jonc

I realise many of you who post here are technically knowledgeble when it comes to all things cars. So I have a question.

What affect does having lighter weight alloy wheels have on the performance of a car? For example, if my standard alloys weigh 17KG per wheel including tyre, what effect will changing them to an aftermarket alloy of the same size weighing 10KG per wheel including tyre?

As there is a total of 28KG difference does this increase the power to weight ratio, thereby giving me better accelleration and decelleration, with the flip side being the engine having to work harder to get and maintain maximum speed due to decrease in rotational inertia. ie. a heavy wheel will spin longer unassisted than a light wheel and therefore a lighter wheel requires more energy to keep it spinning.

I realise that lighter wheel means less unsprung weight which generally means giving a better ride, but, apart from asthetics are there any other *real* noticable benefits to fitting lighter wheels?

ps. appologies for the long winded question.

Paul_M

Acceleration and deceleration should be improved slightly as it's easier to change the rotational speed of an object with lower inertia. Plus the overall power-to weight ratio of the car is improved.

Also the suspension should be able to react faster as there's less linear mass to control keeping the wheels in contact with the road better, although it probably depends on what unsprung weight the suspension setup is optimised around.

I have lightweight 16" Rays engineering forged wheels (around 6Kg per corner) on my Impreza along with alloy wishbones and it certainly seems to perform very well round fast sweeping bends with bumps in them compared to other cars I've driven which felt more unsettled and skipped out more. How much this is down to the low unsprung mass I have no idea as I've never driven a standard Impreza to compare.

RS Grant

I was gonna say exactly the same as whats written above, honest!!

Cheers,
Grant

jonc

Will 7kg difference per wheel have any noticeable effects? Has anyone who changed to lighter wheels noticed increases in acceleration and ride quality?

TopBanana

7kg is a huge reduction in unsprung mass at each corner

TopBanana

You won't notice any increase in acceleration by the way

TaviaRS

IIRC every 5kg lost from the car is the equivalent of a 1hp gain. With unsprung weight this is doubled, so almost the equivalent of gaining 10hp, but at what cost?

It should also improve the suspension and handling responses. Be aware that you can go to far and upset the balance if you go outside the design balance between sprung and unsprung weight.

Dracoro

OK, let me do some maths but I need some figures.

Weight of car
Weight of old wheels
Weight of new wheels
Car's BHP.

mik

I'd agree you'll notice no change in performance.

Only advantage is unsprung weight reduction ~ if you've ever driven a big 4x4 you'll probably appreciate why this is important - huge weight from chassis / transmission & wheels that isn't controlled by springs / dampers so you get a big "aftershock" & shuddering following suspension movement.

Go drive a Chevrolet Blazer to bring this to stark reality

As per TaviaRS - if you go too far you'll upset the balance - ideally you'd want to retune suspension (particularly rebound damping rate) to suit lighter wheels.

And 7kg would be a massive amount to lose. I'd be surprised if you save more than 2-3kg per corner.

[Edited by mik - 8/22/2003 1:39:23 PM]

S.B.

When you get a puncture not so heavy to lift into the boot.

mik

And courier costs for delivery would be lower

Roojai

I not sure if this is the exact figure but I believe losing 1 kg at the wheel is equivalent to losing something like 4 kg of weight in the car.

Roojai

sorry - wont let me edit message.

Just wanted to add:

losing 28 kg from your wheels might be equivalent to losing 100 kg off the car which is significant.

Also, people tend to use aftermarket alloys and tyres with a larger radius than standard, effectively giving taller gearing. This makes quite a difference to acceleration.

One test I saw resulted in a 0.6 sec decrease in the time to 60 mph by removing the after market 19 inchers and replacing with standard 15's.

SCOSaltire

how would the gearing be changed?
Surely its the whole radius - including the tyre.
19" with lower profile tyre to make same as 16" with higher profile tyre?

perhaps the test showed the difference in having to rotate x amount of Kg compared to x+y amount of Kg?

mik

SCOCsaltire ~ don't know the test but I expect it was to highlight to muppets who don't retain the same rolling radius that this isn't actually a great idea

jonc

I don't understand how 28KG loss can equate to 100KG. Surely 28KG reduction is 28KG reduction?

mik

It's a bit like lightening your flywheel ~ less rotational inertia.

Whilst accelerating, most of the energy goes into pushing the car forwards. Some is "eaten" by the effort required to accelerate the wheels from "x" rpm to "y" rpm.

Lower wheel weight means more is left to punch the car forwards.

No idea whether the 1:4 ratio uoted above is true though, and VERY unlikely you'd lose 28kg via lighter wheels.

diver944

Simple test to feel the effects of lighter wheels:

Hold a pencil horizontally between your thumb and forefinger. Waggle it up and down, this simulates your wishbone going up and down with lightweight wheels on.

Now do the same but with a rubber pushed on the end of the pencil.

See how easier it is for your suspension to cope with bumps and dips in the road?

Gompo

Regarding the 1:4 thing.

Although I dont have any proof that it does make a difference, when I was into MTB's there was often a mention that saving weight on your wheels would be the equivalent of saving x3 that weight on none-rolling mass.
Obviously a 1:3 ratio instead, but still better than a 1:1..

Gomp

Roojai

SCOSaltire, you are right, it is the whole radius including the tire that effects the gearing. It's just that very often (but not always) people put on wheels and tyres with a larger radius than standard as they fill the arches better. This makes the gearing taller, slowing down acceleration but increasing max speed.

Roojai

Really not sure about the ratio of wheel mass to effective mass but probably not as high as I quoted before. I have read recently that 1kg extra on the wheels is equivalent to 1.5 kg or 2kg on the body.
Take a kilo of your flywheel though and it's equivalent to taking ~45 kilos off your body mass (in first gear, ~ 20 kilos in second gear, down to a few kilos in fifth).

skiddus_markus

I'd imagine it's simliar to losing weight off the crank,just to a lesser degree.For every kg lost off the crank you gain PAW(IIRC 4-5bhp).That's not to say you're engine becomes any more powerful,just that you lose less,thus increasing the actual useable power.
I don't think it would be that hard to lose 28kg off the wheels.You can save quite a few kg changing your tyres.Those with stiffer sidewalls and more tread are significantly heavier than those without.I compared my S02's to some cheapo Uniroyals and the uniroyals weighed around 2kg less each.Using the above mentioned Rays wheels as an example-I think they are actually lighter than 6kg,more like 4 or 5kg.If your standard wheel weighs 10-15kg,then you can save 4-10kg per wheel just from these.Add some lighter tyres(not necessarrily desireable due to handling characterictics)and it can add up to 6-12kg per wheel.
Lightweight wheels are VERY expensive though-I've seen 16" ones at over £400 each...on offer.
So you gain in a better power to weight ratio and wheels that will accellerate more readily.All IMO.