7 replies to this topic

[Dusky]

Hi

Looking through old stuff I found the old gbox spec sheet from my dads mini.
It has a jack knight sccr box, my dad wrote "1.087" ratio next to it, tho that might be the drop gear ratio?
And it has a 4.9 fdr. Id like to calc the top speed in 4th gear @8.5 k rpm, but these ratios arent featured in guessworks calc and when I tried 50 ish km/h was the top speed, Wich cant be correct ( couldnt win championchips with that!)
Cheers

[nicklouse]

generally the 4th gear is a 1 to 1 of the input speed on most Mini Boxes (commonly called direct drive)


then you have the reduction of the FD ratio and then there is the effect of the drop gear ratio to consider for the input RPM which is unknown but could be a 1:1.1 as that was a common ratio.

[Cooperman]

Dusty, with a 3.76 FDR you get 14.7 mph/1000 rpm in 4th

So the calculation is:

14.7 × 3.76 ÷ 4.9 = 11.28 mph/1000 rpm.

With a 1.087 drop gear ratio, assuming it is set to reduce the overall gearing:

11.28 ÷ 1.078 × 8500 RPM = 88.2 MPH

I hope this helps.

[nicklouse]

I am guessing that is with a 165/70-10? but the tyre and wheel combo is not known?

guessing slicks? 20" or 21" diameter?

like I said there is not really enough info to give speed.

[Cooperman]

I could only use a known figure as a start point. With a 10", 12" or 13" running normal road tyres that figure will be correct to +/- 4%. So the speed at 8500 in top will be 84 and 92 mph.

 

Dusty, if you let me know what wheel/tyre combination you are running and what the rolling radius is I can get closer with a figure.

[Dusky]

Hi! Thanks for the Quick replies, running 13 inch avon slicks. The numbers on them are 6.0/19.5-13 and 4902 WTIG

Cheers

[nicklouse]

19.5 is the diameter in Inches.

so 165/70-10 circumference 1523.7mm

19.5" slick circumference 1556mm

[Cooperman]

Thanks for the numbers Nick

So in round figures that means the speed increases by 2% giving a new theoretical max of almost exactly 90 mph at 8500 rpm