Wheel width......17 x 9 20lbs vs 17 x 9.5 22.3lbs - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 10 Old 05-11-2017, 08:06 AM Thread Starter
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Wheel width......17 x 9 20lbs vs 17 x 9.5 22.3lbs

Has anyone done comparisons for wheel widths/weight like this?

96 mustang spec racer where tire/wheel specs are limited. We run 275/40/17 Toyo RR's and I've used 17 x 9 FR500 knock-offs for years. I just bought a set of 17 x 9.5's and can account for wheel offset with spacers but these wheels are 2.3 lbs heavier. I'm wondering if the extra 1/2 inch is worth the 2.3 lbs per wheel?


If racing is like sex.....why would you want it to only last 10 seconds. (road races last 25 to 45 minutes!)
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post #2 of 10 Old 05-11-2017, 08:57 AM
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Interesting question. Hopefully someone can chime in with good data, but I have a feeling the only way to find out is to try it yourself.

(I'm surprised the FR500 knock-offs were that light.)

Edit: Since the differences may almost negate each other, it might be hard to pick out whether there's an advantage or not. While I know that wider wheels are "better", and heavier wheels are "worse", I don't know what the threshold is for actually being able to perceive a difference (either via the seat of the pants or the stopwatch). I don't believe I've ever seen any actual testing by someone to show how much of a change it takes in one of those dimensions to see a difference on track. Seb Vettel might perceive a 0.5# difference in unsprung weight, but would I? Similarly, Seb Vettel's car may be sensitive to a 0.5# change in unsprung weight, but would my vastly less sophisticated car be sensitive to that change?

Edit2: Don't forget the weight of the spacers when you're comparing wheel weights. Or did the 2.3# difference already account for that?


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post #3 of 10 Old 05-11-2017, 01:11 PM Thread Starter
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When swapping wheels I removed the .5 inch aluminum MM spacers from all 4 corners and added a smaller 5/16 on the rear only. So, while I removed some weight, it was very little.

I'm going to be at my home track (VIR) next weekend where I've run multiple laps within .1 second of each other (2nd and 4th lap were identical to the hundredth). However, atmospheric conditions effect things too. Hopefully I'll be able to learn something from a 3 day weekend!



Yes, I was surprised the FR500's were that light and part of the reason I've been using them for years. Being under $500 for a whole set was certainly nice too! My Fr500's are actually plasti-dipped which probably added a .2 as well.

If racing is like sex.....why would you want it to only last 10 seconds. (road races last 25 to 45 minutes!)
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post #4 of 10 Old 05-14-2017, 08:58 AM
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Closest actual research into the effect of wheel weight I've ever seen was in SCCA's monthly magazine a few years back. Going entirely off memory here, a difference of 5 lb per wheel on a Miata corresponded to a tenth or two over a typical autocross time (40 - 50 seconds?). I'd expect 2.5 lb/wheel on a car that's much more powerful and around 50% heavier to be worth less than that, maybe 0.05 seconds per minute.

I have done some acceleration simulations that consider an estimate of wheel rotating inertia (and the tire a few other rotating inertias). Overall for the wheels, it's not the 1 lb rotating = 10 lbs sprung weight correlation that you might have heard over the internet. It's not even close to that, more like a 1 to 2 relationship (and even that may be a little high depending on the exact situation).


The half inch of wheel width may be worth more, particularly if it causes any improvement in driver confidence. The rotating inertia of a spacer is negligible (small weight times small diameter (squared) => tiny MOI).


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post #5 of 10 Old 05-14-2017, 10:43 AM
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No tech here but a further question about the benefits/impact if you're retaining the same exact tire. Wouldn't the effect of this change have more to do with the relationship of tire width to wheel width have the potential for a greater impact that a few pounds of additional weight?

Per TireRack - For your Toyo Proxes RR
SIZE - RIM WIDTH RANGE - MEAS. RIM WIDTH - SECT. WIDTH - TREAD WIDTH - OVERALL DIAM. - REVS. PER MILE
275/40ZR17 9-11" 9.5" 10.9" 10.5" 25.7" 809

How would the section width and overall diameter change with your move from 9" to 9.5"? How would this change cornering dynamics?

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post #6 of 10 Old 05-14-2017, 11:40 AM
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Section width increases by about 0.2" (5mm) and the sidewall shape being wider at its base (the bead regions) provides slightly better 'bracing' for the tread (which remains pretty close to its original measurement). Overall diameter decreases minutely, so the tire moves toward having a slightly lower operational profile from both effects.

I have no idea what the half inch wheel width is worth by itself with the same tires. But in back to back testing a sensitive driver might notice the wider wheel setup being just a little better-behaved up toward the limit, which might be minutely higher with the wider wheel.


