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Discussion Starter #1
Hi people

I just came a crossan interesting thread on another forum. I could not understand alot of it - but basically it is the question of...

If I have alloys that are a total of lets say 20kg lighter than my originals - will there be any difference to the accelleration response in the car?

The assumption being youare swapping between exactly the same sizes.

Is there a correlation between the crank and flywheel vs the weight of the alloys at a MATERIAL or detectable level by the driver?

If not humanly detectable - will the difference be detected in performance times on lets say a lap of Hockenheim?

Regrds
Jahangeer
 

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Hi ,

Two points :

1) If you car is 20Kg lighter than before, no matter where the weight has come off, this will give you a higher power to weight ratio, giving you (in theory) faster acceleration.

2) Reducing the upsprung weight will change (should improve, but not necessarily) the handling of the car. By unsprung weight I mean everything on the car whose weight is not carried by the suspension, including : wheels & tyres, hub & brake assemblies, rear axle etc.

There is much discussion on the benefits of reducing unsprung weight. Some argue that reducing the unspring weight by 1Kg will yield the same benefit as reducing the sprung weight by 10Kg, while others say this figure may only be around 4Kg, but all say 'its a good thing'.

My point of view is physics is physics, and my point 1 applies. However, the 'response' of the suspension will be improved, simply because each spring/shock has less mass to get back under control when, for example, the wheel hits a bump, and bounces up towards the car.
 

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Reducing any "unsprung" weight is an excellent idea.

Here's an excerpt from the Chris Longhurst Suspension Bible:



Sprung vs. Unsprung Weight



Simply put, sprung weight is everything from the springs up, and unsprung weight is everything from the springs down. Wheels, shock absorbers, springs, knuckle joints and tyres contribute to the unsprung weight. The car, engine, fluids, you, your passenger, the kids, the bags of candy and the portable Playstation all contribute to the sprung weight. Reducing unsprung weight is the key to increasing performance of the car. If you can make the wheels, tyres and swingarms lighter, then the suspension will spend more time compensating for bumps in the road, and less time compensating for the mass of the wheels etc. The greater the unsprung weight, the greater the inertia of the suspension, which will be unable to respond as quickly to rapid changes in the road surface.
As an added benefit, putting lighter wheels on the car can increase your engine's apparent power. Why? Well the engine has to turn the gearbox and driveshafts, and at the end of that, the wheels and tyres. Heavier wheels and tyres require more torque to get turning, which saps engine power. Lighter wheels and tyres allow more of the engine's torque to go into getting you going than spinning the wheels. That's why sports cars have carbon fibre driveshafts and ultra light alloy wheels.




his sites can all be found here..

http://www.chris-longhurst.com/carbibles

hope that helps

cheers
dmc
 

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It's not the extra/lesser weight that is being carried by the car that is important, it's the extra forces required to spin that weight on the hub.

Imagine a shot putter at the olympics - he spins a 5kg ball around but it takes a good bit of effort to get it going, and eventually he is spinning it at his usual rate before releasing - i.e. he accelerates it up to his release speed.

He swaps to a 2kg ball and it's as if there's nothing there and he reaches his release speed twice as quickly - i.e. he accelerates more quickly.

He swaps to a 10kg ball and can't even get up to his release speed because his grip is not strong enough to take the extra strain.

Hence, bigger wheels are usually worse for a car's performance than small ones because even if the material is the same, the larger wheel will increase the rotational mass the engine has to drive.
 

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Discussion Starter #5
Mmmfive, Phil and dmc Many thanks and great answers from all, I will look at the link you sent

This forum has an excellent pool of knowledge that would put most companies to shame.

Also the reason I ask is I got a great deal on a set of genuine BMW E39 M5 wheels which are 18's as opposed to the current E34's throwing stars which off-course are 17's.

I will need to swallow this disadvantage. A case of form over substance.

But I wonder how much the E39 wheels (at the rear 275/35 18) weigh and how much my current (255/40 17) weigh?

Does anyone know or should I write a post on the E39 M5 forum?
 

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If you've got them, why don't you just weigh them? Get the bathroom scales out, but don't let your missus see!! Hee Hee
 

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Discussion Starter #7
ha ha - she's away for a few days.

Have irritated her before by cleaning the alloys in the bath tub!!

AndyE, I have sold my summer tires (the original throwing stars), and have not got the E39 M5 wheels yet, but I will ask a freind to weigh the throwing stars and will off-course weigh the new ones. Problem is I am impatient and very curious (not a good combo).

I might go to BMW in Karlstad and weigh the ones they have in the workshop. If they weigh too much - I may get a set of BBS (ouch!).

Does anyone know who make the E39 M5 wheels?
 

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Don't forget that even if the wheels weigh the same they are bigger, and a higher proportion of their mass may be at a wider point on the wheel than with standard 17s. Think of a 1kg weight at the end of a 1m pole and then imagine the same weight at the end of a 2m pole - it's effectively double the weight. The wheels won't have this much difference, but it all helps.

17" Turbines weigh 26lbs, Throwing Stars weigh 25lbs, M-Parallels weigh ??lbs!

BBS wheels aren't necessarily lighter either - based on 18x8.5 wheels...

RK
(17.5lbs)

RC
(18.3lbs)

RGR
(19.4lbs)

LM
(22lbs - light for a 3-piece wheel)

...are relatively light however the ...

RK2
(24.3lbs)

RA
(25.6lbs)

RX
(25.5lbs)

...are relatively heavy.

The very light-looking
Breyton Imagine wheels are actually over 30lbs!

Compomotive do 18" wheels for motorsport which are very strong, but also 24lbs!.

The lightest I could find were Volk TE37 Mag
at 12lbs - but they are forged magnesium!

Try www.tirerack.com for wheel weights.
 

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phil.cavanagh said:
I have Magnesium split rims on the Westfiled, and these weight 7.48lbs each (only 13" though...!) :1:
They would have been lighter if you had gone for one-piece rims as they tend to be slightly lighter due to not having titanium connecting bolts everywhere - again, not a lot of weight difference though, probably about 2-3oz!
 

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Discussion Starter #11
Mmm-Five said:
They would have been lighter if you had gone for one-piece rims as they tend to be slightly lighter due to not having titanium connecting bolts everywhere - again, not a lot of weight difference though, probably about 2-3oz!
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Well - now I feel like I owe it to myself to get a light set of BBS, but the problem is I want 265 or 275 's at the back. So I'm limited in choice a little compared to the choices available for 235's.

I would go for the Lemans but they cost too much. I would ge tthe BBS RK but they do not have 9 or 10's for the back.

I love the Volk's but are they not for Japanese cars??
 

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Discussion Starter #12
Jahangeer said:
--------------

Well - now I feel like I owe it to myself to get a light set of BBS, but the problem is I want 265 or 275 's at the back. So I'm limited in choice a little compared to the choices available for 235's.

I would go for the Lemans but they cost too much. I would ge tthe BBS RK but they do not have 9 or 10's for the back.

I love the Volk's but are they not for Japanese cars??
--------------------

What is the maximum size tire width on a set of BBS RXii 8,5j?
 

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Jahangeer said:
--------------------

What is the maximum size tire width on a set of BBS RXii 8,5j?
Well, the 8" front wheel on my M5 has a 245 tyre on it, so I can't see a 8.5" wheel not being able to fit a 265 tyre!
 
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