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Replacing sprockets, worth upgrade?

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Hangear
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Joined: 16 Oct 2012
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PostPosted: 00:18 - 14 Nov 2012    Post subject: Replacing sprockets, worth upgrade? Reply with quote
Hi guys.
Did a bit of TLC on my Derbi Senda XTreme R 50 workhorse that gets me from A to B every day. It's done about 3800 miles since I bought it new last july.

Today I noticed the sprockets teeth have been worn and therefore the sprockets need replacing (mainly due to my poor maintenance in the chain).

Basically, since i will be ripping the old sprockets off and changing them, should I try a different ratio? I feel my gears are very similar (I have 6 to do 32 mph) and I would like to vary them out a bit more. I would love to be able to do a higher speed uphill as that's about 18 mph.

What sprockets would you recommend guys? Very Happy
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Kickstart
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Joined: 04 Feb 2002
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PostPosted: 01:19 - 14 Nov 2012    Post subject: Reply with quote
Hi

If you are slowing down that much up hill then either it is a very steep hill or the gearing is too high (in which case changing to a lower gear might help).

An attempt to explain it follows!

You need X thrust to go at a particular speed. X being made up of the amount needed to overcome rolling resistance (ie, down to weight which is small and pretty much fixed whatever speed you are doing) and wind resistance (which increases rapidly with speed).

The engine produces thrust which is down to how much torque you have (NOT power), and then that is changed by the gearing. It is engine torque times the gear ratio (which is smaller in the higher gears). Hence as you use a higher gear (or use sprockets to give higher gearing) you get less thrust pushing the bike forward; but if you use a lower gear at the same speed you might well land up using revs well beyond those where torque is falling away rapidly.

So gearing is a balancing act.

All the best

Keith
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Walloper
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PostPosted: 08:25 - 14 Nov 2012    Post subject: Reply with quote
Lay off the Pies fatty... Karma
Get more supple so you can reduce your profile.
Become more Aerodynamic.
Tuck your knees and elbows in and get your big head down.

Probably find the gearing is set for optimal/general use. Or for what you are doing with it.
It would probably only be advantageous to modify the gearing for use on a track or special special circumstances.

Your main issue is the power (The overall combination of Torque/Brake Horse power Stuff) the engine can produce.

As Kickers says, it is like swings and roundabouts. Lower gearing more appreciable oomf when accelerating and going up hills but rubbish cruise and top speed.
Higher gears will give rubbish acceleration and hill climbing ability but superb cruise and top end. (So long as the head wind is not too severe as wind can act like a hill or heavy weight).

Get down the gym and eat more fruit
and veg. Smile
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garth
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PostPosted: 09:43 - 14 Nov 2012    Post subject: Reply with quote
De-restrict it...?
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Rogerborg
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Joined: 26 Oct 2010
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PostPosted: 12:01 - 14 Nov 2012    Post subject: Reply with quote
It's going to come down to where the peak torque and power are on your bike, and getting the gearing sorted so that you hit them at (or just before) your desired top speed.

Lengthening the gear ratios might give you a bit more top end, but the most likely result is that you'll just never get to the, uh, "powerband" in the top gears, and so won't use them anywhere except on downhill runs. It'll do nothing for hill climbing, that's just a lack of torque and power.

You may even need to shorten the gearing if you want to use 6th effectively. Again: it'll be entirely dependent on your engine's performance characteristics and the current gearing, you'll need to have a think about that yourself.

What I will say is that lengthening the gearing might make the bike feel bit less frantic. Going from 15 to 17 teeth on the front sprocket of my 125 made it feel much more relaxed, but for actual performance increases you'll need more torque and power, not more gearing.
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barrkel
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PostPosted: 12:50 - 14 Nov 2012    Post subject: Reply with quote
Kickstart wrote:
The engine produces thrust which is down to how much torque you have (NOT power), and then that is changed by the gearing.


Power is relevant rather than torque because of the gearing.

Max torque at the wheel is achieved at max power RPM from the engine with gearing to match the road speed.

