This scooter project is really making me realise how much more complicated karts are than I expected!

First off, please don't laugh too much at my crude drawings. I was never very good with CAD, and paint is so much quicker!

So, yesterday I sighted up the chassis with the engine and axle and it strikes me that the engine has to go a long way forward to get the chain at a sensible angle. Here's what it looks like:


Chain problem

The green obviously being the approximate chain angle, although I've not got a rear sproket yet (or figured out how many teeth it will need so I'm guessing at the moment).

The problems are that the lower part of the chain will run very close to the casting where the red arrow is. It should be ok as the sproket is proud of the casing, but there are only a few mm in it. Also it means I can't put the sproket cover on but as it will be covered by bodywork I'm not sure that matters anyway.

Second problem is that the chassis rail is right in the way of the gear linkage, indicated by the blue arrow so really the engine could do with going back by at least 50mm, ideally a bit more to keep as much of it 'hidden' under the bodywork as possible, which would compound the problem of the chain.

Now keep in mind that the axle actually needs to sit the same height as the chassis rail to keep the wheels level, about 45mm down, which is going to make the chain angle even tighter.

So my initial reaction was to move the engine forward, but that's not ideal for a lot of reasons. Or I could move the engine down, better for handling etc (not that this thing will handle anyway so that's a bit of a joke!) but then there will be all sorts of problems with the linkages and it kind of makes the engine vulnerable if there were any bits of debris in the road. Conclusion: ideally the engine will sit at the height in the photo.

The other solution I can think of is to use some kind of idler on the lower part of the chain, like this:


Idler possible solution

(again, sorry for the crap drawing)

That would probably mean I could move the engine back 50-60mm solving the front-end clearance problems and get the major part of the engine under the bodywork. With any luck, the carb will also end up under the seat where it will get a little protection.

Is an idler pulley something that is 'done' on motorbike/kart/etc chains?

Would something like this do? eBay Item

Keep in mind that the chain will run on this constantly. I'm not sure whether an idler on a bike would normally do this or is just there to stop the chain going slack on full suspension movements? Will something plastic like that survive constant use?

Any words of encouragement (or otherwise) greatfully received! If there is an obvious gotcha here, please tell me before I plough forward and make a mess!

Chris

Can't see the Ebay linky Chris cos I'm not in the UK but if it's any help a lot of off road bikes use a frame mounted idler in pretty much the same place. The only real consideration is making the mounting strong enough to deal with loads generated on the overrun.

HTH (a bit)

Mike

Hi Mike

Thanks for getting back to me.

It's basically a little plastic idler pulley with a bearing in the middle, probably designed for the same thing as you mention.

On said bikes, does the chain run constantly over the pulley? Or just on the extremes of suspension travel?

Chris

Hi Chris

What frame are you using? Can you not modify that the get the bits where you want them? "Z" the whole back of the frame?

If it was not for the casing bit fouling you could put the engine as close to the rear axle as you like. I would not worry about how much "wrap" you get on the sprockets. I presume you don't want to/can't cut off the casing bit?

First prize is to position the engine offset from the centre line so that the sprockets can run outside of the chassis, which would maybe solve the gear linkage problem as well?

PS - I guess you are aware you need to make the engine position a little bit adjustable fore and aft to adjust the chain tension? Or if you use an idiler it can move in and out to do that.

Cheers

Fred W B



[Edited on 2/5/12 by Fred W B]

[Edited on 2/5/12 by Fred W B]

quote:

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Originally posted by Fred W B
What frame are you using? Can you not modify that the get the bits where you want them? "Z" the whole back of the frame?

If it was not for the casing bit fouling you could put the engine as close to the rear axle as you like. I would not worry about how much "wrap" you get on the sprockets. I presume you don't want to/can't cut off the casing bit?

First prize is to position the engine offset from the centre line so that the sprockets can run outside of the chassis, which would maybe solve the gear linkage problem as well?

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can you not put the engine "on top of" the axle,... so the drive goes down wards not back?

Just a thought

quote:

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Originally posted by Fred W B
PS - I guess you are aware you need to make the engine position a little bit adjustable fore and aft to adjust the chain tension? Or if you use an idiler it can move in and out to do that.

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quote:

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Originally posted by HowardB
can you not put the engine "on top of" the axle,... so the drive goes down wards not back?

Just a thought

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quote:

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I'm not sure what you mean by 'Z' the back - can you explain a little?

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Got it! Yes, it's possible, but to be honest mounting the axle isn't the biggest issue. It would be creating the 'drop' at the other end, keeping in mind I want to retain the front half of the chassis with the steering etc on it, and making it strong. Also I'd then have to have some way of supporting the bodywork. It's do-able, but it would complicate the project a lot!

Another consideration is that I really don't want to drop the ground clearance too much if I can avoid it.

It's something to think about though.... I might change my mind

By the way: I uploaded some photos. Look in my archive under 'scooter project'.

Chris

[Edited on 2/5/2012 by ChrisW]

[Edited on 2/5/2012 by ChrisW]

quote:

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Originally posted by ChrisW
Hi Mike

Thanks for getting back to me.

It's basically a little plastic idler pulley with a bearing in the middle, probably designed for the same thing as you mention.

On said bikes, does the chain run constantly over the pulley? Or just on the extremes of suspension travel?

Chris

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quote:

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Originally posted by MikeCapon

quote:

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Originally posted by ChrisW
Hi Mike

Thanks for getting back to me.

