Effects of wheel weight and width on suspension

[wootwoot]

I recently had to pass on a great set of used wheels due to "life." Hopefully I'll get a chance to pick up a set later in the year. But I got to thinking....

Lightweight wheels in 17x7 are around 14lbs. That will save 6lbs per corner.

Lightweight wheels in 17x8's are around 15.5lbs. That will save 4.5lbs per corner.

I would like to run a 225 tire.

My question is, would it be better to run a lighter wheel (17x7) or give up a little weight for a better match to my tire selection (17x8)? Or would I be better off putting a 215 on the 17x7?

My goal in all this is to upgrade my tires for more grip and keep things as light as possible to let my suspension work better. I am on coilovers and they are a little harsh over large bumps. I thought the light wheels would help there. The car is 95% street driven. Will likey do 2 to 4 track days yearly. Was thinking of doing some sticky stuff on stock wheels for track days but I don't know. So will the 1.5lb difference be noticeable between the 17x7 and 17x8 in terms of suspension performance?

[PowderfaceTr.]

If your tire is not following the surface its because of a combination of factors. You don't really notice its happening. Its on such a small scale. You really notice it if your suspension is old.

If there is a problem let some air out of your tires if your going to do hard acceleration.

Here is the reason why:

The resonant motion of the unsprung weight moving up and down on the springiness of the tire, is only poorly damped, mainly by the dampers or shock absorbers of the suspension. You get wheel hop on hi power cars. Its important to design the car properly. Wheel hop is caused by the location of the "instant center" in relation to the center of gravity of the vehicle. Lift bars will change the angle of your rear suspensions trailing arms, and usually cure the problem.

Less weight will resonate less.
Having more contact patch will delay the inevitable.

[wootwoot]

After reading more it seems a 225 will be too pinched on a 7" rim and likely lead to excessive sidewall flex. So where is the line of diminishing returns when considering tire size against reducing un-sprung weight? Is it better to get a 215 on a 7" wheel at 6lbs less per corner? Or would it be better to get a 225 on a 8" wheel at 3lbs less per corner? Or should the 215 be on the 8" wheel?

[tyler_win_photo]

Originally Posted by wootwoot

After reading more it seems a 225 will be too pinched on a 7" rim and likely lead to excessive sidewall flex. So where is the line of diminishing returns when considering tire size against reducing un-sprung weight? Is it better to get a 215 on a 7" wheel at 6lbs less per corner? Or would it be better to get a 225 on a 8" wheel at 3lbs less per corner? Or should the 215 be on the 8" wheel?

It depends on how much power you're making. If you're making stock power (no tune), you'd probably be better off on the lightest weight setup, so that 215 on a 7". If you've got headers + tune, the 8" with a 225 would probably match up well.

Also, I'm sure you're well aware of this but the type of tire you're using will likely make a greater difference than just how wide it is.

[wootwoot]

I appreciate the help. It's a tough call. I'm on RSR sport-I coilovers. Intake, catted header, catless front pipe, catback, OFT stage one with upgraded final drive, carbon drive shaft, lightweight flywheel, lines, fluid and pads. I am also on poly bushings with rear control arms and an aggressive alignment. For the street I feel like a 215 MPSS or something similar would be enough. It's a sizable investment and I don't want to make the wrong call and have to do it twice. For track days I feel like I would overheat a 215, but I really don't have a clue...

[cjd]

Originally Posted by tyler_win_photo

It depends on how much power you're making. If you're making stock power (no tune), you'd probably be better off on the lightest weight setup, so that 215 on a 7". If you've got headers + tune, the 8" with a 225 would probably match up well.

Also, I'm sure you're well aware of this but the type of tire you're using will likely make a greater difference than just how wide it is.

That depends how much turning is part of the performance package you want. I expect 245 on a 17x9 will be faster than 215 on a 17x7... considering 245's stretched on a 17x7 is what the CS AutoX guys run.

I've got 225 PSS on 17x8 for my daily (that's an 8" wide tread so not even stretched.) 255 on 17x9 for AutoX. Of course, that 17x8 is ~14.6lbs I think. 15.8 for the 9's. Or so.

[wootwoot]

Originally Posted by cjd

That depends how much turning is part of the performance package you want. I expect 245 on a 17x9 will be faster than 215 on a 17x7... considering 245's stretched on a 17x7 is what the CS AutoX guys run.

I've got 225 PSS on 17x8 for my daily (that's an 8" wide tread so not even stretched.) 255 on 17x9 for AutoX. Of course, that 17x8 is ~14.6lbs I think. 15.8 for the 9's. Or so.

What 17x8 are you running that is 14.6lbs?

In general I like to keep autoX set ups out of my calculus because they are so specialized and, in general, very far from what I'll be doing with my car. If the point was that a 225 will sqeeze on and perform with a 7" wheel because the autoX guys run 245 on the same width... that's interesting. I believe they do that because of class rules and I'm trying to figure out what's optimal.... Which hasn't been easy.