Peak torque RPM from the engine is mostly irrelevant information; when RPMs are at peak torque for any single gear, you can usually get better acceleration / more wheel torque by downshifting. Getting the RPMs to peak power by adjusting gearing gets you what you want (max acceleration or max speed).

An engine could have the torque of a hamster wheel, but at a high enough RPM, it would have monstrous torque when geared down.

Torque curve OTOH is very relevant for what a bike is like to ride.
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Kickstart
The Oracle



Joined: 04 Feb 2002
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PostPosted: 16:47 - 14 Nov 2012    Post subject: Reply with quote
Hi

I was trying to keep this simple!

barrkel wrote:

Power is relevant rather than torque because of the gearing.

Max torque at the wheel is achieved at max power RPM from the engine with gearing to match the road speed.


Max torque at the wheel (which is essentially thrust) is down to torque times gearing. When that equals drag you land up at top speed, while power is a bit of a proxy measure as it already includes gearing. While that might well work out to be peak power rpm = top speed rpm, if you can't pull the gearing before those revs to get there it isn't a useful figure.

If you have a totally flat torque curve then at 10000rpm you have double the power you have at 5000rpm, but the thrust is exactly the same on the same gearing. The thrust is what the rider feels.

Using power to work out thrust to work out gearing just gives you extra numbers to try and remove from the calculations.

barrkel wrote:
Peak torque RPM from the engine is mostly irrelevant information;


Agreed, but I never mentioned peak torque. I just mentioned torque, and that varies with rpm by an awful lot less than power.

barrkel wrote:
Torque curve OTOH is very relevant for what a bike is like to ride.


And is pretty much the critical thing for gearing. Can happen that the torque curve is such that the bike cannot actually reach top speed as the thrust below the revs needed for peak power is insufficient.

All the best

Keith
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Walloper
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PostPosted: 16:54 - 14 Nov 2012    Post subject: Reply with quote
Anyone fancy nipping out for the popcorn before we get started here?
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Kickstart
The Oracle



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PostPosted: 17:31 - 14 Nov 2012    Post subject: Reply with quote
Walloper wrote:
Anyone fancy nipping out for the popcorn before we get started here?


Can if you want, but what he is saying is pretty legitimate. Just that for working out whether your chosen gearing is usable the easiest thing is to take the torque graph and from that get a thrust graph, rather than trying to work out a power graph which can be increasing even when thrust is falling away rapidly.

All the best

Keith
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barrkel
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Joined: 30 Jul 2012
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PostPosted: 18:00 - 14 Nov 2012    Post subject: Reply with quote
Walloper wrote:
Anyone fancy nipping out for the popcorn before we get started here?


I think Keith and I are roughly in agreement, just talking about slightly different aspects of the implications in practice.

I remember once upon a time reading a monstrous thread on some other forum - must have been over 10 pages - mostly between two guys debating whether peak torque vs peak power was more relevant, or some such. What the guy on the torque side missed was that torque on its own is meaningless, you have to take the RPM into account; and if you use gearing to halve the RPM, you double the torque. So if you have the ability to change gears[1], power is the only interesting number, since power is torque x RPM (x some constant to suit your desired unit of power).

[1] In an ideal world, the gearing would be infinitely adjustable and keep RPM constant for max acceleration, and the fastest bikes would all sound a bit like scooters Smile
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Walloper
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PostPosted: 18:05 - 14 Nov 2012    Post subject: Reply with quote
Kickstart wrote:
Walloper wrote:
Anyone fancy nipping out for the popcorn before we get started here?


Can if you want, but what he is saying is pretty legitimate. Just that for working out whether your chosen gearing is usable the easiest thing is to take the torque graph and from that get a thrust graph, rather than trying to work out a power graph which can be increasing even when thrust is falling away rapidly.

All the best

Keith


Oh I can see the bones of a conversation.
But invairably when a Torque v Power chat starts folk confuse the two.

There is, unfortunately no simple explaination for Toque v Power.
They appear on graphs together but really it is easier to think of them as un-related until one understands each property for what it is.
Most folk will tell you about some car or bike they were in that was very powerfull. The language is wrong. They may actually have been better to say the car was very torquey (Not the place in the South of England)

Basicaly, Torque is easier to think of as a long lever versus a short lever with the same amount of load acting on it. The longer lever will apply more 'thrust' then the shorter lever for the same load X.

