Rotational Weight?

[acidrainy]

Once upon a time, in a hexus forum far far away... There lived a little thread (probably started by Zak33!) Talking about the significance of the weight of wheels as to performance. Much math was sloshed around this thread which explained the multiplications of impact that each pound (or kilo) added to your wheels.


Anyway, enough of the silliness. On my car I have factory standard 18" alloys with 225/40's on them. Apart from wanting to replace my tires (so some lovely Toyo's me thinks) these wheels are damn heavy. I've not weighed them, but when I had them off the car they were very cumbersome. Thus, I was thinking ahaa! A thread on Hexus told me that lighter wheels have a performance impact. As I'm trying to justify a nice set of O.Z.'s I'm wondering what things I can expect? I wouldn't be surprised if I am looking at a reduction of about 5-8kg's a corner - perhaps more (I will get the scales out when I'm home! . I've heard of sharper acceleration, improved handling and better fuel economy are all on the cards. Is this the case?


Thanks,


rainy.

[Moby-Dick]

It should probably help due to the reduction in unsprung mass ( allegedly worth several orders of magnitude of weight saving sprung mass )
worth checking the weights as not all after market wheels are as light as you'd expect.

of course smaller wheels would be even lighter provided they clear your brake discs
and finally make sure you get a good offset.

[acidrainy]

I'm looking to replace my TT's 9spoke (RS4) alloys which I suspect are close to 20kg a wheel with O.Z Racing Ultraleggera that are ~8.2kg a wheel.

Also, since posting I've found out that my wheels on the car now are replicas. The previous owner must of put these on to replace the factory ones, presumably due to scuffs etc.

The offset, that is printed on the inside of my current wheels are 35. Although no metric is shown, I presume this is mm.

Just to add to the mix. The brakes that I have on the car currently could take a 17" wheel. But the brakes I'm going to be adding need an 18".

[Zak33]

Wheel size: a tricky issue.

Fr the last few years, wheels have only increased in size really cos they "look" cool, something I'm not convinced of.

The REAL reason to increase wheel size are as follows:

#Get bigger brakes in /alow better airflow to brakes
#Allow the wheel/tyre to skip over small holes in the road wherer a smaller wheel would drop into it, hence alowing for better ride comfort
#Allow the fitment of larger tyres so that they last longer (a small diameter tyre doing 10,000 miles turns a lot more than a large diameter tyre)
#Allow the fitment of lower profile tyres to a car without changing the speedo reading or final drive ratio of the car, hopefully allowing higher lateral cornering force through lack of tyre wall flex


BUT they often weigh more.

Discuss

[hitman67]

The offset, that is printed on the inside of my current wheels are 35. Although no metric is shown, I presume this is mm.

Measurment known as ET that has some fancy German name I forgot.

[acidrainy]

I'm not really looking to increase the size of my wheels, just not decrease them. I am thinking about reducing the weight of them though. I may consider a 19", but only if I am still saving weight. I believe weight is the major factor here, or are the others Zak mentioned more important?

What I'm trying to do is justify a purchase of new wheels for the car to myself. If a lighter wheel means more mpg, better handling, faster acceloration etc... I would feel more comfortable shelling out my cash!

I do remember a post in the past regarding the impact that weight has sprung & unsprung and I'm sure in that thread it was mentioned that wheel weight plays large factor in the scheme of things.

[Butuz]

What I'm trying to do is justify a purchase of new wheels for the car to myself. If a lighter wheel means more mpg, better handling, faster acceloration etc... I would feel more comfortable shelling out my cash!

Every kg shaved off the car = better acceleration, better braking, better handling, better fuel economy. Rotational mass makes considerably more difference than non rotational mass. Eg saving 5kg on the weight of your wheels or flywheel will make much more difference than saving 5kg on the weight of your seats.

A lighter wheel will mean improvements to all of the above. But as a % figure of improvement in each of the categorys above, I would be suprised if you could even notice the difference.

Lighter wheels are great - but a difference will only really be felt if coupled with a strict and prolonged weight saving regieme on the whole car.

Butuz

[Konan555]

#Get bigger brakes in /alow better airflow to brakes
.............
Discuss

Or... you could keep your existing wheels and introduce Mr Grinder in a few choice spots

[dkmech]

Him and Mr Voided Insurance. They get along swimmingly

[Konan555]

Rubbish

Tis chavs that forget to declare such things.

[schmunk]

Or... you could keep your existing wheels and introduce Mr Grinder in a few choice spots

Remove every third spoke. *




* Do not try this on 3-spokes. Also, do not try 3-spokes.

[Zak33]

I may consider a 19", but only if I am still saving weight. I believe weight is the major factor here, or are the others Zak mentioned more important?

I didn't go into weights Rainy, only size increase

Weight is a WHOLE other thing.

Here...let me show you. Imagine the muscles in your arms are shock absorbers.... hold your arm out, elbow straight in front of you, and move your arm up anddown really fast, using JUST your shoulder to do it...dont bend at the elbow.

Think about how easy it is to control and to suddenly change direction. That is how a production car's wheel tyre hub and brakes are controlled on a car.

Now....increase the weight in your hand.....hold a heavy book or a bag of sugar. Now, arm out straight, elbow locked.....do the same sudden up and down and up movement, radom...and see if you can control it properly.
Repeat with more weight....it gets impossible to control...you start the thing moving down and it takes massive effort to even slow it, let alone reverse the travel.

Now...go drive along a road, and watch the wheels of people's cars going up and down, thousands of times per minute.
Those shock absorbers (and springs) have to do that over every ridge and little hole.

Now....tell me why ANYONE would upgrade their alloys and tyres to a huge heavy set without doing major suspension changes?
And we're not even oto WHEEL BEARING stress!

My Astra Rally car has standards 14" alloys on, and except for the fact that I could do with fractionally bigger wheels/tyres for larger brakes, and cos 15" tyres are now cheaper than 14" I'd not even look at them. I am in fact still sticking with 14" s as I have plenty of tyres!

[MarkyP]

When I was tootling about with Scoobie's, Powerdrive reckoned the best mod to make to an impreza was to upgrade the 16" wheels to 17".

I went from 16" standard jobbies to some super light 17" wheels, and I could certainly tell the difference.

Sharper handling, better braking, more control, better ride as well!!!

[Butuz]

Marky explain how larger wheels and lower profile tyres willl give better braking and better ride??

Butuz

[sleepyhead]

Perhaps Marky had runflats on and the "low profiles" are normal style tires? Runflats weren't the most comfortable ride and, even though it may be a placebo effect, braking improved as the normal "low profiles" gave better feedback and therefore confidence.

Just a suggestion.

[dave87]

Either that, or due to the increased size of the wheel the tyres could have a stiffer sidewall - increasing roadholding and resistance to flex - ergo better handling.

Haven't got a clue on the better braking side, unless it allowed larger discs to be run?