The Relationship between BHP and weight??

[Dancing Fish 0]

Hi All,

 

Does anyone know the formula relationship between weight and bhp. Im trying to work out how much my BHP will go up by reducing the weight of my wheels and thus how much it might be worth spending to get really light ones.

 

Where is scrapheap challenge when you need it!

 

DF.

[Kevin Bacon 5]

LOL at Scrapheap challenge!

 

You have to get some pretty exotic wheels to get any significant gains, something like Magnesium split rims, such as the BBS LM funnily enough, my favourite wheel :lol:

 

Out of interest, I weighed a speedline 15" wheel without a tyre on it and I was surprised to find it's 15lb! Heavier than I thought. For comparison, the BBS RC 17" is about 12lb......

 

Compomotive and Speedline do some very light wheels but they can't guarantee them for road use as they're magnesium.

[dr_mat 0]

erm.. your bhp won't go up, per se.

You *will* benefit from reduced transmission losses, which will mean more of the engine's power gets to the road, rather than gets used charging up your alloys with rotational inertia.

It's complicated, very. It's best summarised as: if you're buying new wheels and you don't want to *lose* performance, make sure your new wheels are the same or lighter than your current set. If the new wheels are bigger, then make sure they weigh about (old size)/(new size) or less.

[dinkus 10]

The usual marker is bhp/tonne so you can use a standard point of reference for all cars. So lightening your car you will get a tiny bit more bhp/tonne but it's not going to make a noticable difference.

[LJ 0]

An overall power to weight ratio type thing really.

Car power / (car weight +/- change in wheel weight) 'ish.

Lighter wheels give less unsprung mass which can give better steering feel ...?

 

G60 BBS 6.5J (kerbed and buckled) minus tyre weighed in at 7.4Kg

Passat G60 winter 6J steel minus tyres is 9.4Kg

 

Entertainment factor of ~1996 Astra being attached to a massive twin tailpipe exhaust system that could house homeless families, whilst waiting for my alloys to be swapped for steels. Priceless!

[dinkus 10]

Entertainment factor of ~1996 Astra being attached to a massive twin tailpipe exhaust system that could house homeless families, whilst waiting for my alloys to be swapped for steels. Priceless!

 

Is that with the standard 1.2 engine still fitted? :lol:

[LJ 0]

It was well smart!!

Faded purple colour with black tint windows, and bumpers hanging off at different angles to the rest of the car. Nice :wink: :lol:

[Kevin Bacon 5]

G60 BBS 6.5J (kerbed and buckled) minus tyre weighed in at 7.4Kg

Passat G60 winter 6J steel minus tyres is 9.4Kg

!

 

The Speedlines aren't so heavy after all then if BBS RAs weigh 16.3lbs!

[Den_VR6 0]

erm.. your bhp won't go up, per se.

You *will* benefit from reduced transmission losses, which will mean more of the engine's power gets to the road, rather than gets used charging up your alloys with rotational inertia.

It's complicated, very. It's best summarised as: if you're buying new wheels and you don't want to *lose* performance, make sure your new wheels are the same or lighter than your current set. If the new wheels are bigger, then make sure they weigh about (old size)/(new size) or less.

 

Dr Mat is correct, BHP will not change. IIRC, 1 BHP is the work done to raise a 1 lb weight by 3000ft - ie a measure of power. Lightweight wheels will, i think accellerate faster when torque is applied - but probably the difference wont be noticeable?

[dr_mat 0]

I've heard that it's noticeably slower putting larger (heavier) wheels on, so it should be noticeably quicker putting smaller lighter ones on, but I can't say by how much. It'll make most difference in lower gears. It'll make bu**er all difference in 5th on the motorway... (Apart from costing you more brake pad to slow down!)

[Supercharged 2]

http://www.vwspeedshop.co.uk/14%20mile%20calc.htm

[Guest]

http://www.vwspeedshop.co.uk/14%20mile%20calc.htm

 

That was interesting!

[Henny 0]

they're also very good for buying gauges from... ;) Oh, and if you want a MAD beetle engine building, go talk to 'em 'cos they've built some right nutter engines! 8) (they're just down the road from where I work)

[Roger Blassberg 0]

1 horsepower = 33000 ft. lbs/minute = 746 Watts. That is like carrying about 12 full sized bags of cement, plus your own weight, up one storey flight of steps in one minute. Power is the rate of doing work, so there is a dimension of time involved. If you are reasonably fit, you might be able to work at about one-third of a horsepower but not for very long. So, a VR engine works at nearly 600 times the rate that you do, for hours at a time.

Lighter wheels have the advantage of reducing unsprung weight, so your suspension is more able to control wheel movements, but as others have said here, most road alloys are insignificantly lighter than steel rims and it doesn't make that much difference. Only ultra-light ones will have a perceptible effect, and will be too fragile for the pot-holed farm tracks we call roads. Basically, it's all about appearance.

 

Larger diameter rims will have a greater radius of gyration and polar moment of inertia, so will need more time to accelerate and decelerate, and will have a greater gyroscopic effect on steering.

 

Best wishes

 

RB