21 replies to this topic

[Bulsara]

A class of racing that I compete in is going to a fixed gear ratio of 3.00:1. Are there any advantages to the possible pinion/crown combinations? ie. Is 10/30 potentially better than 9/27 or 8/24. I think that any benefit would be negligible or non existent. What say the gurus.

[slo1quick]

I dunno. But with go karts, I have been told that the higher drive tooth , ie, 10 tooth drive and 80 tooth driven will give better top end power than one with a 9 tooth drive : 72 tooth driven gear ,even though they are both the same ratio. It may be because of the chain tho.

[Steve61]

Bulsara, on Feb 7 2010, 09:17 PM, said:

A class of racing that I compete in is going to a fixed gear ratio of 3.00:1. Are there any advantages to the possible pinion/crown combinations? ie. Is 10/30 potentially better than 9/27 or 8/24. I think that any benefit would be negligible or non existent. What say the gurus.

I'm no guru, but as an engineer I can't help but have a go. The most important thing is that the gear and pinion are a meshing pair - made by the same manufacturer so that both the pitch and the tooth profile match, otherwise the car will run rough and vibrate.

As the gear gets more teeth the dimeter increases, so that can be a limiting factor itself - the gear must be smaller than the tire OD (obviously) and also clear the slot in the chassis if it's inboard of the bearings. It would say go for the largest diameter gear you can get because the forces on the gear teeth will be lower eg a torque of 600 g.cm on the rear shaft will have a gear force of 750 grams at a gear radius of 0.8cm, but only 600 grams at 1.0 cm radius (2cm gear diameter)

With pinions when the no. teeth gets too low, the tooth profiles start to undercut each other - 8 seems to be about the min. you see and you can definitely see the undercut on them. This compromises the normal profile of the tooth and is required so the gear tooth can enter the pinion and rotate and clear itself out. For smother running I'd recommend the largest pinion you can get to get the ratio you need.

These aren't big factors and lot's of cars run well with smaller gears, but if you have the choice - largest pinion and gear you can get in the car.

[rick1776]

Id say it would be marginal to non existent. Get torquemadaslot to test it.

[Bulsara]

Thanks guys. Interesting isn't it.

[SlotsNZ]

No dumb questions . . . but here's a dumb answer, kind of

Alluding to Steve61's comment about meshing, if you are using say Slot.it or Sloting Plus Gears (and hte same probbly goes for NSR and anything wth fixed axle to motor shaft distance)

In 1/24th stuff, they generally work at 64 pitch and move everything, [correct me someone if I am wrong, I just sort of ASSUMED THAT] - butwe don't. Those Slot.it pods for example, all have the same fixed distance from axle to motor shaft.
The gears are built with fixed diameters to suit that fixed axle to motor shaft distance. Therefore the "pitch" of the gears has to change as the number of teeth change.

The "nominal" pitch is 48, but in practice of course, the pitch of the supposed "middle gear ratio" is/would be about 48 pitch, but everything either side of that in teeth numbers moves up or down in pitch, and to compensate so that the gears still mesh "okay" the angles of the teeth are altered. - sorry I'm NOT an engineer, and don't know the term for that

But imagine that say 9:27 Slot.it inline, or an 11:32 sidewinder was 48 pitch, then by the time you get to say 11:27, your pinion has quite a different pitch, so it may not mesh so well.

All this waffle means, getting a pair of gears that are close to "true 48 pitch" is important, so that they mesh well, and waste less energy in rubbing moving surfaces as they engage and dis-engage.

I'd suggest 9:27 is closer to true 48 pitch than say 10:30, and a better cut of gear angle.
Whatever you use, run those gears in with some decent grinding paste for good effect -

Or you could use some 64 pitch gears that happen to be "right"
I think Professor Motor have a 15T pinion and 45 tooth spur gear that mesh correctly (correct diamters) as sidewinder in Slot.it pods........ if sidewinder is permitted in your class, and any brand of gear.....

[kalbfellp]

You will find that the bigger pinions will give you slightly less braking effect.

[Bulsara]

kalbfellp, on Feb 7 2010, 11:55 PM, said:

You will find that the bigger pinions will give you slightly less braking effect.

More to this than meets the eye.

Once again, thanks all for the feedback.

[Steve61]

Quote

But imagine that say 9:27 Slot.it inline, or an 11:32 sidewinder was 48 pitch, then by the time you get to say 11:27, your pinion has quite a different pitch, so it may not mesh so well.

I'm amazed that the slot.it gears mesh so well with such a wide range of tooth pitches - pinions that go from 8 to 11 teeth can mesh with crown gears from 23 to 30 teeth -all without any variation to the diameter of either the gear or pinion.

The tooth profile or shape must be so cleverly designed that it can pull in the mating tooth over a wide range of pitches and soak up the difference without accumulating errors and binding up the gears. The guys at slot.it REALLY know their stuff!!!

[jazzbell]

Bulsara, on Feb 7 2010, 09:17 PM, said:

A class of racing that I compete in is going to a fixed gear ratio of 3.00:1. Are there any advantages to the possible pinion/crown combinations? ie. Is 10/30 potentially better than 9/27 or 8/24. I think that any benefit would be negligible or non existent. What say the gurus.

