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Old 08-09-2012, 09:55 AM   #1
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Questions?? large spur/small pinion vs small spur/large pinion

I have done some searches to find info about spur/pinion combinations but I have not turned up anything useful.

My question is:
What are the pro/con of each combo, small spur/large pinion or large spur/small pinion?

I have mostly been running a silver can spec class with a kit spur and whatever pinion got me close to the FDR for the class.

I am moving on to either a 21.5 or 17.5 class and I have noticed that large pinions are common with brushless motors.

Thanks in advance for any and all info.
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Old 08-09-2012, 01:30 PM   #2
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Search for threads the title words that include "gearing," "gear ratios," or something along those lines.

Anyway, the lower you make your gearing (i.e. higher gearing ratio, smaller pinion, bigger spur), the more torque you'll have. If you're racing, some things that effect gear ratio selection are: track layout (need for acceleration or top end), motor temp, drop off during the race, and timing. Generally, and assuming all else is constant, you'd gear lower (smaller pinion - most of the time it's easier to change a pinion than a spur) for a tighter track and higher (bigger pinion) for a more open track.

I hope that helped.
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Old 08-09-2012, 02:26 PM   #3
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Quote:
Originally Posted by bencason View Post
I have done some searches to find info about spur/pinion combinations but I have not turned up anything useful.

My question is:
What are the pro/con of each combo, small spur/large pinion or large spur/small pinion?

I have mostly been running a silver can spec class with a kit spur and whatever pinion got me close to the FDR for the class.

I am moving on to either a 21.5 or 17.5 class and I have noticed that large pinions are common with brushless motors.

Thanks in advance for any and all info.
This changes your final drive ratio (gearing). Do you drive a full sized car? Well, what are the pros and cons of driving your car around stuck in 1st gear, as apposed to being stuck in 5th gear? That is basically the same thing.

Small pinion + Large spur = 1st gear
Large pinion + Small spur = 5th gear

Your task in gearing, is figuring out what gear combination gives you the best compromise between acceleration AND top speed.
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Old 08-09-2012, 02:29 PM   #4
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I assume bencason is asking for pro/con of having BigSpur:SmallPinion=SmallSpur:BigPinion with same gear ratio.
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Old 08-09-2012, 03:08 PM   #5
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I assume bencason is asking for pro/con of having BigSpur:SmallPinion=SmallSpur:BigPinion with same gear ratio.
That would be impossible.

If that's what you want to talk about, that would be small pinion and spur vs. big pinion and spur, same ratio. There are several threads about that already.
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Old 08-09-2012, 03:12 PM   #6
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Quote:
Originally Posted by 91420l View Post
I assume bencason is asking for pro/con of having BigSpur:SmallPinion=SmallSpur:BigPinion with same gear ratio.
That will never give you the same gear ratio.

It would have to be big spur:big pinion vs small spur:small pinion with same gear ratio.

In that case you change where the weight of the motor is. Either more forward or more rearward. I personally don't ever worry about it. It wont make a difference for me or most people I know.
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Old 08-09-2012, 04:40 PM   #7
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That will never give you the same gear ratio.

It would have to be big spur:big pinion vs small spur:small pinion with same gear ratio.

In that case you change where the weight of the motor is. Either more forward or more rearward. I personally don't ever worry about it. It wont make a difference for me or most people I know.
It's a big difference in pan cars. Really big difference.
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Old 08-09-2012, 04:54 PM   #8
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Originally Posted by 91420l View Post
I assume bencason is asking for pro/con of having BigSpur:SmallPinion=SmallSpur:BigPinion with same gear ratio.
I knew what you meant, and yea there is a.difference.
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Old 08-09-2012, 07:00 PM   #9
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The difference is time and rotational mass ! It takes less time to turn a smaller spur , specially when the spur weighs less.... It's preferable to keep things small....
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Old 08-09-2012, 07:05 PM   #10
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It's a big difference in pan cars. Really big difference.
To clarify I didnt say it didnt matter.

But now that I think about it, it probably will affect my F1 as thats pancar enough. But thats the only class I run where I might worry about it.

What are the effects of having it fwd or back?
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Old 08-09-2012, 08:09 PM   #11
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Thanks to all who responded to my poorly worded question. I just wasn't thinking straight. Let me try again.

Lets say I wanted to get to roughly a 4.3 FDR using 48P gears on an Xray T3 with a 1.9 internal ratio and assuming the gears will fit and mesh.

according to GearChart.com,

one choice is: 90T spur 40T pin which is 4.27
another choice is: 70T spur 31T pin which is 4.29

Since my original post made no sense [even to me when I re-read it], what I was getting at was:

What is the pro/con to big spur/pinion approach vs the little spur/pinion approach for the same FDR?

Odin544's comment about changing the motor location which would affect weight distribution was the sort of thing I was wondering about. I'm not sure the difference would be noticeable at my skill level either
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Last edited by bencason; 08-09-2012 at 08:10 PM. Reason: spelling, punctuation, etc.
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Old 08-09-2012, 08:27 PM   #12
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40T will have 9T more meshing drag per rotation versus the 31T, so maybe less efficient? On other hand too smalll pinions (e.g. 15T) I hear do not mesh well either, causing excess drag as well.
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Old 08-09-2012, 08:39 PM   #13
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Virtually no difference. Smaller gears will be a bit lighter and thus spool up better, larger gears mesh better, pretty much a push. The weight shift would be the biggest difference and that is quite minor on a TC.

For pan cars, you generally want to run the biggest combo you can fit. The closer the motor is to the pivot, the better the car works.
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Old 08-09-2012, 09:37 PM   #14
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you will never get 90t 40t 48p combination on a t3. Biggest you will get is around 75 37. And the difference between that and say 68 33 or so that would yeild a similar ratio is negligble. Some say you get better braking with a bigger spur due to the leverage, but I cant tell. Better to get something bigger as the holes in the spur are generally bigger and let you mount the motor easier. A 68 spur is a pig to get the motor screw through, with the ideal spur I have found is around 72t for 17.5 blinky
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Old 08-12-2012, 01:36 PM   #15
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There are several answers to the question, but if we are talking efficiency, then the closer you get to the two being the same size, the better.

Rotational mass - depends solely on the weight of each part. Rotational mass is about where the weight is in relation to the centre of rotation. A small steel pinion will have less rotational mass than a large plastic spur gear. Assuming the that rotational weights are the same, it makes no difference how large the pinion or the spur is. It is the total rotational weight that affects how fast the transmission spools up, not simply the individual weights.

Tooth strength - is dependent on the design of the tooth and the materials used. Providing suitable materials are used, and the motor is the same, the size of the spur and pinion will make no difference. All RC gears are over-engineered so when meshed correctly will all be the same.

Efficiency - this is where the closer to each other the gears are, the more efficiency you will have. As the gear count gets closer, the teeth move into and out of mesh with the least angle and thus the least friction. Also, in this condition, you have more teeth in mesh which helps transfer the power more efficiently as well as spreading the tooth load thus reducing the likelihood that you will strip a gear.

That's the theory. In practice it makes very little difference. There are so many other issues with the whole power train that any slight gain in the gears will be lost in a small fault in a bearing, a belt or a diff. Choose the spur/pinion combination that gives you the best set-up for your car, and forget the niceties of a little bit of physics! HTH
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