There are numerous comments that 16" wheels offer better acceleration than the 17" wheels becuase the wheels are lighter. It's hard to imagine that the weight of the wheel provides enough rotational inertial to make a statistically significant difference to acceleration. Does anyone have any direct comparision acceleration figures to back up these claims?

Thanks,

ON-ORDER - MCS IB/Wt, Sports, Premium minus sunroof, HK

 

While I haven't seen any numbers, it's easy to visualize how the change in rotational inertia will help off-the-line acceleration and deceleration, as well as quick steering inputs. After changing from my boat anchors to lightweight wheels/tires, I noticed a marked improvement. The best way to describe it is the car feels lighter on its' feet.

Cheers,
Ryan
moving to wheel/tire and brake forum

 

2 possible scenarios here:

1. The outter diameter of the 16" wheel/tire combo is smaller than the OD of the 17" setup so the car will accelerate faster.

2. Removing as much as 15 lbs. of unsprung weight from each corner of the car will lead to better acc/braking/cornering because less energy is required to make the wheels/tires change direction.

Often it is a combination of the two that will make a car with lighter wheels/tires outperform one with heavy S-lite wheels. BTW, it's very hard to get scientific data to back it up but if you were to drive a car with an identical setup to yours with lighter wheels you will feel a difference.

 

at the track you will notice a difference from unsprug weight.

for street use, the biggest difference you would feel is rather in the
braking than acceleration.

 

There was a good article in SCC a few years ago... they tried to quantify exactly this. They switched to a number of lighter wheel/tires setups on a few different cars... and then measured lap times and quarter mile times... they then started adding weight to the car (sandbag to the front and rear) until the times matched the times with the stock(heavy) wheel/tire combo. (Keeping tire compound the same, etc). and then came up with a general formula.

I think they were the original ones that come up with the now, well-known: "saving 1 lb of unsprung weight is like saving 4-5 lbs. of sprung weight".

So saving 20-25 lbs. total on wheels/tires is like removing 100lbs from the car, and a 100 lbs is supposed to gain 0.1s in qtr mile and more on a track.

I'll try to dig up that article and post a reference.

 

Just did a google search trying to find that article and came across a post saying that Grassroots Motorsports did a test for just qtr mile.

Same conclusions as the SCC article:
Here is the link - the post is second from the bottom by Farfrumwork. Everything else is just amusing flame-war:
http://forums.nasioc.com/forums/show.../t-446090.html

 

>>Same conclusions as the SCC article:
>>Here's another angle.
From Car and driver's article on the John Cooper works kit in an MCS:
http://www.caranddriver.com/article....rticle_id=7279
"The last Cooper S we tested (July 2002) took seven seconds to reach 60 mph. The JCW package drops that time to 6.4 seconds. Our stock Cooper S ran the quarter-mile in 15.5 seconds at 90 mph. The JCW car turned the quarter in 14.7 seconds at 95 mph. "

That means comparing the JCW MCS to the stock MCS the additional 37 HP and 22 lb-ft of torque results in lowering the quarter mile time by 0.8 seconds. Therefore for every 4.6 HP there is a 0.1 second drop in the quarter mile.
And for every 25 pounds of rotational weight lost that would roughly equal a 4.6 HP gain assuming the benefits are linear as HP is increased.

My S-lytes are 48 pounds each and my Centerline RPMs are 36 pounds each so for a set of four I save 48 pounds and so it might feel like a 9 HP and 5 lb-ft of torque gain. And when I drive it surely does feel just that.

 

I remember that SCC article. I can't remember what car it was (think it was a civic), but it was yellow. They also put it on a dyno and came up with a few more horses in the lower gears. In effect, having a lighter weight wheel/tire combo gives you more horsepower "after" the wheels. The faster you go, the less of a difference the wheel weights make in acceleration terms, because at higher speeds the car is primarily fighting against wind resistance. In first and second gear accelleration, the lower weight in rotational masses like wheels/tires and flywheels really can make a large difference. There is a ballance you want to achieve, however. You don't see many cars out there with 5"x7" wheels. It has been agreed upon by the mini comunity at large that lightweight 16" wheels offer the best in real world racing and daily driving performance/benefit.

_________________
ssssshhhh . . . don't wake the sleeper!

 

Back in July of last year Brian Garfield tested and recorded w/a G-Tech back to back runs with 17" S-lites w/Run flats to 16" light Enkei's w/Falken Azenis.

His results are Here

What was suprising was that the improvement was greater on the 60-0 times, i.e, the braking than on the 0-60 times.

Don't forget it is possible to go with aftermarket light weight 17" wheels -- as well as light weight 15" wheels.

 

>>Back in July of last year Brian Garfield tested and recorded w/a G-Tech back to back runs with 17" S-lites w/Run flats to 16" light Enkei's w/Falken Azenis.
>>
>>His results are Here

Thanks for the link. Interesting.

>>What was suprising was that the improvement was greater on the 60-0 times, i.e, the braking than on the 0-60 times.

This is not surprising. Given a speed there is mass in motion and a certain force needed to stop it. Less force and quicker stops can be obtained if the wheels are lighter provided you maintain traction and not go into a skid- usually ABS helps with that. So from a safety standpoint if you can stop faster and maintain control of your car then the lighter wheels do double good.

Also it is not surprising to note that the benefits of lighter wheels are to be had at speeds from 0-70+ rather than at top speeds due to wind resistance which gets to be a limiting factor above 120 mph. It is in acceleration/deceleration that those light wheels are helping.

Just get a bicycle with very heavy wheels like a kids bike and compare acceleration with a racing or triathlon bike with very light rims/tires. Naturally the racing bike is also very light but you are the engine and you can tell right away that less effort is needed to go fast with the light wheels.
>>
>>Don't forget it is possible to go with aftermarket light weight 17" wheels -- as well as light weight 15" wheels.
The problem is that with a limited budget you are more likely to find cheaper lighter 15 or 16" wheels than lighter 17" wheels and tires. It's not that it isn't possible. Also with the lightness comes the worry of durability with the 17" wheel and given the cost that can become a concern when roads are not smooth or if road debris is common.