To cut along storey short,my opinion is if it's a fixed gear ratio for the class your running,go for the
10/30 combination.
Motors run cooler and the motor does'nt need to work as hard with the bigger pinion/spur set up
This would be my choice

regards shane a

team thunderbird

[kalbfellp]

Quote

The guys at slot.it REALLY know their stuff!!!

Just shows what an EX Ferrari employee can do!

[rick1776]

A 10/30 gear has the same mechanical advantage as a 9/27 gear unless Im missing something??? I suppose there would be a bee's dick of an advantage in having the pitch as closely matched as possible but the mechanical advantage MUST be the same.

Take the sidewinder gears at 6.5mm and 19mm diameter they would have the following pitch.

9T = 35 pitch
10T = 39
11T = 43
12T = 47

34T = 45.5
36T = 48
38T = 51

[kalbfellp]

Sorry Rick BUT the Slot It gears are all around 48 pitch! I know this does not calculate properly BUT that is what they ar
When you look at the gear teeth the shape is not actually a true 48 P shape! I suppose that is how they get them to work.

[cliffy]

If you are trying to get speed from engine RPM, you first calculate the circumference of the tire using the formula pi x D where pi is 3.14 and D is the outside diameter of the mounted tire.

You divide a mile by the result to get wheel revolutions per mile.

With a 22" o.d. tire the circumference is 69 inches (actually 69.08") and that equals 5.75 feet. Divided into a mile (5,280') gives 917.19 axle/wheel revolutions to a mile.

To go 60 mph, a mile in one minute, with a 22" tire your axle revolutions will thus be 917 axle revolutions/ minute.

(Incidentally, if you go a mile at 60 mph you are traveling at 88 feet per second.)

If you have a 3.54:1 rear axle ratio your engine will be turning 917 x 3.54 = 3,246 RPM.

Of course this ignores wheel slip or spin and any increase in tire circumference due to centrifugal force from tire RPM. Slip is likely a lot more than tire growth with at least the tires I was using.


This is the best description I've been able to garner regarding the calculation of ratios....

As for pitch, you will find a close relation to axle size in slot cars, but not all the time...

[kalbfellp]

Recently I built a Howmett using a Fly chassis and Yellow Scalex motor. The Scalex motors lack brakes and when I geared it 13/38 is was like running with very little brakes But the gear mesh was great.
After the first race I changed the gearing to 11/32 and the braking improved! The mesh was still very good.
The ratio's are very close BUT the difference in braking is noticible! This was all using Slot It sidewinder gears.
I would not have though the change would make much difference BUT it did.

[rick1776]

Did you use the same OD tyre? I have often suspected that the laws of physics that govern all our lives do not apply down in Tassie. God does have a sence of humour after all.

[gzminiz]

i agree that braking does *seem* less with the larger pinion. Maybe the better mesh, less drag, affects this? hard to tell braking distance from one gear to the next, all perception.

[slotbaker]

kalbfellp, on Feb 7 2010, 11:55 PM, said:

You will find that the bigger pinions will give you slightly less braking effect.

gzminiz, on Feb 9 2010, 11:16 AM, said:

i agree that braking does *seem* less with the larger pinion. Maybe the better mesh, less drag, affects this? hard to tell braking distance from one gear to the next, all perception.

Might have something to do with the mechanical advantage of the larger diamter pinion versus the smaller one?

The crown/spur 'gear' contacts the pinion at a radius point from the centerline of the motor.

The larger the radius (big pinion) the easier it is for the 'gear' to turn the pinion. = Less brakes.

Conversley, the smaller the radius (small pinion) the harder it is for the 'gear' to turn the pinion. = More brakes.

[rick1776]

kalbfellp, on Feb 9 2010, 07:03 AM, said:

Recently I built a Howmett using a Fly chassis and Yellow Scalex motor. The Scalex motors lack brakes and when I geared it 13/38 is was like running with very little brakes But the gear mesh was great.
After the first race I changed the gearing to 11/32 and the braking improved! The mesh was still very good.
The ratio's are very close BUT the difference in braking is noticible! This was all using Slot It sidewinder gears.
I would not have though the change would make much difference BUT it did.

Hey Phil does that chassis allow you to move the motor? The 38T is 19mm and the 32T is 18mm. If you didnt move the motor it would have been very sloppy when you went from 19mm to 18mm spur. Perhaps if you moved the motor back 0.5mm this was enough to affect the rear weight bias and improve braking.

Hmm actually there might be some truth to it. The overall mechanical advantage cant be different. A 3:1 ratio is a 3:1 ratio. Put perhaps the peak and trough mechanical advantage varies more with one system than the other just like an armature doesnt produce a constant torque through one revolution but varies by about 15%. Hmmm I need some more think time music. Definetely a job for torquemadaslot, but hes not allowed to use any oil on the bearings.

[jazzbell]

Guy's,

The post was just a simple question on what gear ratio to use.With all the talk
on about what meshes best and tire diameter still does'nt answer his question.
I think i have a fair idea on set with gear ratio's,and some of your replys are
losing me.
My opinion,stick to the 10/30 combination.This is what i'd choose to run.

regards shane a

team thunderbird