rim weight

[elementZ]

I was wondering how much of a difference does the weight of a rim affect overall times for autocross. I am comparing a 15" alloy vs 15" steel rim. Does anybody know?

[-bbTeg]

I highly doubt you will notice a difference.

[shanehutton]

It might not be much. I know the factory alloys on my car weigh a ton. Generally unsprung weight has a much greater effect then sprung but if you are only talking 3lbs a corner it might be about the same as if you had 40 - 50lbs extra in the car. (no flames, this is a very loose guess).

[mikewolf]

In general alloys tend to be wider than steelies. If the weights aren't too much different, I would go with which ever one is wider.

[Kreutz73]

I've noticed a difference on the miata.. not sure about other cars. Upsprung weight is hp according to some. a lot of people go to great measures to get every ounce out of their upsrung weight and wheels/tires is a good place for some low lying fruit. brakes is the other.

[Marsh]

I find it quite noticeable. I'm always surprised at the number people that say they can't feel the difference. The 18" alloys on my daily driver are so much heavier than stock it takes an additional 500rpm to pull away from a stop without stalling.

[shanehutton]

That is true. The switch between my 17 and 16 wheels is noticeable. If they are indeed a lot lighter then it should make a difference. I just know that my factory alloys are not a lot lighter than my steels.

[Travis]

There's a 1 lb difference in my 16x6.5" vs my 17x8", and the difference is subtle but noticeable when accelerating, but I believe it's because most of the mass is further from the center of rotation rather than just the increase in weight. I haven't factored in the weight of the tires, but their section widths are the same.

I'd buy used alloys before buying new steelies, but I'm not sure if used is an option for you.

[elementZ]

If I am going to buy rims, then for sure I will get alloys. I just suggested the steelies because that is what I have sitting in my shed, and they are not being used. My winters are on 14" alloys and summers on 17" alloys. I could break down the winter tires and run those rims (which are 12 lbs each), but tire options are limited for 14". The 205/50R15s are everywhere.

[-bbTeg]

There's a 1 lb difference in my 16x6.5" vs my 17x8", and the difference is subtle but noticeable when accelerating, but I believe it's because most of the mass is further from the center of rotation rather than just the increase in weight. I haven't factored in the weight of the tires, but their section widths are the same.

I'd buy used alloys before buying new steelies, but I'm not sure if used is an option for you.

You probably notice a difference because a 16" wheel compared to a 17" wheel will make your gears feel shorter provided it doesn't have the same overall diameter.

I believe the thread starter was referring to a same size steelie and same size alloy.

I very much doubt in a blind test, that with the same tires you would notice a difference on an auto-x.

[scrap master]

For my Volvo the steelies are 19.5 lbs and alloys are 11lbs. Both are factory rims and identical size (14x5.5) and the difference is incredible between the two. With only 100hp the car really comes alive with every pound it sheds.

[AngeloX19]

(Puts flame suit on):) For auto-cross speeds any decrease you'd see in time will come from the fact that the car may be 20-30 lbs lighter, and that the suspension will work better over bumps, and not from any improved acceration. I remember an old magazine that did a comparo on a '69 Vette using oe steel wheels and similarily sized aluminum wheels and they ran it in the 1/4 mile. IIRC the et was only .3 of a seconds faster in the high 90 mph range. Considering how few wheel revolutions there are at 40-60 kmh, engine enertia will have a bigger effect.

[Marsh]

(Puts flame suit on):) For auto-cross speeds any decrease you'd see in time will come from the fact that the car may be 20-30 lbs lighter, and that the suspension will work better over bumps, and not from any improved acceration. I remember an old magazine that did a comparo on a '69 Vette using oe steel wheels and similarily sized aluminum wheels and they ran it in the 1/4 mile. IIRC the et was only .3 of a seconds faster in the high 90 mph range. Considering how few wheel revolutions there are at 40-60 kmh, engine enertia will have a bigger effect.

While true that wheel weight takes more energy at higher speeds, so does aerodynamics and thus wheel inertia as a percentage of the total power used is not neccessarily going to increase with speed (at the very least it certainly won't be linear). To say that wheel weight is unimportant is the same as saying weight is unimportant. Wheels are the single most important form of weight on the car. They're rotating and unsprung. So they negatively effect every possible quantifiable measure of a cars performance, no matter what the speed. Oh, and only 0.3sec in the quarter mile? That sounds pretty darn significant in a quarter mile run. Especially when it's nothing to sneeze at in a 60 second autoslalom run. I would consider 0.5 seconds in an autoslalom by just changing wheels to be astronomical.

[h-bomb]

I notice a difference between my 13lb Kosei and 10 lb OZ's with my Type-R.
Both had the identical tire on them.

Hanif

[shanehutton]

13lbs. 10lbs. Just thought I would mention that I hate you. My wheels are at least 25lbs. :(

[Kreutz73]

(Puts flame suit on):) For auto-cross speeds any decrease you'd see in time will come from the fact that the car may be 20-30 lbs lighter, and that the suspension will work better over bumps, and not from any improved acceration. I remember an old magazine that did a comparo on a '69 Vette using oe steel wheels and similarily sized aluminum wheels and they ran it in the 1/4 mile. IIRC the et was only .3 of a seconds faster in the high 90 mph range. Considering how few wheel revolutions there are at 40-60 kmh, engine enertia will have a bigger effect.

WOW! they are coming out of the wood work for this one! Nice to see you are still kicking around Angelo! Do you have enough brownie points in the vault to make it out to a few this year?

[AngeloX19]

Hey Paul, maybe, with a surprise;) I might not be able to run but I'll definatly swing by now that the car's running again.

I found the article and my age is showing. Boy I got a bunch of facts wrong on this. Grassroots from Jan. 95 did a big wheel feature where they ran 2 hypothetical cars on a simulator. 1st was a modded out 69 Mustang, the other a stock 90 Vette. They factored in car weight, final drive, tire sizes, conditions, etc and changed wheel weights to represent steel wheels, or heavy cast oe wheels for lightweight 3pc wheels. The results for each measured to only 660 ft though was .08 seconds for the mustang and .1 for the vette. Trap speeds were .8 and .9 mph faster. Seem rather minor to me based on the fact that wasn't even a real test. Remember too that this was from a standstill and run to speeds about twice what the average slalom runs. I don't know.

And if you ask what's on my car, yes lighter (but not super light) aluminum wheels, cause I think they look better:)

[TOYSRUS]

Although debatable I have always used the rule of thumb 1 lb of unsprung weight = 6lbs of sprung. Doing similar math you can calcualte theoretical HP gains as well. From seat of the pants experience (including personal research) I concur with Angelo. Losing 8lbs per corner on my 96 Talon completely transformed that car in terms of handling, transitions in particular. UNSPRUNG AND RECIPROCATING MASS IS CRITICAL IN AUTOCROSS, SAVE IT WHERE YOU CAN!:)