Spreadsheet blows lid off lightweight wheel debate!!!

[DaveNagy]

Okay, not really, but it's certainly grist for the mill. And I just love a gristy mill.

I'm in midst of picking out some aftermarket tires/wheels for my not-yet-delivered MCS. I've narrowed it down to a very lightweight 17" wheel from Centerline, or perhaps a freakishly lightweight 16" wheel from SSR. Like most everyone else, I like the look of the bigger, heavier wheel more, so I'm trying to get a feel for how that last few pounds of weight will influence may car's performance.

Well.

I went over to the i-club forums and discovered that they debate that concept at least as much over there, as we do over here. (They even quote Garfield's MINI thread where he G-Tech'd a couple different wheel sizes.) Anyway, in my searches I happened across this thread.

If you follow the link in the first post, you can download a spreadsheet that purports to calculate the (average?) torque required to spin a wheel up 60MPH. (You plug in how long it takes your car to reach 60MPH "normally", and then it figures out how much torque is required to just get the wheels spinning that fast. i.e. How much torque would it take to spin (all four of) the MINI's wheels up the 60MPH, while the car was up on jacks? (I'll leave it up to the reader to figure out how you get the rear wheels spinning at all in such an experiment.)

Better yet, the spreadsheet calculates all this for two different tire/wheel combos, and displays the difference between them. Cool, cool, cool.

I went ahead and plugged in the specs for the standard 17" S-Lite <cough> wheels and runflats, as well as those for the super lightweight 16" SSR Competitions shod with lightweight (and smallish) Toyo T1-S tires. I've attached that spreadsheet for your enjoyment. Check out lines 30-34. That's where the 'output' is. (Hmmph. I see the attachment didn't work. I'll try to attach it in the next post.)

The results? Oh. My. God. Unless I've messed up something somewhere, it takes almost 25 ft/lbs less torque to accelerate a 30lb tire/wheel package, as compared to the 50lb stockers. 25 ft/lbs. That's a lot. Like fully 16% of the peak torque the MCS is capable of. If the MCS doesn't have to spend that torque spinning (and accelerating) its wheels, it can spend it accelerating the rest of the car. To give a feel for that number, if the whole car was to accelerate 16% faster, (it wouldn't, quite) a 7.2 0-60 time would improve to near 6 seconds flat.

(That wouldn't happen in real life since you'll always be traction limited down in the 0-20MPH range.)

The spreadsheet also says my wheel-change is equivalent to dropping 399 lbs off the chassis. I don't know if I believe that, but yike. Oh, that's just 16% of the MINI's 2500 gross weight. Makes sense, I guess.

Yes, I entered pretty "extreme" lightweight wheel and tire values, and my 16" tire has a pretty small diameter. 23.2" vs the stock 24" of the recommended tires. But I think the main factors that contribute to these glorious improvements on the MCS are that:

• The stock wheels are pretty darn heavy.
• The stock tires are ridiculously heavy, and that weight is out on the "edge" where it hurts the most.
• The MCS is fairly light.
• The MCS is not overly endowed with torque to begin with.

Crazy stuff.

One concept I had trouble with when interpreting the spreadsheet was, "What does it mean when it say it takes x amount of torque to accelerate the wheels?" Accelerate it how? When?

I believe what the spreadsheet does is take the amount of time it takes for the car to accelerate to 60MPH. (i.e. 7.2 sec.) It then figures out the "average" acceleration experianced during such a run. Then it figures out how much torque is required to accelerate just the wheels at that same "average" rate, and bases the rest of the figures off of that. In real life, we accelerate really quickly in the lower gears, and more slowly in the higher gears, so the amount of torque required to spin up the wheels varies a lot. The moral? Light wheels help a ton when accelerating quickly, but don't help much at all when accelerating slowly. Frankly, I'm only interested in what happens when accelerating quickly!

Anyway. Discuss!

-Dave

[DaveNagy]

Here's the Stock 17" vs. Lightweight 16" spreadsheet:

[olddad]

Add to that the extra force required to overcome higher flywheel effect of the heavier mass when braking and the less trouble the suspension has keeping the tires in contact with the road and you have rediscovered why racers have always taken such care in reducing unsprung weight.

I see you've picked out the 225/40 Toyos. That is my plan as well as the specs indicate they will just fit. Wish the final drive of the S was much lower, low enough to peak in 6th at about 135.

[jlm]

acceration is made more difficult by more mass; controlling the wheel over bumps or trying to speed up or slow down a rotating wheel, or changing the direction of your valves, pistons and rods.

olddad: gonna have to get that lsd with the 4.24 final ratio.

[olddad]

A low ratio LSD would be great except I keep thinking about the waranty issues and the many years since I've owned a new car . Would hope to autocross in DS with just the filter element/exhaust/shock mods for a while. We're still trying to get a light weight wheel for autocrossing before next season, but folks seem to be waiting too long.

Would have been great if MINI had homologated a low ratio LSD and a 7" wheel.

[DaveNagy]

olddad: No, I'm leaning more towards a 205/45/16 tire on those SSR Comp rims. A 225/40/16 is also an interesting choice since it has an even smaller diameter. (4.5% less than stock. There's your new final ratio right there!) But I wonder if it might be a little too much tire for a 7" wide wheel. Toyo recommends something between a 7.5" and 9" wheel for that tire. Plus the 225/40 weighs a full pound more than the 205/45!

Toyo's T1-S spec page

Seriously, I think I want to stay with a "conservative" tire size in case I want to do a little lowering at some point in the future.

-Dave

[mike58]

The michelin Pilot Sport Cup road race tyre is substantialy lighter than a regular road tyre .

[jlm]

just got the OZ 16x7 superlegerras fitted with Kuhmo V700 205/45-16. 34lbs each, mounted. very nice sort of pewter colored aluminum.

dia is just over 23", circumference about 73", about a 4% reduction in speed from stock 24".

[DaveNagy]

jlm: Do you happen to have any pics of those tires mounted on your car? That's the size I'm leaning towards, but was hoping to see how they looked before I put down my money.

I've actually got a different thread on that subject, so if you (or anybody) has some pics, that would be the place to post them.

Thanks!

-Dave

[jlm]

right now, i'd have to hold them up to someone else's car; mine hasn't been delivered yet. i will post a photo of the wheels tommorrow and the wheels on the ar when it arrives...supposed to be this week!

john

[jlm]

just got the car saturday and mounted the OZ today:

"just got the OZ 16x7 superlegerras fitted with Kuhmo V700 205/45-16. 34lbs each, mounted. very nice sort of pewter colored aluminum.

dia is just over 23", circumference about 73", about a 4% reduction in speed from stock 24".

as far as stock sport wheels and all season runflats (goodyear): 50 lbs each, mounted. dia 24.5", 78" circumference.

photos before and after to follow.

that is a savings of 16 lbs unsprung per corner!, not to mention a 64 lb overall weight reduction.

[jlm]

s-lites with goodyear runflats:

[jlm]

s-lite/goodyear vs OZ superleggera 16x7 with Kuhmo 205/45;
note rolling diameter reduction.

[jlm]

final trim, 16x7 superleggeras, 205/45 Kuhmos

[jmardy]

Quote:

Originally posted by jlm
final trim, 16x7 superleggeras, 205/45 Kuhmos

Those look EXCELLENT!!!

How do they feel?