31 replies to this topic

[Horsepower]

I came across a handful of 72 pitch gears and was wondering if they have a specific use? The pinions in the group are cut on an angle. I can't see any use for these that 64 pitch would not handle.

[Jaeger Team]

I guess they use that pitch in Eurosport class.

[swodem]

72p lets you run lower gearing without going to silly small pinion or a spur gear so big you need to run larger diameter tyres.

Send them to me if you don’t want them!!

[Bill from NH]

They use them in wing car racing, too. I think the Boxstock class.

[Cheater]

All of the above applies, but I never had much luck with them. They certainly weren't any faster or smoother than 64p for me.

 

The gear mesh is ultra-critical with 72p gears. Get it wrong and they self-destruct rather quickly.

[zipper]

Gr 27L for me - easier to get the right gearing and probably a tad less friction.

[W. J. Dougherty]

I have used them for all my cars except Retro (48p only) and Open Wing cars. Smoother and faster in all applications, for me anyway...

[Ramcatlarry]

I can use them if you do not want to keep them.

[Ecurie Martini]

I received some advice years ago to the effect that 64DP crowns often mate best with 72DP pinions. My experience has confirmed that in several builds.

 

EM

[Bill from NH]

Alan, wWhose 64 pitch crowns were you using? I've heard that before about the plastic crowns JK once sold.

[MSwiss]

In the early 2000s, the I15 guys were super-convinced of the advantage of 72Pp

 

I tried to do as scientific comparison test as possible of a 64p vs. 72Pp back in probably 2003 or 2004.

 

I used a C12 car, that was either punched, or virtually punched, on the Koford King after it moved to Speedworld.

 

I compared 10/38 64p (3.8-1) to 11/42 72p (3.818-1).

 

I didn't see any performance difference whatsoever.

 

If that .018 difference, on the ratio, was enough advantage for the 64p to run the same lap time, I can't objectively say.

 

I was just surprised as vehemently as 72p was touted, it wasn't clearly the winner.

[gotboostedvr6]

I agree. 64p for me, unless we are talking GT12, B-production, or other classes that require 7 tooth or smaller.

[Ecurie Martini]

Alan, whose 64 pitch crowns were you using? I've heard that before about the plastic crowns JK once sold.

 

I don't recall – one set was a US-made bronze crown. I've also used them with a (Hungarian?) plastic crown. They did not work well with a large (~44 tooth) aluminum drag crown.

 

EM

[Bill from NH]

Sounds like Sonic and JK.

 

Thanks.

[Fast Freddie]

I found the biggest advantage of 72P over 64P gears was smoother power, better performance, but most of all less stress on the motor.  I may be totally in left field but I have what I call a  "stress number"  The bigger the number the less stress on the motor.  Take Mikes examples of 10x38 and 11x42.  To get the stress number subtract the pinion teeth from the spur teeth 38-10=28 and 42-11=31.  You could also do a 3.50:1 gear set in 64 and 72 pitch which is more natural comparison.  35-10=25 and 42-12=30 both are 3.50:1 gear sets and would be a more accurate example in such a comparison.  I think you'll notice a difference in performance and a difference in heat generated by the motor.  The reason I chose this particular ratio is because it's a natural ratio that can be had by both 64 and 72 pitch gears. There may be other natural ratios but this one is easy to see.   By the way my "stress number " gets bigger with 80P gears and smaller with 48P reflecting the stress applied on the motor using those gear pitches.

[Slot-Racer]

So what is the actual factor of the "stress" on the motor that brings you to that it's a "stress number" factor? The larger diameter or force/torque needed to turn the gear/wheels or the rotational mass or what? The diameter of the spurs are the same, but teeth obviously are not in count and mass is tiny so, I'm interested in the science of what the "stress number" you may think relates to or is the cause? 

[Fast Freddie]

Mario, just so you'll know.  I measured the 64P 35T and 72P 42T gears I have and here's what I found the diameters to be.

35T..PS-.574, Cah.-.576, JK-.571,and Parma-.581

 

42T..JK-.603, Cah.-.602, and ARP-.603

All of the 42T gears are angled since I don't use straight cut 72P gears 42T and above.  I did, however, have some straight cut 41T 72P gears and they were, on average,  between .598 and ,600.  So even a 1 tooth less gear had a larger diameter than the 64P gears.  I would guess that a straight cut 42T 72P gear would be even larger.  What all this means is leverage favors the 72P gears especially when identical ratios are involved.  Try it for yourself compare a 10x35 64P gear set to a 12x42 72P gear set using the same tire diameter and see what you come up with.  I've already done the test that's why I use the 72P gears when ever I can. 

[MSwiss]

Fred,

What brand infrared thermometer did you use?

 

What was the difference in temperature, degree wise?

 

If your theory is correct, I wonder why the manufacturer recommendations for gearing don't state a lower numerical ratio is possible with 72P.

[Slot-Racer]

Hi Fred, I guess it's hard to believe that .001 or .002 of a size in diameter difference would effect stress that much to have a measurable effect,   very interesting. 

[gotboostedvr6]

A few pics for your consideration

[Ecurie Martini]

The "leverage" argument does not work.  Of course, for a given force applied to a tooth mating surface the torque will increase as the diameter of the gear increases - it is a simple lever - however, on the pinion side, given a constant torque output of the motor, the force available at the pinion will decrease as the diameter increases - in effect, a wash.

 

A general formula for (overall) gear diameter is:  D= (N+2)/P  where D= diameter, N= # of teeth and P=diametrical pitch, or, commonly, simply pitch

 

Thus a 10 tooth 64DP pinion will have a diameter of 0.1875"  and a 12 tooth 72DP pinion will measure at 0.1944"

 

The increase in the pinion diameter will result in a 4% loss of available force while the corresponding increase in the spur diameter will result in a 5.5% increase in torque.  Given all of the other variables in power train efficiency,  I would suggest that this is not a meaningful difference,

 

EM

[Ecurie Martini]

In point of fact, if one does the calculation using pitch diameter rather than OD, the loss/gain is the same, -7%, +7%

 

Em

[old & gray]

While the torque may not vary with gear pitch the length of the “line of action” increases as the pitch increases. In practical terms this means there is less force per tooth (friction is a function of force) and less sliding contact (also reducing power loss).

 

I got a very practical introduction to the difference between 48 and 64 pitch on flexy cars when my 48 pitch geared car failed just before a race and a friend lent me his 64 pitch geared car for the race. The smoother power transmission was immediately evident in the turns and resulted in higher cornering speeds.

[MSwiss]

Was your 48P car 1/8 axle and the 64P car 3/32 axle?

[old & gray]

Was your 48P car 1/8 axle and the 64P car 3/32 axle?

 

Using a 3/32 axle was a further improvement but the car I but the car I borrowed had Parma 1/8 inch 64 pitch gears (local rules for 1/8 axle).