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New custom wheels - aim for widest track width?


turbogrill
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Hi,

 

I am having a set of wheels made for me, I can basically chose whatever backspace I want.

 

Should I just get something similar to what I have now that I know works or is there any gain in getting 1-2" extra track width?

 

Is it one of those things where more is always better? 

 

With our car there is really no thoughts or calculations about GC, roll centers and suspension geometry. It's more about following guidelines and hope it turns out well :)

 

I guess wheel bearings could take a hit, that would be annoying.

 

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Roll Centers, Scrub Radius, etc are all important.

 

If you add track width, you will end up adding more load to the wheel bearings, changing effective spring rates, and other things.

 

You should really consider carefully before clicking "order"

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I went through a similar exercise when I put the big 245-15 tires on my car which is also a strut front end car, keeping the tires off the struts is the one thing you absolutely must do. In my case I wasn’t designing custom wheels, I wanted to use Miata wheels since they are very inexpensive and light, and available in 9 or 10 inch wide options, so I was designing custom adapters. In my case I modeled everything up in CAD but there are good wheel fit calculators out on the internet. If you know how much clearance you have now you can plug in new tire and wheel options and know how much clearance you will have with the combination you come up with. In my case I had to make very thick adapters for the front to keep the tires off the struts, then a lot of beating on the fender lips and I was just able to keep the tires inside the bodywork. In the rear since I had room I used 20mm thinner adapters to tuck the rear wheels in more.

 

59e4eee46a1dc_IMG_20161029_0847410101.thumb.jpg.828581dd6b7cad522af039c101cde746.jpgfront_adapt.jpg.73dd593694b01ee298dda15121875ccf.jpg

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 Any weight transfer equations divide the  mass by track width . So wider  has faster potential -  every time.  You may have  steering wheel kick back if you get too far outside of the steering p ivot line. Googlate  "Scrub radius."  

 Sight the line from the top pivot through the ball joint pivot to the ground.  It should pass near the center of the stock positioned tire. 

 As you get more tire outside of this pivot line you get more steering effort/kickback . Taken to extreme you can break the wheel studs /axle/ spindle off, when jumping the car, as the load  path is way outside of the  designed  center line.    But yeah it goes faster. 

 My SCCA cars are offset to the right and right at the max track values. . Enoughthat I measure by the book every qually session and have the fixtures to measure at the hub CL, Not the ground.  

Edited by flyinglizard
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17 minutes ago, mhr650 said:

I went through a similar exercise when I put the big 245-15 tires on my car which is also a strut front end car, keeping the tires off the struts is the one thing you absolutely must do. In my case I wasn’t designing custom wheels, I wanted to use Miata wheels since they are very inexpensive and light, and available in 9 or 10 inch wide options, so I was designing custom adapters. In my case I modeled everything up in CAD but there are good wheel fit calculators out on the internet. If you know how much clearance you have now you can plug in new tire and wheel options and know how much clearance you will have with the combination you come up with. In my case I had to make very thick adapters for the front to keep the tires off the struts, then a lot of beating on the fender lips and I was just able to keep the tires inside the bodywork. In the rear since I had room I used 20mm thinner adapters to tuck the rear wheels in more.

 

59e4eee46a1dc_IMG_20161029_0847410101.thumb.jpg.828581dd6b7cad522af039c101cde746.jpgfront_adapt.jpg.73dd593694b01ee298dda15121875ccf.jpg

 

We also tried with Miata wheels, but we need wider than 30mm adapters (rule 4.3.2). 

 

37 minutes ago, Huggy said:

Roll Centers, Scrub Radius, etc are all important.

 

If you add track width, you will end up adding more load to the wheel bearings, changing effective spring rates, and other things.

 

You should really consider carefully before clicking "order"

 

Hmm ok. I might just get a little extra to make sure I have room for the struts.

 

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3 minutes ago, enginerd said:

Keep the wheels inside the fenders, there are many examples of wheel to wheel contact which end poorly. If these had been body to body contacts, only a bit of paint would have been lost. 

This.

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26 minutes ago, enginerd said:

Keep the wheels inside the fenders, there are many examples of wheel to wheel contact which end poorly. If these had been body to body contacts, only a bit of paint would have been lost. 

 

hmm that is another good point. A Miata did hit our left rear fender last race!

 

Maybe I should rethink this.

 

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6 hours ago, wvumtnbkr said:

I am under the impression that narrower trackwidth will have more corner grip.  I could be wrong.

 

My days of racing rc says that this statement is accurate.  Not sure if it translates well to big cars.

It doesn't. Fat is phat. :P

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 One of the few times that narrow might go faster is Karts.  If you can get the inside rear tire up , the center turn is a lot faster . So on a low grip paved track you narrow  up until the tire just comes up. 

 Other times might be a very low grip situation where PSI on the ground improves bite. Like snow or mud.   

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Rob,

 

Most vehicle dynamics center around the concept that more wheel load will give back proportionately less grip. This is the primary driver for lower cg and lower vehicle weight handling better. 

