I am currently running a square set of DSIIs which are about 23 lbs each if I remember correctly. I conceptually understand how a lighter wheel affects acceleration, braking, and the inertia of the suspension components under compression and rebound, but I am wondering how much of a quantitative difference it makes on performance. How much of an improvement could I expect to see in my times if I were to run a set of rims like ARC-8s in the ~17 lb range?
You cannot really quantify how much time can be shaved each lap with a lighter wheel. This weight loss is unsprung mass, which will improve how the car handles, brakes and accelerates. This weight savings could be good for 0.05 seconds or 1.05 seconds, it depends on many other factors. Static mass vs. rotational mass is calculated around a 1:5 ratio.
Meaning that for every 1 lb at a stand-still, that is about 5lbs when rotating. If you are saving about 6 static lbs per corner, that means in total, you're saving 24 static lbs. Rotational mass of this would be 24 static pounds x 5 (1:5 ratio) = ~120 rotational pounds.
Total weight saved while in motion would be around 120lbs! So imagine you are basically saving the total weight of a smaller adult woman just by moving to a lighter weight wheel.
Is this something you would notice? It is hard to quantify exactly how many seconds a lap it would be worth, but the weight savings do improve the entire driving experience as a whole, so there is going to be significant gains in performance.
I hope this was helpful!
- Max
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I've done almost exactly what you're contemplating - switched a set of DSI for a set of ARC-8 on the same track on the same day with the same tires (Nitto NT01). I actually weighed the wheel and tire combinations and the DSI with the Nittos was pretty heavy at 45 lbs/wheel and the arc-8 was 38lbs - so there is a significant difference. Also keep in mind that the DSI is a 7.5" wheel and the arc-8 is 8.5. So lighter AND wider. Kind of a knife to a gun fight.
I'm a pretty experienced driver and drive very consistent lap times - usually within a couple of tenths once I settle in for the weekend (half dozen laps on a new track, less on a track I've been to before). What I found is on a 1:30 ish track like summit main or NJMP I see about a 2 second a lap difference plus I feel the car is more responsive and easier to manage with the lighter weight wheels. Lime Rock would be less, Watkins glen, more, if only because those tracks are shorter or longer. That being said, any difference you might experience would depend on how much you could take advantage of the handling differences the lighter wheels might offer.
In competition, a couple seconds a lap is a lifetime. If you're driving DEs it will be a noticeable difference but buying a set of arc-8s just for fun will set you back $1k. (I bought a second set of arc-8's to replace the DSI's. Ha!)
Last edited by emoore924; 02-07-2018 at 08:07 PM.
Not sure how it happened here but....
the top screen is on a 8.5 apex with a 245/40/17 r6
the bottom is on 8.5 ds2 square with 245/40/17 re71r
nothing changed with the car
result is the same exact lap time. It really baffled me hence why the video was made.
https://youtu.be/Q8xQ83H_mhw
Really different weather? Just muscle memory on the slower tires? It did look like there was much less "mid-corner adjustment" on the R6's vs. the RE-71R's. So maybe all the superior grip of the R6's just wasn't been utilized.
It wouldn't be the first time I've seen it on people with lots of laps on a given car/setup. They bolt on stickier tires or a bigger wing etc. and turn the exact same lap times. Then there usually needs to be some coaxing to get them to utilize that extra grip where it's available.
Weather was about the same, r6’s only had one heat cycle. I can do 2:08s on r7s so I don’t think it was driver mod. Only thing I can think of is the taller diameter of the r6’s (taller gearing).
Really need to to put the same tires on a heavy set of rims and then a light set. That would be the only way to know for sure. I should know more this season now that I’m going to heavy 18’s. Setup will be about 5 pounds heavier than the ds2’s. Hoping it won’t slow the car down too much.
Grassroots Motorsports magazine did this test, as well. I think the article was in a magazine last year??? Maybe the year before? I tried a quick google search and could not find the article on line, but maybe deeper digging will provide a copy.
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No way 7lbs per wheel/tire is worth 2s on a 1:30 lap even with the 1" width advantage. 2s is huge. People in my class have added 150lbs of ballast to cars with less than a second difference on a 2:20ish lap.
If your DSIIs are 8.5" all around, i'd run 'em and put the money toward faster rubber. 17x9" ARC-8s are heavy anyway, but they are pretty tough.
I just checked and see they're 17.8lbs in a 17x9 et42. That is pretty good, for some reason i thought they were 19lbs. I have ARC-8s and K1s and the K1s feel to weigh significantly less. Granted they're 1/2" less wide but the different in lugging around is real. I guess its just the tires that i have on them.
Another factor to keep in mind is wheel rigidity. Performance gained in weight can be lost in the flex. There was a Japanese outlet that tested RPF1's (lighter) vs Fn01r-c's (heavier), and the heavier wheel was faster. They determined it was due to wheel flex and the effect that had on the tire. Ideally you want lightweight and strong. My preference is one piece forged wheels, and Rays are what I typically use.
Last edited by Amir@AvusAutosport; 02-14-2018 at 03:04 PM.
Almost none. People like to talk about unsprung weight, rotational mass, etc - and certainly a lighter wheel is better than a heavier one.
But in practice, if you swapped the same tires onto a 1lb lighter wheel, there's more variance in driving than in the weight difference.
In my experience, anyway. Maybe you will be a MEEELION miles faster than everyone and go to ludicrous speed right in front of us.
(Save your money.)
(Aslo, a fast car is an awful lot of small things that add up to a big difference. Lighter wheels are just one of the small things.)
Another thing to think about - even a lap around a skidpad would be absolutely no faster with lighter wheels. Nothing relative has changed. So, your cornering speeds won't be any different.
Last edited by CoMZ302; 02-17-2018 at 02:21 PM.
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