatt]

Okay, so I bought a set of 13 inch rims. 9 inches wide in the front, 8 in the rear. Now I need rubber.

I was hoping to go with RA1, but they don't make them that wide in 13's. Is there an other tire that has the long life of an RA1 that would fit those rims?

I'm thinking 235-245 in front and 225-235 in the rear. Any other suggestions/recommendations?