Tire diameter as part of overall gear ratio

[Viper488]

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.

 

[GregLocock]

I would have thought tire behaviour was absolutely dominant, and you'd then choose an axle ratio to suit the best tire. But I know nothing of drag racing.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kenre]

A taller tire will give more contact patch for traction.

Ken

 

[Viper488]

Well, related topics aside, I'm just wondering if there's a difference between a stiff gear turning a tall tire and a not-so stiff gear turning a shorter tire. When both set ups more or less equal out mathematically when plugged into a mph formula. Or any variations of that formula.

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.

 

[GregLocock]

Not to any appreciable extent. I'd have said everything except aerodynamics was in favour of the larger tire.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

The overall gearing and therefore the overall torque multiplication are equal.

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

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[Fabrico]

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.

 

[Warpspeed]

I would agree, the larger tire would be advantageous in regard to traction, and that is where the low ETs come from.

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.

 

[patprimmer]

Warpspeed

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
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[sbc438]

Wrapspeed,

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.

 

[GregLocock]

If anyone would like to do the calculation of the angular momentum of a tire rotating at a given roadspeed, they might get an horrible shock when comparing a larger tire, with most of its mass in the tread, and a smaller one.

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.

 

[Rob45]

Greg:
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

 

[Metalguy]

A possible item is that a higher ratio rear is usually stronger than a lower one. A big fat pinion gear is what I like. Smaller tires are also easier on the axles.

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill

 

[truck]

Remember - if you have enough torque to make this question worthwhile, the tire will squat quite a bit under load so you will effectively have a variable overall ratio.

Truck

 

[Warpspeed]

But does the total circumference of a tire decrease if it goes a bit flat on the bottom under vertical load?

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 ?

 

[truck]

The moment arm decreases

Truck

 

[Warpspeed]

So does the hub then rotate faster than the tire contact patch around the circumference?

 

[truck]

Kind of depends - in my estimation all bets are off when you use wrinkle walls - the moment arm goes down - there is a torsional spring effect, etc

Some of this has to do with why the winners are generally old enough to have tons of empirical data.............

Truck

 

[Warpspeed]

I don't know the answer to this either.

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.

 

[truck]

the magic of bleach.............

Truck