It's basically a little plastic idler pulley with a bearing in the middle, probably designed for the same thing as you mention.

On said bikes, does the chain run constantly over the pulley? Or just on the extremes of suspension travel?

Chris

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Yep that's the sort of thing Chris. From (ageing) memory the chain was just about touching at ride height so all the bump movement resulted in the chain resting on the idler. So the chain was touching the idler for the lower 75%ish of the wheels movement.

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I think you are asking that chain roller to do two things - best to follow a tried and tested setup IMO

Bikes use the following setup



chain roller close to the front sprocket and a rear chain guide to make sure it stays aligned even when a little slack (dont forget if you have the engine on runner system there is the potential it might twist slightly, putting sideways load on the chain (without the rear chain guide you could be looking at a plastic smashing derailment - both can be bought really cheaply here

http://www.pitbikeparts.co.uk/chains-guides-chain-adjusters-guides-tensioiners-rollers-c-33_60.html

the above setup means you dont have to run the chain supertight, this will reduce the load on the chain roller significantly

[Edited on 2/5/12 by Dangle_kt]

Hi

I don't think I'll have the space for a rear chain guide!

My point, I guess, is that my scooter doesn't have any suspension, so the chain will run on the roller all the time. Are such plastic rollers strong enough to hold up to this?

Chris

as i mentioned, it depends on the tension of the chain, and if its supported on both side - normally its only supported on one side, but running the chain too right can bend the "bolt" it runs on (this has happened to me once when I change rear sprockets and didn't adjust the chain)

Also the chain roller in the middle is unlikely to stop a derailment -but I guess it depends what you want the roller to to - (or just fit two if space is tight? one near the engine, one near the axle?) - also it doesn't need to be a bought chain guide, two bits of round either side of the sprocket would prevent a derailment, but as you were looking for off the shelf stuff, thats what I linked too.

On further research, I think I might be grossly overestimating the diameter of the rear sprocket.

I'm told that a pit bike would typically have a 14-15 tooth front sprocket and a 39-41 tooth rear. That gives a ratio of 2.6-2.9:1. A typical pike bike has a 450mm diameter rear wheel, so 1.4m circumference.

The scooter wheels are 330mm diameter, so roughly 1.05m circumference.

So, if my maths is right, I need roughly a 2:1 ratio front to back to achieve the same performance as a bike.

It seems the biggest front sprocket I can get is 18 tooth, so rear needs to be 36 teeth.

So something like this: eBay Item

Going on the picture, I guestimate that being 120mm diameter, which is a lot less than I was expecting.

I think another sight-up is in order to see whether, with a smaller sprocket, I'm creating more problems for myself.

Chris

if you find you need any pit bike bits or pieces your struggling to get cheap give me a shout, I have loads of old bits knocking round.

quote:

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Originally posted by Dangle_kt
if you find you need any pit bike bits or pieces your struggling to get cheap give me a shout, I have loads of old bits knocking round.

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Having now looked at the pictures, it makes more sense. If you are looking to get the motor as low as possible I would reposition that cross member under the engine and hang the motor so that the lowest point is flush with the bottom of the frame. A sheet of steel under it (welded to the frame) as a bash plate will protect it if necessary.

I have to ask, you are going to replace the seat with a tin top race car style one?

Re the gearing, I don"t know what RPM the motor turns, but at 5000 rpm/2.1ratio*1.037 circ = 2469 m/s, which equals = 148 km/hour, or 90 mph, so you may want to rethink the ratios?

Cheers

Fred W B



[Edited on 2/5/12 by Fred W B]

quote:

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Originally posted by Fred W B
A sheet of steel under it (welded to the frame) as a bash plate will protect it if necessary.





[Edited on 2/5/12 by Fred W B]

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but hang on, I haven't considered any internal gerabox reduction from crank speed to output shaft speed

Cheers

Fred W B

Am I missing something, or could you not just remove the bit of casting that is causing the clearance issue?

Erm.....
http://item.mobileweb.ebay.co.uk/viewitem?itemId=140743412499

Interesting find. I'd have had a bid on that if I'd have known!

eBay Item

^^ link for those of us on a PC. Owelly's link goes to mobile eBay.

Chris

quote:

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Originally posted by Fred W B
Re the gearing, I don"t know what RPM the motor turns, but at 5000 rpm/2.1ratio*1.037 circ = 2469 m/s, which equals = 148 km/hour, or 90 mph, so you may want to rethink the ratios?

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maybe I'm missing something but if it's rigid rear end then chain angle doesn't matter as it's in constant tension ( as in a chain drive diff ) to tension/adjust the chain, you only really need say 2 links play can you not slot the mounting plate for the rear axle bearing carriers

The problem was more that with the tight chain angle I was worried that it would hit the engine casing, but I think I've resigned myself to the idea that, if it proves necessary, adjusting the clearance using an angle grinder isn't the end of the world. I didn't really want to do it that way, but it's a whole load of effort to go to for a 'proper' fix.

Chris

If you are worried that a plastic guide wheel will wear, why not just use a small free-running sprocket?

Gearing sounds right now.

I would also prefer a proper idler sprocket, rather than a plastic roller.

With such a short chain run, you have to be carefull to keep the sprokets well aligned in the longitudinal direction. It is easier to move the engine to adjust the chain slack, rather than trying to keep the axle square in the chassis if you move that for chain adjustment.

With the solid axle you need to keep the axle square in the chassis so that the contraption runs straight.

Cheers

Fred W B

[Edited on 6/5/12 by Fred W B]