I keep hearing that I need to ditch my 17in 5Zeigans for some shorter wheels. I am wondering what this will do for me. Isn't the contact patch bigger in a taller tire than a shorter if the width is the same? I have no idea how heavy they are and they aren't chrome stunnas or anything, they are just taller racing wheels (they appear to be anyway), thoughts thanks

 

They still weigh more than a corresponding 15, 14, or 13.
Unsprung weight bad.
They increase your effective gearing with a longer circumference.
Taller gearing bad. (for accelleration)

 

How much of a difference is there as far as gearing? I am using very lo-pro tires (ziex unfortunatly) and the wheels are paid for so if I just switch tires and kept the wheel would i be sacraficing alot? The car is much faster with the swap anyhow and I am competeing with Truenos... any more thoughts?

 

Well I had run with 17x7 wheels with 205/40-17 Yokohama A520 tires and 15X7 wheels with 205/55-15 Kumho Ecsta 712's.... The car handled better with the 17" package and my times seemed to be around the same (though I'm comparing two different courses). It all depends on the 15 tires I guess....if you get some good tires on the 15" wheels then they probably will grip better than 17". However, Grassroots motorsports did a comparison on this...and it seemed bigger was better - > Would you to Super Size that order?

 

the 17s are probably WIDER thats why the contact patch is bigger. wider rims = more rubber on ground = better handling.

 

It's so hard to say...it's hard to get an R compound tire in a big 17 inch bling size. Go with your stock wheel size and you'll have better luck with the proper race tire. Any 15" R compound tire will outperform any 17" high performance street tire. That is fact. Plus, like they're saying, weight is your enemy. I run 15" tires on a 7.5" wheel, and my 205/50/15's look wide as hell, due in part to the wide-*** rim. Get a wide rim, you'll love it (also helps with tire roll).

 

first of all, the unsprung weight has nothing to do with the acceleration.
second, if you choose your tyre carefully, the overall diameter will remain as stock.
the unsprung weight affects how your tyres do their work. as the weight not supported by the springs increases, the wheel/tyre combination aquires more momentum and inturn has slower resonce time to the road. thus loosing traction. i think using suprelight wheels that allow for wider tyre patch and improving your brakes (they are also unsprung)as in lighter calipers is aprovable of using 17" or even 18 " wheels. if i had a bigger car, and good sum of money i'd get 17" ssf manufactured wheels (only SSR makes those BTW) get willwood calipers and slap some 225+ tyres on those.
but i have a CRX and 15" is big enough.
just my thought.
stan


[Modified by 90crxsi, 10:12 PM 9/13/2002]

 

I thought if you had a heavy *** 18" chrone wheel, your car will accelerate slower because there is more rotating mass. Other unsprung non-rotating weight i think doesnt affect acceleration, but these are the wheels.

 

if you choose very light 18" rims (in the area of 10-25lbs) it won't matter much. of course it is all a matter of compromise, the acceleration WILL be compromised, due to heavier wheel (in respect of having the SAME wheel in 15") and as the tyre gets bigger it gets heavier (usually). very carefull planning and choices can find that happy medium. patch size vs. silly light unsprung weight. that is why draggers use 13" and simplified brake systems (lighter) and road racers use big(ger) wheels (bigger, wider tyre) and beefy brakes. but all should have the same rotating diameter if the wheel and more importantly the tyre, are chosen right. the SSR 17" weighs less than say 15" cast american racing wheel. so using 17 SSR with 225 tyre is much better in all respects than the abovementioned AR wheel with 195 tyre.
stan
who wishes that he could afford HRE wheels


[Modified by 90crxsi, 2:46 AM 9/14/2002]

 

Disadvantages of 17's?

Answer: You can't a set Potenza SO-3's for $350 shipped. Good enough reason for me.

 

Stan, I have to disagree with your statement there. Any increase in unsprung weight (the weight not supported by the suspension) will mean that the force (power) required to produce equivalent acceleration will be increased. The second disadvantage in larger rims is that the rotating mass is further outward from the center of rotation, requiring a greater turning force to accelerate.
Then there are arguments about tire size/choice/weight/cost/etc. The only true advantage to big wheels is the ability to use HUGE brakes, but that is likely to be overkill (maybe, depends on use and weight of the car).

Steve

 

So very untrue

 

what wheels do you have?

 

Any weight hurts acceleration, but rotational mass is the worst... 15" street tires will take a little responsiveness out of the steering (instead of 17" wheels), but will grip just as much if you get a comparable size.. Just turn sooner...

Disadvantages of 17" wheels/tires

*rotational mass
*expensive tires
*expensive wheels
*ride rough
*expensive
*easy to damage wheels/tires
*unspring weight
*expensive

 

are you talking about all unsprung weight or unsprung rotating mass? Calipers for instance I think arent supported by the suspension but yet it's not rotating mass. Why does it increase the force required to produce an equivalent acceleration? I'm drawing off first year physics formulas and can't find one to explain why that would be so.

