Is there a difference?

Michael Polizzi · Joined: 18 Jul 2001 Posts: 1565

Maybe I missed this topic but...

If a 15/22 is 1.47 and 19/28 is 1.47

Do you get a difference in applied torque to the rear wheels. or is a 1.47 ratio a 1.47 ratio????

Thanks in advance
Michael Polizzi 69n
Full Bore Karting

John Learmonth · Posted: Sun Oct 14, 2001 8:56 pm Post subject: Is there a difference?

quote:
Originally posted by Michael Polizzi:
Maybe I missed this topic but...

If a 15/22 is 1.47 and 19/28 is 1.47

Do you get a difference in applied torque to the rear wheels. or is a 1.47 ratio a 1.47 ratio????

Thanks in advance
Michael Polizzi 69n
Full Bore Karting

Mike,
I think a ratio is a ratio, and will multiply torque from the motor to the wheels the same way regardless of the sum involved. You might want to deduct a smidgin of horsepower for pulling the chain around a smaller driving sprocket, but the difference must be tiny I would expect.

Chad Stapleton · Joined: 19 Jul 2001 Posts: 4403

Mike,
yes you sure missed that debate (several times over !)
Lots of opinions & debate but no hard facts to show anything other than the theoretical answer as John L said. Torque (as with revs)is proportional to the ratio - irrelevent of the actual sprockets used.
Stand by for more comments !!!

Pete Muller · Posted: Sun Oct 14, 2001 9:14 pm Post subject: Is there a difference?

quote:
Originally posted by Michael Polizzi:
If a 15/22 is 1.47 and 19/28 is 1.47

Do you get a difference in applied torque to the rear wheels.

No.

Ian Jones · Joined: 20 Aug 2001 Posts: 154

It seems the consensus is "no" to a change in total torque output, but would that total torque output be applied differently? Faster or slower depending on the size of the driven gear? My guess is "no" becuase the chain is making the two into one as the motor sees it. Where does tire circumference figue into the formula? Can someone give a formula?

Chaz Clover · Joined: 18 Jul 2001 Posts: 879

I've laid awake nights pondering this issue. Not for long, but I have laid awake for a while..........

Accepting what john (and others) will say on this subject, another question comes to mind. If one procket combination is the same as another given equal ratios, why then do they make more than one size drive sproket? To keep the rear from cutting a trench in the track surface?

quote:
Originally posted by John Learmonth:

Mike,
I think a ratio is a ratio, and will multiply torque from the motor to the wheels the same way regardless of the sum involved. You might want to deduct a smidgin of horsepower for pulling the chain around a smaller driving sprocket, but the difference must be tiny I would expect.

Chuck Bunnell · Posted: Mon Oct 15, 2001 6:49 am Post subject: Is there a difference?

A ratio is a ratio and I would definitely agree with the "NO" votes. The difference between the two gearsets is the final application. The larger diameter gears will have a larger weight which must be linearly accelerated and a larger diameter which must be rotationally accelerated. Both things will reduce the net acceleration of the kart. There will also be more chain to accelerate which further slows the acceleration of the kart. On the plus side the chain will be bending less as it goes around the larger sprockets and should take less energy. Alignment will also figure in with other variables. And keeping the rear gear off the pavement or out of the dirt is definitely a speed secret too. Everything is a compromise and everything affects everything else on a kart.

For those looking to play with gear ratios and tire diameters and stuff, I have free software available on my website that saves you from doing the math and lets you do the thinking and playing. http://www.chuckbunnell.com/kart/soft/ The software is called Gear Selector and is part of the Gear Selector family of programs. Many karters find this software helpful.

Ian Jones · Joined: 20 Aug 2001 Posts: 154

Thanks Chuck, I'll see if I can get that to load up. That's what I was looking for.

Oscar Aguilera · Joined: 18 Jul 2001 Posts: 1614

IMO, A ratio is a ratio, it is math and there is no difference. There is no difference in torque either because that is what the motor produces and not what the gearing produces.

