Gear ratios

[MWR]

hi to all drag racer's and fan's
i don't drag race myself but do compete in autograss racing and i'am a huge john force fan.
anyway i do have a question about how you work out your gear ratio's.
is there a math's formula that i can use to work out the distace it would take to reach maxium rpm in any one gear.

 

[obi_waynne]

It is pretty complex as you need to take into account the
gear ratio
final drive ratio
wheel/tyre size

When you are looking at distance to get max RPM you will then need to take into account the weight and power of the car and factor in transmission losses.

Perfect setups will invlove a rolling road session to highlight the power band and then you calculate your cog sizes for speed/rpm to coincide with your power band.

I did see a downloadable ratio calculator but although it is complex it is unlikely to do everything you want.

I built a quarter mile time calculator in the Tools section and this is pretty complex so I can only imagine the maths involved in gear ratio's.

If someone can come up with a formula I should be able to build this into a calculator for you

 

[MWR]

cheers wayne
i do have the formula to work out maxium speed in any gear that needs alot of different factors and also the formula for the drop in rpm between gear changes just thought some one might have this formula to hand.
it does sound like it is more complicated to work out than the others though.

 

[obi_waynne]

Post up what you've got and I'll see what I can knock up. Its always easier to have a calculator rather than use paper/pencil to work this sort of thing out.

 

[MWR]

right wayne these are the two formulas that might be a help to some of the motorsport guy's or just someone that wants to work out there gear ratio's


GEAR RATIO FORMULA

MPH = (RPM x Tyre diameter (in inches) / (Drive Ratio x 336)
where drive ratio = (ring & pinion ratio) x (Gear ratio)

For example,with a tyre diameter 0f 24 inches,a final drive ratio of 3.38
and a top gear of 0.82.the road speed at 4000 rpm would be:

MPH = (4000 x 24) / (0.89 x 3.88 x 336 )

= 96000 / 1160.2752 = 82.7.

not the easiest thing to workout but can be very helpful to workout top gear for drag racing or just working out certain gears in your rally box.
hope this benifits someone on here it has certainly benifitted me in autograss racing.
i will try and find the other formula the works out the rpm drop between gears i have got it somewhere.

 

[pgarner]

:blink::blink: think al leave it to the pros to work out

 

[obi_waynne]

Great stuff I should be able to build it into a calculator and I can probably work this from tyre size rather than diameter so that will make it easier.