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same ratio dfferent gears

6337 Views 64 Replies 19 Participants Last post by  stoner
hy would some one like to explain the different combo,s, and why. take a 3-1 ratio, the standard is 9-27. going to extremes, what about 7-21 or 11-33 there must be a difference and i,d like to know. a euro sport runs 6-1 6-36 why. john
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QUOTE (Ember @ 24 Apr 2012, 09:16) <{POST_SNAPBACK}>I don't really think that's the question Mr Cahill, sorry.

Is there an engineer in the house?


hi, Embs
Are you sure about that? *grin* I believe I introduced energy effects, profile efficiency, stress and geometry and questioned how deep Stoner wanted to go into this before introducing other issues.
As one of several Mechanical Engineers here, I could have moved further into the weeds, but I'm still skeptical that John wanted any more info than first presented..or whether it would be helpful.
Energy losses from profile are pretty trivial, for example, in comparison to differences in material deformation and hysteresis (plastic, brass). I still think rotational inertia and overall geometry
(clearance and spacing) are the main issues, unless we're talking high performance cars and high rpm motors where efficiency becomes more of an issue. That notwithstanding, as has already
been said...mechanical advantage is mechanical advantage. It's not the ratios but rather the ancillary issues that predominate here.

I don't believe John/Stoner has yet clarified his intentions...I want to be careful not to hijack his thread.


PS with respect to rubbing, there shouldn't be any if the tooth profile is correct and not deforming under load (and endplay limited, ball bearings etc)...but that's a big if....definitely an issue separating higher quality/precision cars from...others.
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John, the Mr Cahill was only to save some confusion. I do hope no offense was taken with that.

I also hope others are finding the information contained in this thread as interesting as I am.
Come on, we are talking about a toy car here?
QUOTE (RikoRocket @ 24 Apr 2012, 17:03) <{POST_SNAPBACK}>No, torque multiplication is simply a product of the gear ratio (excluding losses) not to do with gear sizes

You'll have to explain that to me.

Assuming gears have 64 pitch and if an inline pinions are at 5.5 then 11/33 would have a 0.5 inch dia. spur but at 8/24 the spur would only be .375 dia. So the force acting on a 33T has a moment of 12% (.25-.18/.18 ) larger than the 24T.
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hy guys, i dont think i made myself very clear. forget the 5.5-6.5 gears with different teeth counts. what i was trying to say, would a larger dia pinion say 8mm 13t and a larger gear 39t be any different to a 5mm pinnion 6t smaller gear 18t be any different, in performance of the motor. bearing in mind their both 3-1 ratio with vastly different mechanical leverages. i do know the effect of changing ratios. but i,d like to know the effect of different size pinions and gears with the same ratio. forget the cars ect, this is a hipothectical question, to do with leverage and torque changes. i think its a very basic question with i suspect a complicated answer, which i cant work out myself. so keep the explananation simple for me. thank john
Well a 6 tooth 5 mm pinion will obviously weigh less than a 13 tooth 8mm do it may well have more acceleration ...
Hi Stoner
You are asking about gears that are the correct diameter for the size of teeth rather than these standard diameter pinions with different numbers of teeth?

Two different possibilities
1 The tooth size is changed so the diameter remains the same (for example 9:27 in 48 pitch and 12:36 in 64 pitch are approximately the same size and both the same ratio)

2 The tooth size is constant so more teeth means bigger diameter.

Taking those in turn
1 Doesn't make much differance. I have some practical experience of this - it doesn't make any noticeable differance. (Although some makes mesh better than others)

2 Bigger diameter gears are inevitably heavier. Theoretically that'll reduce acceleration and braking.
Bigger diameter gears that are delivering the same torque put less side load on the bearings Theoretically that'll reduce friction.
Both effects are probably so small so they don't matter much
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The simple answer is that a mechanical advantage of 3 is a mechanical advantage of 3. This is whether it is achieved using 27/9 or 30/10 or whatever other combo you want. Anyone that says something different failed year 11 high school physics. Remember though that this is the AVERAGE mechanical advantage, as the each individual tooth engauges and dis-engauges this value will oscillate by a small amount both above a mechanical advantage of 3 and below a mechanical advantage of 3 (by a very small amount). The larger the tooth count the less the mechanical advantage will vary from 3.

Any one that says they can notice the difference in acceleration using a spur gear that weighs 1.2g compared to 1.1g is pulling there short one too hard. A while back it was fashionable to use hollow axles because they had less rotational inertia. I did some calculations and on the assumption that the hollow axle was 1g lighter, if you removed 0.003g from the outer edge of the rim/tyre it reduced the rotational inertia by the same amount. Smoke and mirror stuff. Not disputing the axle weighs less and that this could increase the accelleration, but the gain would not be due to less inertia!!

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does a big dia pinion give the motor an easier time than a small dia pinion.john
QUOTE (Flange @ 25 Apr 2012, 00:28) <{POST_SNAPBACK}>Life's not all just maths , this is from experience of the way cars go

It's a good point. After all at one time (the 1930's) it was theoretically impossible for a Bumble Bee to fly (according to accepted aerodynamic theory at the time) but try telling the Bumble Bee that!

I'm not knocking the engineers - but perhaps there should be a practical experiment undertaken to test acceleration and braking using the same mechanical ratio but with different tooth counts. Maybe a long straight with a dead strip would be a good start. After the dead strip could be a separate controller - use it open to measure distance travelled after the dead strip as a test of top speed reached at the dead strip and with the brakes on to see whether the percentage shortening in that stopping distance represents a significant difference in braking power. You obviously need the same start line with the same car and ideally an on/off switch instead of a throttle.