The difference is how quick the motor revs up. A 15 front driver will rev a motor up quicker than a 19 front driver. As for the circumfrence thing someone mentioned, it is the reason that the gear ratio exists. I don't have two sprockets in front of me but a 35 pitch sprocket has a sister 219 sprocket for certain sizes where the circumference is the same. The 219 offers smaller increment changes and thus more gear ratios.

Chad Stapleton · Joined: 19 Jul 2001 Posts: 4403

Oscar, if you agree a "Ratio is a ratio", why do you believe a smaller driver would let the motor rev up faster ? just because of the less weight/inertia in the sprocket ?

Oscar Aguilera · Joined: 18 Jul 2001 Posts: 1614

It is like putting a ball on a short string and twirling it and using a long string and twirling it. The motor can only spin it so fast, max rpm, but the short string will spin the rear sproket to speed quicker than the long. The motor will reach max rpm sooner but the final ratio is the same.

Ian Jones · Joined: 20 Aug 2001 Posts: 154

Your giving up overall kart speed to gain rev speed and torque at the rear axle(smaller countershaft gear and larger axle gear),but keeping the ratio the same. There is give and take. I got confused when I started thinking of the countershaft gear and axle gear as somehow one unit. They are independant of each other.

Chad Stapleton · Joined: 19 Jul 2001 Posts: 4403

BUT Oscar, with the smaller front sprocket you must use a smaller rear sprocket(to keep the Ratio), which makes it HARDER to spin the axle up to speed.
What you gain with the smaller driver, you lose with the smaller rear sprocket.
The only possible variation would be from the reduced rotating mass of the smaller sprocket set, and i dont believe these gains would be detectable.
Only realistic way to check would be on an inertia dyno.
I don't recall seeing any correction factors being needed to allow for sprocket size (as opposed to ratio) on dyno calculations.

[ October 16, 2001: Message edited by: Chad Stapleton ]

Oscar Aguilera · Joined: 18 Jul 2001 Posts: 1614

I think you may be right on the smaller front and smaller rear but a motor can pull a smaller front easier than a larger front. If you have more HP and torque for example you can RR with a larger front tooth sprocket and then gear higher if your motor can sustain the rpm's uphill or into the wind, you will be faster.

The new motors have a very broad power band to make them more driver friendly. The idea is to run a larger front sprocket so that the driver-the guy in the kart, has more time between shifts. If your motor can not pull with a broad power band then you will be forced to opt for a smaller front driver with the same ratio so that you can pull out of the corner without getting passed on the straight.

EX: You are shifting in the middle of the corner to third gear, you would rather not shift in the corner. You put a taller back gear and find that third is now too tall a gear to be in, it won't pull and second is too much gear, no way. You go back and change the front gear driver to the next smaller size and the back gear to keep the same new gear ratio, and hopefully find that you can go to 4th gear on the exit and ride 3rd through the entire corner. or better yet you can take the corner in fourth and carry even more speed on the exit. Of course it never works that easy but maybe this explains it better.

From my experience the motor will spool up quicker with a smaller front driver. In Formula A or Y the 219 chain was better because you could run a smaller diameter front gear driver than with 35 chain. That little added pull was needed with no clucth.

Mike Goebel · Posted: Wed Oct 17, 2001 5:30 pm Post subject: Is there a difference?

quote:
Originally posted by Chad Stapleton:
BUT Oscar, with the smaller front sprocket you must use a smaller rear sprocket(to keep the Ratio), which makes it HARDER to spin the axle up to speed.

[ October 16, 2001: Message edited by: Chad Stapleton ]

No! if the ratio is the same the acceleration is the same not including things like efficiency and inertia. If you run a 100 tooth up front and a 1000 rear you would accelerate the same with a 10 and 100 front and rear respectively. However the smaller the gear the more the chain is being pulled off the sprocket by centripetal acceleration and it is also less efficient.

Mike G.