Norm

Mine: '08 GT, 5MT, black/light graphite, un-Fstock (DD, occasional track day)
Wife's: '10 Legacy 2.5GT (DD, six-speed manual)
Spare:'01 20AE Maxima, 5MT (also my bad weather alternate)
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post #7 of 10 Old 05-15-2017, 03:41 PM Thread Starter
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Originally Posted by Norm Peterson View Post
Section width increases by about 0.2" (5mm) and the sidewall shape being wider at its base (the bead regions) provides slightly better 'bracing' for the tread (which remains pretty close to its original measurement). Overall diameter decreases minutely, so the tire moves toward having a slightly lower operational profile from both effects.

I have no idea what the half inch wheel width is worth by itself with the same tires. But in back to back testing a sensitive driver might notice the wider wheel setup being just a little better-behaved up toward the limit, which might be minutely higher with the wider wheel.


Norm
Kind of what I'm hoping for. I'll be at VIR this weekend where I can run back to back laps withing a tenth (although we were there 2 months ago in perfect weather and I beat my own track record by 8 tenths!) By the end of a 3 day weekend I hope to determine if I am better off. I'm at 3,300lbs and 260rwhp.....and slide the #### out of the car! (Kinda have to.....my rear is welded like a spool! )

.5 wider wheel, same 275/40/17 RRs
22.5lbs vs 20.2 wheels, track width spaced out to be the same
Tires "look" wider and sidewalls look more vertical (less bulged)

If racing is like sex.....why would you want it to only last 10 seconds. (road races last 25 to 45 minutes!)
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post #8 of 10 Old 05-15-2017, 07:21 PM
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Can you post a couple of pictures of those wheels and tires?

I like the thoughts, I hunted for a used set of forged wheels for my SUV since the cheap 18's I got ages ago were 72.5lbs per wheel/tire. I settled on a 17x9 Volk wheel that came in at 21lbs, specs said 20lbs. The 275/55/17's I bought are 37lbs each, so I'll be back to the 58lbs level, which is what I had with the 93/94 Cobra wheels I first used on it(255/60/17). I have those wheels on my Lincoln now, and the 255/45/17's fit the car well. But a 275mm tire would make me happy if it could fit the front too. That 275/40/17 might be my best choice, so any pictures would be helpful.

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post #9 of 10 Old 06-12-2017, 09:28 AM
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Originally Posted by t500hps View Post
Kind of what I'm hoping for. I'll be at VIR this weekend where I can run back to back laps withing a tenth (although we were there 2 months ago in perfect weather and I beat my own track record by 8 tenths!) By the end of a 3 day weekend I hope to determine if I am better off. I'm at 3,300lbs and 260rwhp.....and slide the #### out of the car! (Kinda have to.....my rear is welded like a spool! )

.5 wider wheel, same 275/40/17 RRs
22.5lbs vs 20.2 wheels, track width spaced out to be the same
Tires "look" wider and sidewalls look more vertical (less bulged)
So how did things go @ VIR?
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post #10 of 10 Old 06-28-2017, 10:04 AM Thread Starter
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So how did things go @ VIR?
Oops, Results.......and remember, weather conditions make huge differences in results.

also FWIW: Nov 2015 I ran a 2:09.8 with the FR500 knock-offs in PERFECT conditions but had never been able to duplicate it...or even really get that close. March 2017 I managed a best of 2:10.8 in a race and 2:10.0 in morning practice, again with FR500 knock-offs.

So at the May event I'm on these new 9.5 wheels but it's fairly hot for Friday and Saturday......and I'm stuck in the 2:13's with a car that is overheating! Not enough for fastest laps but still end up winning the Sat race due to other drivers errors. Sunday race the weather is much cooler and I may have solved my overheating problem. I immediately get out to a big lead and several laps later, out of boredom, I decide to see just how fast a lap I can lay down. However, since I'm well out in front I repeatedly "throw-away" corners allowing other class racers to get by me clean so I don't screw up their race....but the car is staying nice and cool!

20 min into a 45 min race I end up laying down a 2:09.8. Same thing I ran in 2015. Was it the wheels, weather, or bigger balls I really don't know. The car didn't feel any different on track. I was pumped though because my old 2:09 was during a club day and this 2:09 is an official race.....so it counts as a track record!!!!

Since it's been over a month, we also raced at Summit Point 2 weeks ago. It was somewhat hot, my overheating problem came back and while I won, I was 2+ seconds off my personal bests. The cooling system is in the middle of getting a complete flush, inspection/replace before our next event which isn't til August!


My opinion: If I buy another set of wheels it will be these heavier 17 x 9.5's. I like the way they look better than the FR500's.......but that seems to be the only difference on track using them .


If racing is like sex.....why would you want it to only last 10 seconds. (road races last 25 to 45 minutes!)
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