13" Does size matter....?

[dogvet]

Many of you claim that 13" wheels are a vast improvement on these cars but how much is subjective?

Presumably altering the overall drive ratio by going from 14" to 13" wheels will allow more of the rev range to be used, but will this give a noticeable increase in acceleration and top speed in top gear?

Overall weight should be reduced particularly unsprung weight,which presumably improves speed of handling?

Or is it only if one drops down in size from 16"

wheels that the difference be noticed?

Thanks in advance for all comments.

 

[Peter Carmichael]

I am sure a lot of it is down to the amount of tyre and the ability for a light car to get tyres on 16in wheels up to temperature. Unsprung mass is important too.

 

There is also the matter of tyre availability, with A032Rs and ACB10s being readily available in these sizes. There is no point in comparing a car on 16 inch wheels and concrete Michelins with a car on 13inch ACB10s.

 

As for gearing, the rev drop will be the same between the same gears, so any extra acceleration will depend on bringing the upper gears into play more, where the ratios are always closer between 4th and 5th than between 1st and 2nd. This is a marginal effect and it will be more significant if you have gearing which has you approaching maximum revs in whichever gear at the end of each significant straight on any given circuit. Any improvement in top speed depends on the car being overgeared to start with.

 

I think it would be quite difficult to claim a definitive and costworthy improvement in dropping from 14inch wheels to 13inch wheels. Remember that if you change something as fundamental as wheel/tyre spec, you have to find out how to optimise the chassis all over again.

 

From recent experience, I think it is clear that the 16inch wheels are very compromised.

[Peter Carmichael]

I am sure a lot of it is down to the amount of tyre and the ability for a light car to get tyres on 16in wheels up to temperature. Unsprung mass is important too.

 

There is also the matter of tyre availability, with A032Rs and ACB10s being readily available in these sizes. There is no point in comparing a car on 16 inch wheels and concrete Michelins with a car on 13inch ACB10s.

 

As for gearing, the rev drop will be the same between the same gears, so any extra acceleration will depend on bringing the upper gears into play more, where the ratios are always closer between 4th and 5th than between 1st and 2nd. This is a marginal effect and it will be more significant if you have gearing which has you approaching maximum revs in whichever gear at the end of each significant straight on any given circuit. Any improvement in top speed depends on the car being overgeared to start with.

 

I think it would be quite difficult to claim a definitive and costworthy improvement in dropping from 14inch wheels to 13inch wheels. Remember that if you change something as fundamental as wheel/tyre spec, you have to find out how to optimise the chassis all over again.

 

From recent experience, I think it is clear that the 16inch wheels are very compromised.

[PACR]

Your acceleration will be much much better - as long as you forget to change the speedo drive....

 

Top speed is tricky - the aerodynamics, power available and gearing all roll together. Depends on whether you are limited by hitting the red-line or not. At top speed in my 1.8 SS 6-speed - about 130 mph indicated I was feeling that the gearing was pretty spot on - not many more revs to go but I was well into the power band to fight the awful aerodynamics. Reducing the gearing would have hit the limiter perhaps - but then again acceleration at the very top end isn't very good and plenty of road (sorry, air-field) is needed so in real world terms I'd say that as long as gearing allows 120 mph say sod it and just worry about acceleration.

 

Piers

[Jason Plato]

I've changed from the standard fit ACB 10 to yokohama AOO8 175 / 50 / 13 ( not R ) on my superlight 1.6 running 13 " wheels . This was mainly to lower the gearing for sprinting and to enable the use of sticky rubber - only available in this size .

 

the gearing whilst perfect for most events this year , has dropped the top speed . Measured with a bicycle speedo on ACB's at 130.40 mph , which was banging off the rev limiter towards Avon rise at Castle Combe . The top speed has fallen to 124.30 mph on the same section of circuit again banging off the limiter ! .

The top speed / limiter is also reached about 200 m sooner than before .

 

Downside = 5500rpm at 85 mph ( true speed ) as I remember ( sat here and not on the motorway ! ). And a drop in ride height of about 20mm , which has its benifits for handling but not my sump ( see seperate posting - sump guard ) .

 

if my project engine comes together this winter then I will have to go to larger wheels / tyres as the limiter will be reached even sooner :-( .

 

Dave

[Peter Carmichael]

Dave, I found your posting somewhat confusing.

 

You changed from ACB10 to Yoko A008 to use stickier rubbersmile.gifquestion.gif

 

Those are big speeds you talk about. I have never found anywhere that lets my car get to the limiter in 6th. 2 mile straight at the Nurburgring (117) was slower than the plunge down the Fuchsrohre (transl: "Foxes Throat" not "Foxhole") (122) and the run to Schwedenkreuz was quicker (124.8). The Lavant straight at Goodwood gets me up to 125.5mph and I guess I won't be going anywhere near Combe's noise limits now!

 

Interesting.

 

[dogvet]

Looks like you have saved me some money Peter!

[Jason Plato]

Peter ,

I changed from ACB's to Yoko's due to the ACB's not being on list 1A for sprinting.

The speed at Coombe is correct . Measured with bycycle speedo .

Trackday yesterday at coombe and the car was bouncing off the limiter in sixth

200 m before Avon rise and all the way over it ! = 123.70 mph .

WITH A PASSENGER ON BOARD !! - fellow 7 driver ; Kevin .

The car runs a 3.62 diff and aero screen if that makes any difference ??

 

I have no idea why you are unable to get your superlight off the limiter !!??

 

Dave