Your power VS tyre width ratio

[Gaspode]

175bhp on 165 tyres = 0.53 bhp/mm

[Gaspode]

papercup said:

Well, the RX7 has 520bhp and 530ft/lb and runs a 255 at the rear.

520bhp / 255 wide tyres = 2.04bhp per MM

530 lb/ft / 255 tyres = 2.08 lb/ft per MM

Its quite lively.

It would be, with only one of your rear wheels providing drive. Do you go round in circles a lot?

Edited by Gaspode on Tuesday 11th December 12:23

[FD3Si]

In with a 478 on 225 at the mo...
1.062

[Streetym89]

I have 90bhp on 225s, 0.2

[LeoSayer]

2010 X5 241bhp / 255 tyres:
0.47 bhp/mm or 0.23 bhp per tonne/mm

1996 993 285bhp / 255 tyres:
0.56 bhp/mm or 0.41 bhp per tonne/mm

Both have the same width tyre. The bhp/tonne measure above feels more realistic. It's at least twice as hard to unstick the X5 as the 993, but then again, the 993 is rwd, he X5 4WD.

I'd have to run the X5 on 135s to get the same ratio as the 993 which means I'll need 3 space savers. I'm willing to give it a go, should be fun.

[LeoSayer]

Do car engineers use these kind of metrics to help decide what is the appropriate tyre size for a new car they are designing?

Or do the just go with what looks best?

[djdestiny]

RS4 on 4x255 @ 420bhp = 0.41

[orangesrule]

current shed:

60/(175x2)=0.17 yes its rather underpowered lolz

[stevesingo]

279hp and 225 tyres(2) 0.62hp/mm

Or 279/1.2ton/(225*2) 0.52hp/ton/mm

[KMud]

Monaros are crazy:

~378.5 bhp /(2x235) = 0.8053
~397.1 lbft /(2x235) = 0.8449

Numbers from last dyno.

[shirt]

shirt said:

jeep on imperial tyres:
190/317.5 = 0.6bhp/mm

not as bad as i thought. Only wish those 190horses weren't so bloody lame!

seems i was doing that wrong.

in 2wd:

190 / (317.5*2) = 0.3 bhp/mm

in 4wd:

190 / (317.5*4) = 0.15 bhp/mm

in other words, woeful.

[mat777]

Series Landy - rwd (on road) on relatively skinny 7.50 tyres:

65ish/ (2*190.5) = 0.17

Disco 2 - more power, but AWD and huge tyres:

136/ (4*255) = 0.13!!!!!!!

[Mr E]

156bhp/ (225*2) wide tyres = 0.347 bhp per MM.

Edit to add, and this is a proper sports car too.

[bennyboysvuk]

I'm going to play on behalf of a friend with a Mustang that's had a few engines in its time, but remained on the original size wheels and rubber. The power/torque figures are conservative estimates:

600bhp / (225*2) wide tyres = 1.33bhp per MM.
550 lb/ft / (225*2) = 1.22 lb/ft per MM.

Edited by bennyboysvuk on Tuesday 11th December 13:15

[Dilligaf10]

300/595*2=.252bhp per mm.

[chris182]

Mercedes 560 SEC: 300/205 = 1.46bhp/mm. On Pirelli P6000s with so much low range grunt the back tyres take rather a beating.

[papercup]

Gaspode said:

papercup said:

Well, the RX7 has 520bhp and 530ft/lb and runs a 255 at the rear.

520bhp / 255 wide tyres = 2.04bhp per MM

530 lb/ft / 255 tyres = 2.08 lb/ft per MM

Its quite lively.

It would be, with only one of your rear wheels providing drive. Do you go round in circles a lot?

Edited by Gaspode on Tuesday 11th December 12:23

Yes, but not because of my maths

[TobyLaRohne]

530bhp/(335mm x 2)= 0.79 bhp/mm

[After_Shock]

Used to have:

650 / 245*2 = 1.32 hp/mm in a Monaro, interesting to say the least.

[Captain Muppet]

LeoSayer said:

Do car engineers use these kind of metrics to help decide what is the appropriate tyre size for a new car they are designing?

Or do the just go with what looks best?

An interview with a ride and handling engineer I read a few years back said that they sized the driven tyres for traction (I assumed with some kind of target in mind) and the other tyres are sized for handling.

All the OEMs have huge teams of ride and handling engineers who will muck about with tyre sizes, compounds and pressures for months before selecting a production tyre.

Even the OEMs that produce bland dreadful cars put a lot of effort in to exactly how bland and dreadful the handling should be.