# Naked Science Forum

## Non Life Sciences => Technology => Topic started by: fkneg1 on 21/06/2016 09:04:51

Title: If the gear ratio is the same, does sprocket size matter?
Post by: fkneg1 on 21/06/2016 09:04:51
I have an electric car we have made with a chain drive from the motor to the axle. If I have the same gear ratio but can have different sprocket sizes does that make any difference? For example would a 12:36 ratio have any difference in performance to a 16:48 ratio?
Title: Re: If the gear ratio is the same does sprocket size matter?
Post by: chiralSPO on 21/06/2016 20:04:45
speaking as a non-engineer (so I may be wrong): I say yes. Obviously with the same ratio, many things will be the same.

However, if we extend this to the ridiculous limit: is there a difference between 16:48 and 3:12, how about 400000:1600000? The number of teeth on the wheel will have some bearing on the stability of the system and how well it handles stress. Also, if the links in the chain are a fixed size, then this means the sprockets with more teeth will have a greater diameter (and hence greater mass), so there may also be some increase in the inertia of a larger system, as well as more energy expended in spinning it up (which may be negligible, but again, not if taken to the ridiculous extreme...) Or, as the number of teeth increases, the chain needs to have smaller links to fit, which may eventually cause problems in performance (or manufacturing expense).
Title: Re: If the gear ratio is the same, does sprocket size matter?
Post by: ProjectSailor on 01/08/2016 16:02:26
Without stating the obvious..

The sprocket size matters since you are looking at teeth per metre on the sprocket wheel, and the teeth must be of similar size, which means to decrease or increase numbers of teeth the sprocket size must change.

If you change both sprockets to different sizes you need to assess the stability and reliability of the change. If you go bigger you get higher rotational velocity of your wheel and hence greater need for balancing, if you go smaller you generate higher rotational acceleration giving increased load on the chain and less reliable performance. but this will not relate to translating faster speeds etc. you will probably find that you gain some torque as the sprocket size increases from the inertia but it will be minimal compared to the motor torque.

Hope this helps
Title: Re: If the gear ratio is the same, does sprocket size matter?
Post by: Tim the Plumber on 01/08/2016 17:23:26
The result will be the same for the gearing, the speed of the motor vs the speed of the car but to make the drive work without loosing lots of energy to friction in the gears and chain you have to do it right.

The detail of this is more than I know but I do know it's a lot of detail.

Good luck.

P.S. if you are just driving it around the garden to keep the kids happy don't worry about it.
Title: Re: If the gear ratio is the same does sprocket size matter?
Post by: William McC on 05/08/2016 06:05:11
speaking as a non-engineer (so I may be wrong): I say yes. Obviously with the same ratio, many things will be the same.

However, if we extend this to the ridiculous limit: is there a difference between 16:48 and 3:12, how about 400000:1600000? The number of teeth on the wheel will have some bearing on the stability of the system and how well it handles stress. Also, if the links in the chain are a fixed size, then this means the sprockets with more teeth will have a greater diameter (and hence greater mass), so there may also be some increase in the inertia of a larger system, as well as more energy expended in spinning it up (which may be negligible, but again, not if taken to the ridiculous extreme...) Or, as the number of teeth increases, the chain needs to have smaller links to fit, which may eventually cause problems in performance (or manufacturing expense).

Usually the larger sprocket will give you better performance, if done in moderation. One reason is the bearings on the smaller sprocket, the smaller the sprocket the more force is exerted on the bearing. It can be substantial, especially if you are gearing up, from the motor or transmission output, for example a 4:3 motor to drive train ratio. The reason is the force of a chain, is more quickly turned into rotational energy, rather than, energy wasted on the bearing. The larger sprocket will cause more mass that needs to be positively accelerated, or negatively accelerated, depending on the usage of the vehicle. Things like slowing down, or down shifting with larger sprockets might require a recalculation of braking needed.

One thing overlooked by most car manufactures today is the size of the brakes. Originally car engineers when designing cars built cars with brakes that could not only stop the weight of the car at whatever speed the car was rated for. The brakes were also designed to stop the car at its maximum rated speed with the full throttle horsepower applied.

Sincerely,

William Mccormick
Title: Re: If the gear ratio is the same, does sprocket size matter?
Post by: syhprum on 28/10/2016 00:10:26
The latter is not a bad Idea occasionally cars have a problem with the throttle pedal jamming wide open, this is scary! the correct response is to hit the ignition key and turn it off but of course but unless you have rehearsed it this it does not always come to mind .
you respond in two ways you put out the clutch and the engine goes to full limited revs (or if you have no limiter blows up!) or you hit the brakes hard which works if they are adequately rated.
This has happened to me once in 60 years of driving, I did not respond well
Title: Re: If the gear ratio is the same, does sprocket size matter?
Post by: alancalverd on 28/10/2016 08:36:08
The bigger the sprocket, the less tension in the chain for a given torque, so you can use a lighter chain with less frictional loss and wear, or transmit more torque (better acceleration) with a given chain.