 

In a practical sense, you can find some examples where the grip is proportionately the same with added load (weight), and rare cases where it is proportionately better. This is big picture stuff, you can get tuning differences that make the car better or worse from balance change when increasing or decreasing wheel vertical load, although i am assuming you would alter setup to fix this. "Fixing" the setup, more weight difference across an axle, like you get with narrow track, will reduce that axles lateral grip.

 

Generally not getting proportionately more grip with less load is a sign of the tire being very wrong (too stiff) for the car.

 

If the proportionate grip does not change with load, crossweight, noseweight, dynamic cross (weight change when rolling and moving) and shock tuning would all have next to no effect. Pretty critical part of conventional vehicle setup tuning.

 

To the OP, run the wheels with as little backspacing (close to stock) as you can. Your forearms (steering effort and brake\torque steer), steering parts and bearings\bushings will thank you later.

Edited by Black Magic
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6 hours ago, Black Magic said:

Rob,

 

Most vehicle dynamics center around the concept that more wheel load will give back proportionately less grip. This is the primary driver for lower cg and lower vehicle weight handling better. 

 

In a practical sense, you can find some examples where the grip is proportionately the same with added load (weight), and rare cases where it is proportionately better. This is big picture stuff, you can get tuning differences that make the car better or worse from balance change when increasing or decreasing wheel vertical load, although i am assuming you would alter setup to fix this. "Fixing" the setup, more weight difference across an axle, like you get with narrow track, will reduce that axles lateral grip.

 

Generally not getting proportionately more grip with less load is a sign of the tire being very wrong (too stiff) for the car.

 

If the proportionate grip does not change with load, crossweight, noseweight, dynamic cross (weight change when rolling and moving) and shock tuning would all have next to no effect. Pretty critical part of conventional vehicle setup tuning.

 

To the OP, run the wheels with as little backspacing (close to stock) as you can. Your forearms (steering effort and brake\torque steer), steering parts and bearings\bushings will thank you later.

 

^^^ All true, pay attention to the last part if you have no power steering. We don't have PS and there are complaints almost every race, they get worse the bumpier the track is. We have scrub radius = too much. to fit wide wheels.

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Wheel wise, We just went with what we had in the garage (oddly this is true)...Its all for looks.  If we're going to lose, might as well look good doing it...

 

In all honesty, we had a set of Z06 wheels laying around and we checked the fit...That is one good thing about having a UUGE car is that it comes from the factory on a wide tire.  The fenders weren't the issue, we just had to ditch our front wheel spacers and flatten out our brake ducts a touch.  Helps to spread out some of that weight.  Had a bit of wheel well scrub at Sebring coming under the bridge, but nothing a mallet and 5 minutes couldn't fix.  We did Ruin a wheel at Sebring, Because Miata.....

 

Anytime you run big spacers, you're putting more load on the bearings, so make sure you check them frequently.  It's a weak point on our car. 

 

Ken

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16 minutes ago, Quicker10u said:

 

Anytime you run big spacers, you're putting more load on the bearings, so make sure you check them frequently.  It's a weak point on our car. 

 

Ken

 

Scenario 1:

15x10 high offset + big spacer

 

Scenario 2:

15x10 low offset no spacer

 

Both tires sit in the exact same same position

.

Will scenario 1 be tougher on wheel bearings? Why? Is it the lever created by the spacer that puts extra load on the bearings?

The tire sit in the same position so the force on the wheel bearing should be the same? 

 

28 minutes ago, Quicker10u said:

 

This can't be true.  I don't believe this.  Miata's don't hit people or make any mistakes, because Miata....

 

Figured out a solution, more hp. That way you will hit them instead.

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4 minutes ago, turbogrill said:

 

Scenario 1:

15x10 high offset + big spacer

 

Scenario 2:

15x10 low offset no spacer

 

Both tires sit in the exact same same position

.

Will scenario 1 be tougher on wheel bearings? Why? Is it the lever created by the spacer that puts extra load on the bearings?

The tire sit in the same position so the force on the wheel bearing should be the same? 

 

 

Figured out a solution, more hp. That way you will hit them instead.

No difference in load, as you suspect.

 

And yes, much easier to avoid other cars when you're driving away from them. :)

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31 minutes ago, turbogrill said:

 

Scenario 1:

15x10 high offset + big spacer

 

Scenario 2:

15x10 low offset no spacer

 

Both tires sit in the exact same same position

.

Will scenario 1 be tougher on wheel bearings? Why? Is it the lever created by the spacer that puts extra load on the bearings?

The tire sit in the same position so the force on the wheel bearing should be the same? 

 

 

Figured out a solution, more hp. That way you will hit them instead.

 

Yes, if you have that option to choose your offset, there will be no difference in load. Most don’t have that option though. 

 

More power would be nice but we’re maxed out where we are without digging into the motor. We can play spec piñata in the slow corners if we have to. We just do our best to avoid contact. We got a “Mr. Clean” award at Sebring.  Hopefully Everyone will behave at Road Atlanta. Apparently the event has a bit of a reputation. 

Edited by Quicker10u
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