[xwd]

If you are just doing track stuff the wider wheels and tires may not do much for you unless you have the power to negate negative effects on long straights. Tracks have a lot of straighter power sections and obviously no quick transition elements. Heavy wheels and tires with big contact patches on a 200HP car are going to make you slower than faster.

Autox is specialized. You are stuck with a 7" width rim, and the reality is a pinched 245 is still faster than a smaller tire, even though the steering feel isn't great. Also depends on the tire.

How much power are you making?

[wootwoot]

Originally Posted by dwx

If you are just doing track stuff the wider wheels and tires may not do much for you unless you have the power to negate negative effects on long straights. Tracks have a lot of straighter power sections and obviously no quick transition elements. Heavy wheels and tires with big contact patches on a 200HP car are going to make you slower than faster.

Autox is specialized. You are stuck with a 7" width rim, and the reality is a pinched 245 is still faster than a smaller tire, even though the steering feel isn't great. Also depends on the tire.

How much power are you making?

Thanks for the reply. I definitely understand what you are saying. I listed my mods a few posts up. It's basically a full bolt on car with a more aggressive final drive. Never been on a Dyno so no solid numbers.

I know I don't need much tire given the cars weight and low power level. So, given that, would a MPSS in 215 on a 7" be enough (saves 6lbs per corner)? Or would a 225 on a 8" be better (save 3lbs per corner)? Or would a 225 on the 7" work just as well? Where is the balance between saving un-sprung weight, tire size and tire fitment?

[cjd]

Originally Posted by wootwoot

What 17x8 are you running that is 14.6lbs?

In general I like to keep autoX set ups out of my calculus because they are so specialized and, in general, very far from what I'll be doing with my car. If the point was that a 225 will sqeeze on and perform with a 7" wheel because the autoX guys run 245 on the same width... that's interesting. I believe they do that because of class rules and I'm trying to figure out what's optimal.... Which hasn't been easy.

I think I'm a little shy on that number after checking, but not much... 14.8lbs is the reported weight (Wedsport TC105N)

CS folks are definitely trying to optimize beyond what most of us will do (frankly, I'd not want to run a 245 on a 7" on the street around here,) but my point was that "fast" in this discussion seemed to exclude the impact that more tire width and a properly sized wheel will have on the amount of speed you can hold through a turn, which will offset any particular gains from lighter weight.

That said, you DO have to consider all the factors and choose based on what you're doing. It's likely that weight will be a secondary decision: do you spend more for a lighter wheel in the size you've determined is optimal for your use.

Obviously, I chose 225 PSS on a 17x8, but would have considered 17x7.5 too. The 8" ended up lighter than any of the 7.5" I liked and was interested in paying the cost for... and a 215 can fit a 7.5" quite as well as a 7".

[wootwoot]

I had looked at 7.5's as well but the weight difference between the 7.5's and 8's was often less than .5lbs. The difference between the 7's and the 8's, however, was often 2lbs. That's why I was trying to choose between the 7 and 8. I suppose a 7.5 would be a nice compromise and allow me to run either size tire.

What if I approach this from another angle and decide on the tire and then choose the lightest wheel I can afford to go with it. Given that, how does one go about matching a tire size to suspension and driving habits? Again, looking at a MPSS....

[PowderfaceTr.]

You are on the right path.

I would suggest you chose the Brake system first. Its weight has the most impact on suspension. The you chose how soft of a spring you need depending on the characteristic handling you want based on the surface. Softer (less resonance) for bumpier tracks is faster. The tire qualities will depend on how soft your setup. Slicks will do nothing to make you faster if you don't have at least 1000lbs springs. (Stickier Requires more spring rate). Then you get a tire with big sidewall and use air to tune the whole setup.
Rims are last keep them small enough to just fit calipers. Just wide enough to prevent lift off oversteer. So the tire width would be proportional to the way the car reacts to the type of power your putting down before the turn. No more as you will be plowing tread.

[wootwoot]

Spring rates are:

6kg/mm (335lb) front
7kg/mm (390lb) rear

Brakes are currently stock with winmax w2 pads, upgraded lines and fluid.

Surface the car will be on: tight and technical twisty California coastal roads

Given all that, what's the best move for a street tire?

[PowderfaceTr.]

I have 225. But my tire is 300 Treadwear. The 3* neg camber i have up front to balance the car seems to eat tires. I keep a more durable less sticky tire to get more seat time. I have this car setup for what it does best down hill gravel.

You may want more narrow as to not overheat the tread and keep a stickier tire lasting longer. Remember its lighter and will be more gradual at the traction limit. Dont not overdrive the tire. They should last 1000 miles at 10/10ths. Irecomend you drive them in the rain for the first spirited drives. That wi show how they will perform at their max but at a much lower safer speed.

http://www.avonmotorsport.com/road-l...erformance/zzr
215/40R17 83W 7.0