jasonius

iTrader: (1)

I know what unsprung weight is (basically any part of the car below the spring- wheels/tyres/hubs/brakes etc..) however, does reducing weight in these area's have a significant effect on performance, more so than sprung weight..?

What I'm getting at is this, would a lightweight wheel such as OZ super legs or super 'T's improve acceleration, handling and braking..?

I weighed a wheel and tyre on my 05 wrx and was shocked that it weighed nearly 40lbs (18kgs)..!!! The new STi wheels and tyres are over 48lbs (22kgs)..!!! HEAVY

Cheers,

Jason

Fat Boy

The drop in weight alone would help performance obviously, but lowering unsprung weight helps ride/handling at a ratio of something like 1.5 or twice any equivalent reduction in sprung weight. I put a bigger brake kit on my car which was heavier than the rubbish 4 pots and then brought it all back the right way and more with Superlegs

AvalancheS8

The major benefit of lowering unsprung weight is that it improves the ability of the wheels to follow the ground and therefore improves the performance of the suspension. Having the unsprung weight low allows a better compromise between ride and handling and simplifies the setting up of the damping, since the requirement to use the dampers to control the wheel movements is greatly reduced, so they can be tuned more specifically towards body control.

Specifically reducing the weight of the wheels can have a dramatic effect on performance, handling and ride. Weight in the wheels has a double effect on acceleration, since it not only has to be accelerated linerally, along with the rest of the car, but also rotationally, and the majority of the weight of the wheels is in the very worst place, right at the outside. Effectively having lighter wheels is both reducing the weight of the car and the flywheel effect that resists acceleration.

The lighter wheels will also have a greatly reduced gyroscopic effect, which will sharpen turn-in and have less of a smothering effect on feedback through the steering wheel.

Fat Boy

Yeah , what he said

superstring

This is a timely discussion for me as I'm thinking of "upgrading" my wheels and tyres on my Classic from the stock 16" size to 17". My main motivation is performance (and second, of course, is looks ).

It's been my understanding that 17" rims with lower profile tyres will sharpen the turn in and provide better grip. Witness the RB5, whose only suspension mod was 17" wheels and tyres. Prodrive's Mike Wood is on record as saying this upgrade on a Classic will produce more performance benefit than changing the suspension!

But, for sure, the 17s will have more unsprung weight than the 16s and according to AvalancheS8

If true it would appear I could end up with the opposite of what I'm looking for!

Any further comments, practical experience? Cheers!

ScoobySport (SdB)

Great topic

It is the ratio of sprung vs unsprung weight which is important, not the actual figure itself, as it is (as stated) the ability of the suspension to accelerate the wheels which improves road holding, and ride quality.

Regarding 17" wheels being heavier. Firstly, check the weights, as bigger doesn't necessarily mean heavier, but also the weight of the tyre is often forgotten, and the fact there is less sidewall can reduce weight. I say "can" as the sidewalls *may* be thicker on some lower profile tyres which could counter this benefit.

Only way to be sure is to check the weight of both the wheels and tyres in combination.

Make no mistake though.. increasing wheel size at the expense of keeping unsprung weight low and you will regret it in road holding / handling.

Cheers

Simon

mark_my96

I always thought it similar to the gyroscopic effect- when i was 10 i owned a gryoscope and it could be plainly felt that once spun , clear resistance to turning it was felt, So back to reality, what you experince with a heavier wheel or brake calipers is excess moment of inertia and therefore greater gyroscopic effect.

Therefore Keeping the wheel assembly lighter allows the suspension to follow bumps and humps and more importantly easier turn in!- Less laws of physics to overcome.

jasonius

iTrader: (1)

Cheers everyone..!


I've always been keen on keeping the weight down and sort of knew the importance of unsprung weight but needed clarification. I thank you.

Could we start a survey of wheel/tyre weights..? Now that would prove very informative..!

I know Super leggs (wheel only) weigh 6kgs in a 17" x 7.5j (i think)
Jason

superstring

http://www.wheelweights.net/

911

For what it is worth:

Love threads like this!

I hill climb a 1997 Sti, and have raced on P1 wheels @17'' and stock 16's both fitted with the same make/type of tyre (Kumho v70A's)

For the road, the car is great on both sizes, the 17's are nicer as the car looks so much better (sorry)

However, on the track (hill climbing is 2 x more twisty than your favorite country lane)
the 16's win hands down, so much quicker to change direction, better braking and the car sits lower so looks good (2 x sorry)

I have raced this cat this way for 3 years and drive it on the roads 3000 miles a year to qualify my opinion.

Hope that might help.

If you use the car for the road and love it, I'd get some (light) 17's. Like so:



And 16's like so:



911

Engineer@Uni

As always interesting posts attract knowledgeable people. 911 and AvalancheS8 both posted wisely, as did all you others.

I'd like to point out that as far as the Gyroscopic effect is concerned, wheel weight isnt as important as moment of inertia. Think of this: Two wheels, both the same weight, one larger with all the weight at the periphery and thin spines radiating from the hub, the other smaller but the thick spines make it more center-heavy. The losses in energy by spinning both are clearly higher in the larger wheel, although they both weigh the same. Using this as a guide, and estimating that high-performance wheels all, roughly, have similar weights, the larger wheels would be disadvantageous because most of their mass (tyre and rim) are further away from the center. This explains what 911 discovered in his hill-climbing antics (lucky so-and-so).

