Tire diameter as part of overall gear ratio
Tire diameter as part of overall gear ratio
(OP)
I know that tire diameter is part of various formulas like:
MPH =
RPM x Tire Diameter
------------------
gear ratio x 336
But just in case.. I wanted to ask if all other things being equal, does a ring & pinion ratio of say 3.45 with a talle(r) tire perform better in drag racing than a r&p ratio of 3.07 with a shorter tire?
Even if the MPH/1000 rpm with both setups is the same on paper? Or at least the MPH at 6000rpm is the same for both..(or near enough the same)
Is there some advantage in a drag race to having the 3.45 ratio doing the work? Or,does the tire diameter indeed have as much a part in it as any other item in the sequence as the formula would suggest? - Other than just contributing to MPH calculations.
MPH =
RPM x Tire Diameter
------------------
gear ratio x 336
But just in case.. I wanted to ask if all other things being equal, does a ring & pinion ratio of say 3.45 with a talle(r) tire perform better in drag racing than a r&p ratio of 3.07 with a shorter tire?
Even if the MPH/1000 rpm with both setups is the same on paper? Or at least the MPH at 6000rpm is the same for both..(or near enough the same)
Is there some advantage in a drag race to having the 3.45 ratio doing the work? Or,does the tire diameter indeed have as much a part in it as any other item in the sequence as the formula would suggest? - Other than just contributing to MPH calculations.





RE: Tire diameter as part of overall gear ratio
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tire diameter as part of overall gear ratio
Ken
RE: Tire diameter as part of overall gear ratio
A functional difference that is. The taller tire being harder to turn due to its' diameter notwithstanding.
Let's say I have a choice between a taller slick and a 3.55 gear, and/or a slick a bit shorter in diameter to use with a 3.33 gear. - Just to put the question into clear definition.
Does the 3.55 gear's giving more torque multiplication to the axle than the 3.33 somehow net a stronger effect than the 3.33 short tire combo.
Even though they both would give say the same rpm at a given mph.
RE: Tire diameter as part of overall gear ratio
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tire diameter as part of overall gear ratio
With the taller tyre you get better traction but you have a higher polar moment, but you need to turn the tyre slower to get the same peripheral speed.
Like Greg said, there is not enough in it to worry about.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Tire diameter as part of overall gear ratio
There is no such thing as "all things being equal" unless the car and power are also equal. Traction has priority and determines minimum tire requirements. Gearing is chosen according to tire size. Smaller tires and lower numerical gearing are more efficient but can only be reduced to a point short of not getting the job done.
RE: Tire diameter as part of overall gear ratio
Any disadvantages such as weight, inertia, aerodynamic, or increased axle torque reaction forces, would be secondary.
If possible, use the largest diameter and width tires that your class will allow.
RE: Tire diameter as part of overall gear ratio
I disagree to a point. Traction is where lower ETs come from is true, but once you have enough traction to effectively "hook up" extra potential traction is wasted and the lesser influencing factors to that point then become the dominant factors, although they do have a much smaller potential for gain or loss.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Tire diameter as part of overall gear ratio
From experience in drag racing most bracket choose the smallest tire they can get away with and still hook up. This reduces rotating mass which has some advantage. At this lever most use minor size differences to fine tune their engine rpm when the gaps are too large between rear gear options.
In the Really fast sportsman classes and pro's, tire growth becomes a large factor in varying the final (effective) gear ratio to load the engine past the 330' mark. This can have a huge effect on the amount of fuel a car can burn and in effect make large changes in ET (large meaning .05 to .10 sec in extream cases). An example of this is, we went from a 33" to a 34.5' tall tire, the full growth circumferance was about 13% larger compared to the 4.5% dia differance at rest. This made a large change to the whole system and loading on the engine.
RE: Tire diameter as part of overall gear ratio
Still, that's boring old maths, not fancy technical sounding explanations.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tire diameter as part of overall gear ratio
And nobody seems to come to these threads for any of that boring old math, eh?
I once long ago did some calculations on my Triumph Spitfire ( I *said* it was long ago!) and found that, accelerating in first gear, right about half the engine power was expended in simply accelerating the flywheel - 30 hp worth!
I subsequently changed to smaller diameter rear tires and picked up a significant gain in acceleration, and changing to an aluminum flywheel gave a huge improvement.
regards,
- R
RE: Tire diameter as part of overall gear ratio
"When the eagles are silent, the parrots begin to jabber."
Winston Churchill
RE: Tire diameter as part of overall gear ratio
Truck
RE: Tire diameter as part of overall gear ratio
The circumference certainly increases if the tire expands due to centrifugal force at the fast end of the track.
But can vertical load make it less ?
RE: Tire diameter as part of overall gear ratio
Truck
RE: Tire diameter as part of overall gear ratio
RE: Tire diameter as part of overall gear ratio
Some of this has to do with why the winners are generally old enough to have tons of empirical data.............
Truck
RE: Tire diameter as part of overall gear ratio
But even wrinkle wall tires will have a fairly rigid belt to limit the tractive surface itself from distorting too much.
My thesis is that the whole mess can distort, compress, wrinkle, and perform all manner of distortions at launch. But one full turn of the hub will cause one full turn of the tire, and the circumference does not change (much), even though the instantaneous hub to track distance may change fairly dramatically.
In fact, if you watch a top fueler at the instant of launch, they tend to rise straight UP several inches. All very mysterious.
RE: Tire diameter as part of overall gear ratio
Truck
RE: Tire diameter as part of overall gear ratio
Norm