Gear ratio calculator
Discussion
dodgepot said:
Can anyone suggest an accurate gear ratio calculator to calculate speed from gearbox ratios and diff ratio
Tried a couple online and got differing results
Thanks
O level maths?Tried a couple online and got differing results
Thanks
(Engine revs per minute times 60 times rolling circumference of driven wheel/tyre in mm) all divided by (gearbox ratio times final drive ratio times 1.609344 times 1000 times 1000 times 0.9941). Will give the mph.
The 0.9941, is a factor I have put in to calculate with a half-worn tyre.
ETA rolling circumference can be calculated here:
http://www.wheelcalc.com/
So for my 335d in top (assuming no torque converter losses) that is
(1000×60×2016)÷(0.667×2.563×1.609344×1000×1000×0.9941) gives 44.2 mph at 1000 rpm
Edited by Pica-Pica on Saturday 8th July 10:36
TRIUMPHBULLET said:
The biggest variable I find is with the diameter of the tyre which changes as speed increases to a degree.
If you have the tyre size to hand measure from the ground to the centre of the wheel rather than take calculator chart size.
...or better still, measure the actual circumference... If you have the tyre size to hand measure from the ground to the centre of the wheel rather than take calculator chart size.
...that can even be done by marking the tyre and moving the car forward one revolution, then it really is the 'rolling circumference'. TwinKam said:
TRIUMPHBULLET said:
The biggest variable I find is with the diameter of the tyre which changes as speed increases to a degree.
If you have the tyre size to hand measure from the ground to the centre of the wheel rather than take calculator chart size.
...or better still, measure the actual circumference... If you have the tyre size to hand measure from the ground to the centre of the wheel rather than take calculator chart size.
...that can even be done by marking the tyre and moving the car forward one revolution, then it really is the 'rolling circumference'. TRIUMPHBULLET said:
The biggest variable I find is with the diameter of the tyre which changes as speed increases
Have you measured that? It seems unlikely that it would be enough to be significant given how stiff the belts are on radial ply tyres.TRIUMPHBULLET said:
If you have the tyre size to hand measure from the ground to the centre of the wheel rather than take calculator chart size.
That is definitely going to introduce significant inaccuracy. The circumference of the tyre is relatively constant regardless of radial deflections. Deflections within the tread blocks will have some small effect, but to a good approximation the circumference of the tyre is constant regardless of the shape of the tyre.GreenV8S said:
That is definitely going to introduce significant inaccuracy. The circumference of the tyre is relatively constant regardless of radial deflections. Deflections within the tread blocks will have some small effect, but to a good approximation the circumference of the tyre is constant regardless of the shape of the tyre.
There is lots of variation.Some 275's are wider than other 275, and likewise sidewalls are not the same.
Some do grow with speed, others less so.
But measuring actual circumference and using that for any calcs will be the best option.
Using diameter or guesses is far more open to differences.
TwinKam said:
Yes, I would like to see some data on the 'growth' of tyres at (road legal) speeds, we're talking standard 'belted' road tyres, not Hoosier Slingshots!
Even 888's would register a higher speed under load on my car which seemed odd as they would be considered pretty stiff all round.However that was the rear tyres, the front's didn't seem to exhibit the same behaviour.
And even more strangely the Michelin PS4's I have on now, the increase in speed under load vs when letting off is lower.
Overall it is a small increase, but it is quite apparent from datalogs. That is unless even when it appears there is not wheel spin, and power is as low as it can be to ensure no loss of traction....it is still a very small degree of slippage causing it.
But it certainly does not appear like wheelspin in those cases.
At legal speeds harder to say, my car never really grips properly at legal speeds
stevieturbo said:
Even 888's would register a higher speed under load on my car which seemed odd as they would be considered pretty stiff all round.
However that was the rear tyres, the front's didn't seem to exhibit the same behaviour.
You seem to be seeing either load sensitive tyre growth or negative wheel slip, neither of which seem intuitive. How did you determine the wheel slip at each end? I'm wondering whether some quirk of the measuring technique is making positive slip at one end look like negative slip at the other.However that was the rear tyres, the front's didn't seem to exhibit the same behaviour.
GreenV8S said:
You seem to be seeing either load sensitive tyre growth or negative wheel slip, neither of which seem intuitive. How did you determine the wheel slip at each end? I'm wondering whether some quirk of the measuring technique is making positive slip at one end look like negative slip at the other.
wheel slip is calculated between driven and undriven wheel speeds. But the changes for the rear vs front are visible on the graphs even without a calculated slip. ( only Front left though, never did add a front right, but to have both rears ( and prop speed ) )The difference between driven and undriven is highlighted more, as when you lift off the two come back into sync. As said, even with 888's this was apparent, more so than the PS4's. Which was weird.
Obviously with drag tyres there are significant changes, so no point even trying to think about those.
stevieturbo said:
The difference between driven and undriven is highlighted more, as when you lift off the two come back into sync.
Just to make sure I'm not misinterpreting you: you're saying that the driven wheel runs at a lower rpm relative to the undriven wheel when under load? Obviously, tread distortion would usually cause the opposite effect.GreenV8S said:
Just to make sure I'm not misinterpreting you: you're saying that the driven wheel runs at a lower rpm relative to the undriven wheel when under load? Obviously, tread distortion would usually cause the opposite effect.
driven wheel speeds go higher than undriven when under load.Off load they fall back in sync with each other.
GreenV8S said:
Shirley that's exactly what you'd expect due to tread block distortion under load.
No idea what to expect really. But I'd expect an 888 to be more robust in this sense than say a regular PS4 ? Not an awful lot of tread block on an 888, although in general I don't rate 888's that highly anyway, at least certainly not on road. Haven't really used them on track/circuit muchGassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff