Tires and Rims

[johnpr]

Originally Posted by abshnasko

Yea well somehow my civic came stock with 15" steel rims. Don't ask me how. Seems to me that the RMI would be smaller if the tire is thinner, hence the reason all the ricers get huge rims and 1 millimeter thick tires.
Are my assumptions above correct?

stock steel rims on a civic are no mystery, remember, civics are a budget car, thats why they are so popular with ricers.

also, ignore ricer logic, they mostly go for looks. once your OD exceeds your origional OD you are affecting your gear ratio and adding unnecesary mass.if you want a larger rim go with 16's you should still find tires that are the same width as what you have on there now, and in that case you should be able to reduce total wheel and tire weight. but you have to be careful with your wheel selection, as i mentioned earlier, some AL wheels weigh more than steel.
-john

[johnpr]

although i should add that *any* change in OD affects your gear ratio, even going smaller

[abshnasko]

I was thinking of keeping the OD the same, I would just be substituting a larger rim and a thinner tire. So that extra 1" of tire diameter that was there before is now air between the rim spokes. Does anyone follow what I'm saying? I really feel like larger rims and thinner tires would be lighter AND have lower RMI

"ricer logic": oxymoron?

[johnpr]

yep, we're on the same page. but remember get the lightest possible wheels (maybe some real racing wheels and not some sears special - those tend to be pretty heavy)

check out summitracing.com they have a decent selection, or tirerack.com, you could easily get a decent set.

[Shawn D.]

Originally Posted by abshnasko

I know that a thinner tire is better, but if I get a larger diameter rim do I have to get a wider tire?

by thin and fat i'm referring to the thickness between the rim contact and the outer circumference, not the width.

But I'm just asking in general: is large rim/thin tire or small rim/fat tire lighter?

No, if you get a larger diameter, you do not "have" to get a wider tire, but lower-profile tires are usually wider. If you really care to do the math, check out this spreadsheet I made for BMW wheels/tires and use the "Size Calculator" page (note that the "READ THIS FIRST" page is so named for a reason and you have to follow the instructions at the end).

In general, a smaller-diameter wheel with a taller-profile tire is lighter than a larger-diameter wheel with a lower-profile tire. In general, the lower the profile, the stiffer, and therefore heavier construction it will be.

Originally Posted by johnpr

yep, we're on the same page. but remember get the lightest possible wheels (maybe some real racing wheels and not some sears special - those tend to be pretty heavy)

check out summitracing.com they have a decent selection, or tirerack.com, you could easily get a decent set.

Note that the lighter the wheel, the less aerodynamic it usually is, and it's also less likely to be a narrow wheel (the manufacturers get more $$$ from building racer parts than eco parts).

Search for Flik, Rota, BBS, Gram Lights, and Kosei, all of which are known for light (albeit usually larger) wheels.

Also, check out http://wheelweights.net/

You could always go super-expensive and get custom wheels made by Fikse and get the exact size you wanted.

[johnpr]

the wheel weights site is a great resource, thanksfor posting it, and you are right about the aerodynamic standpoint.
the reason i recomended 16's is because it will be easier to find something in the same size range, although like you pointed out, lower profile usually come in wider widths.

[aerohead]

Some of the members have thrown out some really good points.Off the aero forum there's discussion about rolling resistance and inflation pressures,and I've been accumulating stuff to share but not ready yet.When I saw your post,I'd been reading a few nights ago and here's a few notes from the proprietary GM concept tires for the Ultralite 100-mpg car.An overall larger diameter ( 18-inch )tire has reduced R-R due to the fact that each tread element "see's" the road surface fewer times for a given road speed.Surface speed is constant,but greater circumference reduces revolutions per mile ( kilometer ) and each tread block and it's accompanied sidewall elements bends and recovers fewer times per revolution.Inflation pressure is 65-psi to pump up the "air-spring" which a tire is,which helps hold the tread flat against the road ( traction,braking ,and less deformation).The sidewall is very short ( to minimize deflection under load ) and has a designed-in curvature to help it deform,when striking road irregularities and hazards so as to absorb vibration which would otherwise be transmitted into the cabin of the car.The tread is kept narrow to minimize aerodynamic loading on the car,and each tread block is "hinged" so as to deform as freely as possisible when it strikes the road surface ( this also makes for a" noisy" tire,now regulated in the US by EPA ).Rubber compounds are balanced to allow as much hysteresis ( the ability to absorb,then release energy without heating-up ) and still allow wet and dry traction.Michelin used silica in it's "Green-X" tires for reduced rolling resistance and they ended up shocking people with static electricity and had to be re-formulated.The whole business is fraught with trade-offs.By the way,while under-inflation can remarkably lower mpg and shorten tire life,over-inflating,beyond the car-maker's recommendation shows no appreciable gain in mpg and you run the risk of reduced contact-patch and trouble associated with all that.Magnesium is out-lawed in the U.S. by DOT as it breaks when struck.Forged-aluminum is best bet for mass reduction.Polar-moment-of-inertia is an issue,and you want mass away from the extremities of the wheel/tire unit.The steel wheel can take wheel-covers,which is good for aero.Cast aluminum wheels can be as heavy as steel.Take a scale with you.I do! Hope this doesn't muddy the waters for you.I spent $1,000 to put larger aluminum wheels on my CRX so I could run the Michelin tires,and they made no improvement in mpg.They did have excellent wet and dry traction,so the experiment wasn't a total loss,however no mpg for "one-large" really hurt! Good luck!

[Vince-HX]

those stock 15 inch steelies weigh 18 lbs apeice btw (no tire)

Hx rims weight 12 lbs and will fit with a small spacer or a little grinding of the front caliper of your EX.

You can usually get a set for under $200 in good condition with tires.

[johnpr]

great post aero head, lots of usefull info!

[abshnasko]

so in conclusion (correct me if im wrong):

1. a larger rim + tire combo CAN be lighter if attention is paid, helping FE
2. larger rim is generally less aerodynamic and takes a wider tire, hurting FE
3. larger rim size means that more of the LRR tires will fit (possible negating the effect of #2?), helping FE
4. larger rim and OD reduces rpm's, helping FE

about a larger rim being less aerodynamic... would some of this be countered some if I installed dams in front of the wheels? Or would it have to be skirts...