Oh and before somebody points out the Bumble Bee thing was the result of a drunken argument between a biologist and an aerodynamicist - yes it was but the underlying problem was that the aerodynamic theory being used was appropriate to fixed wing aircraft and not rotational wings. My point here is that what works in theory starts to get muddied by the fact that materials used behave differently (flex for example), that frictional coefficients have a bearing (HAH - INADVERTENT PUN!) and that we are talking about tiny little cars that probably weigh less in total than the smallest cog in a conventional automobile gear box yet rev about 5 times faster than their giant brothers.

We all know that scale speed and wind resistance don't scale down to 1:32 in a way that natural logic would expect. I suspect the same applies to the gears we use.
I think Flange has the right approach - forget the theory - give it a go and find out what works for you.
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Exactly and thats coming from an apprentice electrical/mechanical engineer. These are toy cars after all , and yes hollow axles do make a difference
inline or sideways or angle winder?

what motor are you talking about?

what sort of track is it?

ie is the heart set for a marathon or running on a tennis court?

easiest way is for you to set up a chassis with the gearing and time yourself you will soon know what is best for your parameters.
QUOTE (Mr Modifier @ 26 Apr 2012, 00:29) <{POST_SNAPBACK}>I think Flange has the right approach - forget the theory - give it a go and find out what works for you.
Of course, there is always the problem of observation being coloured by expectation, unless you went through double blind testing.
Sometimes it's easier to stop talking theoretical engineering and simply experiment with gear ratios on diferent track layouts and with different cars and motors. The rules and theory of gear ratio are simple; as described in the first response but the only way of finding the perfect setup is through sticking the gears in your model car and pressing the controller trigger.

That's when you find that it's very difficulty to find gears that mesh at the extreme ends of size, and that large axlegears items either jam on the bodywork, or rip into your track. In other words, there's not a huge range of sizes and ratios to experiment with.

Just buy a pack of assorted sizes from SlotIt and experiment. And have fun!
the best ratio for a neo drag car is 14-54- 64 pitch. you can use a smaller pinion and gear to get the same ratio,s the drag boys do hundreds of gear changes to get the right combo, so why go with the bigger heavier gears, when weight is a big issue in drag cars, same as 1/32 and 1/24. what i.m getting at does a large pinion with more leverage slow a motor down, or is it such a small amount in the leverage ,not make any difference. ive seen old 16-24 pinions, why make them if theres no advantage. john
QUOTE (stoner @ 26 Apr 2012, 09:38) <{POST_SNAPBACK}>ive seen old 16-24 pinions, why make them if theres no advantage.
Probably to get a mesh when used in an old 60's sidewinder kit!
To go back to stoner's OP.
I think question 1 has been answered fairly comprehensively - draw your own conclusion
Question 2 - seems to be best manufacturing tolerances come in the non extreme gear rations (9/27 in the OP)
Q3 - different types of racing have different optimum ratios.

I'm new to this game and I get several different answers to any given question every time I ask one up club - several wildly different answers from people that do very well at club and national level.
Basically, I try a few suggestions and see what works for me, so as suggested several times already, go and have a play and see what works for you. ;o)

Great thread stoner - I've really enjoyed it.
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QUOTE (stoner @ 26 Apr 2012, 08:38) <{POST_SNAPBACK}>the best ratio for a neo drag car is 14-54- 64 pitch. you can use a smaller pinion and gear to get the same ratio,s the drag boys do hundreds of gear changes to get the right combo, so why go with the bigger heavier gears, when weight is a big issue in drag cars, same as 1/32 and 1/24. what i.m getting at does a large pinion with more leverage slow a motor down, or is it such a small amount in the leverage ,not make any difference. ive seen old 16-24 pinions, why make them if theres no advantage. john

We seem to be going around in circles. If I understand you correctly one of the questions youre asking is whether there is for example, a difference between a 6.5mm 9T pinion coupled to a 19mm 27T spur compared to say a 5.5mm 9T pinion coupled to a 18mm 27T spur. On the assumption that they mesh properly then both MUST provide the same mechanical advantage which will be 3. The leverage as you call it will be the same in both cases, and that is 3. Now granted that one gear set may have a better mesh and so have less inherant losses (less friction) compared to the other. But that is a different question altogether.

The other question is, if I unerstand it correctly, why a particular ratio and not another? That really depends on the torque of the motor, the rpm, the wheel diameter, weight of car, is top speed important or is acceleration, short track, long track etc. You choose a ratio that gives the best performance for your situation. Sometimes its as simple as the physical room you have that dictates the gear sizes used.

Why make a 16T pinion? Well the motor may be a low RPM type with huge torque, so you can afford to go to a 16T with say a 36T spur. Put that in a Scaly motor with about 90-100 of torque and the car will be very sluggish.

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that wasn,t the question rick take a 14t pinion 9mm in dia and mate it to a 42 gear 3-1 then take a 6t pinion 5mm in dia and mate it to an 18t gear. whats the advantage, does a big pinion drop motor torque or does a small pinion increase motor torque.pretend that wheel, ground clearance isnt an issue. why would you go for the different pinions and gears to achieve the same ratio 3-1. there must be a very good reason judging by the racers and scratch builders setups. i hope this is a bit clearer. john
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