Power is how far you have to pedal to get to the end of the levers to put the load on.

So if a engine with a high torque gives thrust at low RPM say 5000rpm
A Low torque engine will have to run faster to give same thrust. 100000 RPM.


The gearing effect of torque is related to the throw of the crankshaft.
(which is the same principle as the sprocket teeth.)

More 'effort' (Bang from the combustion process) applied to a long throw crank will provide more torque than a weaker 'effort'.
The difference between turbocharged and naturally aspirated.

I am not doing a Tefers on this one sorry.

Better to GOOGLE dis shit. Smile

Facetube has animations etc. to better illustrate the fernomimums involved.
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Last edited by Walloper on 18:27 - 14 Nov 2012; edited 1 time in total
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Kickstart
The Oracle



Joined: 04 Feb 2002
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PostPosted: 18:08 - 14 Nov 2012    Post subject: Reply with quote
barrkel wrote:
What the guy on the torque side missed was that torque on its own is meaningless, you have to take the RPM into account


Common mistake people make, and one those who like diesels (or high torque low revving cruiser bikes) seem to be particularly prone to. Not helped by Clarkson! Knowing the rpm that torque figures equate to is probably more important than for the rpm power figures equate to.

barrkel wrote:
[1] In an ideal world, the gearing would be infinitely adjustable and keep RPM constant for max acceleration, and the fastest bikes would all sound a bit like scooters Smile


Ideal for performance, not for fun though Laughing .

All the best

Keith
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Walloper
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PostPosted: 18:24 - 14 Nov 2012    Post subject: Reply with quote
barrkel wrote:
Walloper wrote:
Anyone fancy nipping out for the popcorn before we get started here?


I think Keith and I are roughly in agreement, just talking about slightly different aspects of the implications in practice.

I remember once upon a time reading a monstrous thread on some other forum - must have been over 10 pages - mostly between two guys debating whether peak torque vs peak power was more relevant, or some such. What the guy on the torque side missed was that torque on its own is meaningless, you have to take the RPM into account; and if you use gearing to halve the RPM, you double the torque. So if you have the ability to change gears[1], power is the only interesting number, since power is torque x RPM (x some constant to suit your desired unit of power).

[1] In an ideal world, the gearing would be infinitely adjustable and keep RPM constant for max acceleration, and the fastest bikes would all sound a bit like scooters Smile


Yes agree and understand this.

But we were presented with:

"Basically, since i will be ripping the old sprockets off and changing them, should I try a different ratio? I feel my gears are very similar (I have 6 to do 32 mph) and I would like to vary them out a bit more. I would love to be able to do a higher speed uphill as that's about 18 mph.

What sprockets would you recommend guys?"

I would NOT phaph about with the bike in question as the power/torque range is limited/narrow.
For a small engined commuter it's just a case of dealling with it. Coz if you make it good on hills you'll probaly hate it on the flat. And visa versa.

For a track we can fanny about with gearing to match 'Ooomph (the noise for 'Push' not Zero Zero Zero Miles Per Hour) from the engine to expected gradients/bends. For the high street I don't think I'd waste the beertime.

The issue with the bike in question is that the range of the power/torque delivery is too narrow.

To save a thirty page thread....
If hills are predominant on the commute then smaller front or larger rear.
But only one tooth either front or rear not both.
Try it and see. If you like it then good if not then swap back to OEM specs.

BTW I hate Popcorn. I'd much prefer a Hotdog. No ketchup but tons of mustard. Please. Ta.
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YBR Ric
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PostPosted: 18:47 - 14 Nov 2012    Post subject: Reply with quote
I wouldn't want to influence your descision in any way, but all being equal, a smaller sprocket wears out quicker than a larger one Smile
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barrkel
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Joined: 30 Jul 2012
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PostPosted: 18:51 - 14 Nov 2012    Post subject: Reply with quote
Walloper wrote:
But we were presented with:

"Basically, since i will be ripping the old sprockets off and changing them, should I try a different ratio? I feel my gears are very similar (I have 6 to do 32 mph) and I would like to vary them out a bit more. I would love to be able to do a higher speed uphill as that's about 18 mph.