 

Not true. Wider tires do not place any more rubber in contact with the ground. Wider tires may provide a patch that is wider in one direction, but it *will* be shorter in the other direction for a net of zero added contact patch.


Maybe, maybe not. The formula for rotational inertia includes not just the weight, but the distance from the center of rotation. A light 18" wheel may or may not have less rotational inertia compared to a heavier 15" wheel.

Matt- struggling to find good 14" tires to go on his light 14" rims.....

[Modified by MaddMatt, 11:50 AM 9/14/2002]


[Modified by MaddMatt, 11:51 AM 9/14/2002]


[Modified by MaddMatt, 11:51 AM 9/14/2002]

 

but for a given diameter, while moving, a wider wheel will have more rubber contact on the ground per rotation of the wheel vs a skinnier wheel wouldnt it? The sitting contact area when the car is at rest would probably be the same, just more lateral area and shorter in the other direction.

 

No. The area of contact is primarily determined by weight of the vehicle, and to a lesser extent, air pressure. You may change the *shape* of the contact patch(which very well might improve cornering g's. It's a slip angle thing), but you do not increase the *area*.

How is the contact area different while moving? Sure, if you have a weight shift under acceleration/deceleration, you can change the area under the front wheels. For example: Ever noticed that is you simply pound on the brakes, it's quite easy to lock up the front tires? Because you've locked the tires before the full effect of the weight transfer. More weight on the front tires = more contact patch *on the front tires* (less in the back) and gives you more grip for better braking.

But the overall sum of the area of all 4 corners will always be the same no matter if you're sitting still, accelerating or decelerating.

 

hmm... im not sure how to explain the way i see it, but here goes:

yes, at any given moment the contact area should be the same regardless of the width of the wheel (just diff dimensions), and under braking/acceleration/cornering it will differ due to weight shifting.

but what i see is that say at X-mph, a given wheel will spin 100 times. For each of the 100 revolutions, the amount of tire that contacts w/ the ground is circumference * 100 revs * width of the tire (just assume the entire width touches the ground), right? so if you increase the width of the wheel, total contact over a given period of time should be higher vs a skinny tire, even if the area of contact for any given moment will always be the same (just diff dimensions).

and so w/ more overall contact per revolution of the tire, there should be more total rolling resistance, and i'd guess better braking and launching traction? As far as cornering, not too sure how to explain the effect on that.


-edit-

btw, i meant to say "tire" instead of wheel in some of the earlier posts, brain fried. xP


[Modified by TypeSH, 12:13 AM 9/15/2002]


[Modified by TypeSH, 12:16 AM 9/15/2002]

 

I think you're trying to relate this to overal surface area of the tire tread, and I'm pretty sure that's meaningless since the total area in contact with the ground does not change.

 

fair enough, but let me clarify, the unsprung weight is composed of the wheel/tyre and rotor/caliper combination. having heavy (stock) calipers does not affect the acceleration as they are not spun but the torque. i honestly doubt that having more mass (as in less weight of the wheel closer to the spindel and more weight outward in the tyre) has life impact on the acceleration. the difference is rather small. and i suppose at high speeds the weight will grow exponetially, but during slow start acceleration is is unaccountable.
that is why i consider 15" more than enough on my rex. and only will go 16" if the future brakes i get won't fit in the smaller wheel.
regarding the tyre size, the patch in question is not the diameter related length (that makes sense?) but the width of the tyre, providing more grip in the lateral accelerarion (as in the turn). i relate everything to the typre of racing i prefer.
very good reading BTW, you guys are kicking my **** i am digging deep for knowledge i long ago forgot.
stan


[Modified by 90crxsi, 2:46 AM 9/15/2002]

 

I think this is a can of worms subject that is not easily answered. Bottom line, my 17s are paid for so I think I will stick with them, and they add a bit of grip, or so it seems, new tires are cheaper than new tires AND new rims, now if I could just correct my camber....

 

The biggest concern I would have is your gear ratio. With those big 17's on you must have a hard time getting to the limiter (you autocross I assume). As we all know honda engines make the most power up top so it would suit you well to get there fast. I use 13's on my car bacuse it allows be to keep well above 4k and acceleration is amazing.

As for traction patch, that's only effected by rim width and tire selection.

You can ride along on one of my rides at the SFR SCCA event on the 29th if you want to see the difference. Then you will see why all the DSP Integras run 13's.

 

Ok cool, what do you run? I was at the last event running in FSP with my blue civic, I must say I dislike Golden Gate Fields... I could not get traction on that surface, especially with all the stones... email me at tgeny@hotmail.com when we get closer to the event and we can hook up.

 

do/did the realtime itr's run 17 inch ssr comps when i saw them at laguna seca?