911

Oh, and racing slicks (crossply) are about 60% the weight of a road tyre too!
911

jasonius

iTrader: (1)

Thanks again everyone,

Glad to have insitigated such an intersting thread..!

This is SN at it's best (especially with no pslewis..!!!)

Cheers,
Jason

superstring

This is great! 911 and Engineer@Uni, your input is EXACTLY what *I* was looking for!

In case anyone's interested, to the best of my knowledge, the stock 5 spoke wheels on 911's car weigh 17 lbs. and the P1s weigh 20.4 lbs. (Don't know about the Kumhos). And, of course, as Engineer@Uni has pointed out, the distribution of mass is different.

Oh, and BTW 911, nice car!

911

Thanks for the kind words on the Sti, but the realy nice car is here:

Owned, rebuilt, hillclimbed, rebuilt (and retired it 3 years ago) since 1988!



Now i should have about a 360/380 bhp engine mapped next week, and I can then press-on with the 911 a bit more.

Lots to do.



911

Engineer@Uni

Wow I'm really flattered... Sniff-sniff-sob... So sweet... Thanks!

jasonius

iTrader: (1)

It would appear that there's a massive difference in the weight of tyres..!

The 5x (split) spoke wheels and tyre (almost new) on my 05 wrx weighs in at just under 40 lbs. According to the web page http://www.wheelweights.net/ the wheel only weights 16 lbs meaning the tyre (bridgestone re050 215/45 17) weights 24 lbs..!!! That seems a bit high to me.

As pointed out by Engineer@Uni the weight at the periphery is more of a problem, do tyre manufactures state the weight of their tyres..?

Keep it coming,

Jason

superstring

Jason, just for comparison, Toyo gives the weight of a 215/45-17 T1-S as 20.1 lbs., so 24 lbs. for the Bridgestone doesn't seem too out of line.

911

That's just the point I'm making about the slicks, flyweight in comparison.
The road tyre has to be so tough to take the curbing, potholes and suchlike it has to be re-enforced so much = weight.

i think split rims are that much heavier too.
Rims are split in design to save money on tooling the wheels for casting with different off-sets.

My old 911 has 15'' rims and So2 pp tyres and are so light compared to the 16'' Sti wheels, and the P1's x 17'' are that much more weight again.

We haven't considered the weight gain with AP 4/6/8 pot calipers yet!

Graham.

jasonius

iTrader: (1)

Ah, it does seem about right. It's hard to believe the tyre is so much heavier than the wheel..!

Where do you get the tyre weight from for the toyo's..?

Thanks again everyone,

Jason

superstring

http://www.toyo.com/

The site seems to be down at the moment, but, when it's working again, choose your tire and then "get the specs".

jasonius

iTrader: (1)

Thanks

AvalancheS8

That's possibly slightly less of a problem, since although the calipers are unsprung weight, they at least aren't also rotating, so they are a little less important than the wheels. The other thing to bear in mind here is that bigger, better quality brakes are not necessarily heavier. On my own '97 car I went from the stock Subaru 2-pots and discs to a set of 4-pot Brembos with 304mm, 3-piece discs. They look far bigger and chunkier, but each wheel assembly is lighter, quite noticably, than the stock ones. I guess the improvement comes from having the Alu bells on the discs and the alu calipers.

Engineer@Uni makes an interesting point about the weight distribution of the wheel, and the effect that this will have on moment of inertia once it is spinning. If you could get the info, this would be another thing to look at with regard to wheels, as there could be significant differences between 2 wheels of equal overall weight. I'd be surprised if you could get this info though, you'd really need to see a full set of engineering drawings for each wheel to see where the material is.

911

Good points.

A pressure die cast wheel, ie the Prodrive P1 wheel weighs a lot to enable enough alloy to tolerate the pot holes of UK roads etc, but forged alloy wheels are a lot lighter and same (better?) strength to survive the impacts.

I think some of the latest high quality wheels are noe 'Squeeze pressed'

ie pressure die cast, then high pressure squeezed in dies to compress the casting.
Sure I've read that in the tech blurb in the Team Dynamics wheel spec.

Another thought, is this why Sti/RA lower front wish-bones are alloy, or is it simply for strength over the steel fabricated arms?(they are part of unsprung weight ?)
Porcshe 911 rear suspension arms were steel, and were changed to alloy to save weight, improve strength, lower cost.

911

AvalancheS8

I'd guess it's for the (un-sprung) weight savings. However, I've not seen the 2 equivalent parts side by side, if I had I could make a judgement on what the relative strength / stiffnesses will be like. Size for size, aluminium alloys are about 1/3 the stiffness of steels, but about 1/3 the weight, the strength can be more comparable. So, replicating the steel part exactly in alu would make it 1/3 the weight, but also 1/3 the stiffness. However, the light weight of alu makes it possible to get the stiffness and the strength higher than they would be for steel parts at the same weight, or the same strength and stiffness but lighter.