What sprockets would you recommend guys?"


Yes, to answer the OP, to absolutely maximize your speed on that hill, you'd need to figure out (either with mathematics or playing around) how much power exactly is needed to overcome friction + drag, then have a gear that maps peak engine power to that speed, + some close gears below that to bring the speed up so that gear could be used.

But such a setup would be optimized for that exact hill and for the assumed wind speed, and would no longer be optimal elsewhere. So there's little point in it. Only so much you're going to get out of a 50cc engine, and if the gears are already fairly close (which they will be since the speed range isn't very high), there's likely little to be gained. Switching sprockets will just shift the optimums around a bit - you may get lucky for that specific hill, but there will be another you're slower on.

And you'll almost certainly either lose low-speed acceleration or top speed, depending on which way you go - and it's not obvious which one is the way to go, because the gearing that maps that road speed to max power RPM will currently be somewhere in the middle, not at either end, so finding the optimum with sprockets rather than gearbox changes is just shifting around the peaks, as it were.

I toured Wales (on my way to Ireland) on my SH300 scooter, 27HP 280cc engine. It still fell back to 40mph at full throttle on one very steep hill.
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Walloper
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PostPosted: 19:05 - 14 Nov 2012    Post subject: Reply with quote
barrkel wrote:
Walloper wrote:
But we were presented with:

"Basically, since i will be ripping the old sprockets off and changing them, should I try a different ratio? I feel my gears are very similar (I have 6 to do 32 mph) and I would like to vary them out a bit more. I would love to be able to do a higher speed uphill as that's about 18 mph.

What sprockets would you recommend guys?"


Yes, to answer the OP, to absolutely maximize your speed on that hill, you'd need to figure out (either with mathematics or playing around) how much power exactly is needed to overcome friction + drag, then have a gear that maps peak engine power to that speed, + some close gears below that to bring the speed up so that gear could be used.

But such a setup would be optimized for that exact hill and for the assumed wind speed, and would no longer be optimal elsewhere. So there's little point in it. Only so much you're going to get out of a 50cc engine, and if the gears are already fairly close (which they will be since the speed range isn't very high), there's likely little to be gained. Switching sprockets will just shift the optimums around a bit - you may get lucky for that specific hill, but there will be another you're slower on.

And you'll almost certainly either lose low-speed acceleration or top speed, depending on which way you go - and it's not obvious which one is the way to go, because the gearing that maps that road speed to max power RPM will currently be somewhere in the middle, not at either end, so finding the optimum with sprockets rather than gearbox changes is just shifting around the peaks, as it were.

I toured Wales (on my way to Ireland) on my SH300 scooter, 27HP 280cc engine. It still fell back to 40mph at full throttle on one very steep hill.


It's a Ped man..... There is not much to play with in there without remapping ...everything.
You cannot get two pints out of a one pint bottle. (walloper's 2nd law of Binge-ing)
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Hangear
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Joined: 16 Oct 2012
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PostPosted: 20:48 - 14 Nov 2012    Post subject: Reply with quote
Wow cheers guys, what a detailed response!
I was just wondering if it would be worth changing any sprocket ratio's. Judging by your answer, I should stick to the same ratio that the factory is.

Many thanks for the advice chaps!
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Walloper
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PostPosted: 10:52 - 15 Nov 2012    Post subject: Reply with quote
Hangear wrote:
Wow cheers guys, what a detailed response!
I was just wondering if it would be worth changing any sprocket ratio's. Judging by your answer, I should stick to the same ratio that the factory is.

Many thanks for the advice chaps!


Well if you are going to change your sprocket ratios you'll need to get the front sprocket off the gearbox output shaft.
To do this either get someone to sit on the bioke and hold the rear brake on whilst you pull on a long bar with a socket on it.

Bla bla bla bla...... Smile
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