Drilling out a sprocket

Clutches, Chain & Belt Drives

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alexanderfoti
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Drilling out a sprocket

Post by alexanderfoti » Fri Oct 16, 2015 8:14 pm

Hi all

Having issues getting a custom sprocket to fit.

This is the layout for the rear sprocket on the Cb250N superdream:

Image

I obtained a hole saw and drilled a 70mm hole in the center of a 08b 76t platewheel, using a pillar drill.

I then placed the old rear pulley on top of the sprocket, centered it, and drilled the mounting bolt holes.

Unfortunately, I ended up with something that doesn't run true on the rear wheel, it shifts forwards and backwards approximately 5mm

So obviously this one is scrap, as it must mean the center hole is not central.

The pilot bore is a quite large 25mm, and the hole saw only has a 6mm drill bit in the center.
In addition to that, it turns out the wheel actually has a center boss diameter of 65mm!

any ideas on how to make the next one a bit better!

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Re: Drilling out a sprocket

Post by gilburton » Sat Oct 17, 2015 10:22 am

Cutting on a lathe is the only answer I can give.
Using a pillar drill is not really accurate enough.
Personally I would have just obtained a 19mm standard industrial sprocket to fit the final drive shaft and fitted the standard honda sprocket on a simple trial basis.
I'm no expert on gearing but most bikes are not far off with a diesel engine fitted.
My own MZ was only a tooth out to get better accelaration.
The other option( if you are sure of the ratios required) is to get the Honda centre machined out and a sprocket of the correct size mated with the Honda centre.
Of course this will cost something but there are plenty of custom sprocket people out there who could probably do this or make a complete custom sprocket.

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Re: Drilling out a sprocket

Post by sbrumby » Sat Oct 17, 2015 7:25 pm

I did it on my superdream, got the new sprocket and the original sprocket and decided where the best place to join up then lathed out new sprocket then lathed out old sprocket to suit make tight fit then place on flat plate and tack weld, fix to wheel and check ok before final weld.
Sam

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Re: Drilling out a sprocket

Post by alexanderfoti » Sat Oct 17, 2015 8:21 pm

Thanks for the input all.

The final drive ratio was 1:5.07 with the old belt.

To keep that I have to to have a 15t front sprocket with 76t rear. I would like to keep something around that ratio as the Aixam cvt drive is directly to the rear secondary shaft, which has the pulley on it, not a huge amount I can do there.

I wouldn't mind going higher or lower just a smidge, but there is nothing available off the shelf in those sizes.

In addition to the above issues, the superdream has a 70mm center hole on the sprocket, but uses a spacer to centrally locate it. Mine is missing the spacer, and the belt pulley I removed off the rear has a 65mm center hole, so either or should work really.

I have also made the chain smaller (428 from 520) as the sprocket size for a 520 chain on the rear would have been massive (400 cm!)

I have found a company online who can make a custom 428 sprocket with 70 rear teeth, which gives me 1:4.6, which will lower the gearing slightly, which is the opposite of what I wanted to do really. I have ordered this, as its not much more than the platewheel, and have give them the dimensions form JT sprockets catalog. I will install that next and see where it takes me, if the gearing inst where I like I will try a local bike place near me with a lathe, they may be able to make up a sprocket.

I think it all boils down to me needing a lathe. I had a place near me that used to do lathe work for me but they closed down which is a shame! I have another bike shop that may be able to make a 76tooth sprocket up.

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Re: Drilling out a sprocket

Post by alexanderfoti » Sat Oct 17, 2015 8:30 pm

Just realised, I can get a 1:5.0 ratio with a 14t front sprocket (the smallest I can go on a 08b(428) taperlock sprocket).

I will wait for the customer 70t rear to arrive and see what its like, if it doesn't negatively affect fuel economy or drive ability then I will do that! :)

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boutje
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Re: Drilling out a sprocket

Post by boutje » Sat Oct 17, 2015 8:37 pm

I `am just curious but how much cost a custom sprocket?

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Re: Drilling out a sprocket

Post by alexanderfoti » Sat Oct 17, 2015 8:39 pm

£51.44 without hard anodising
£61.44 with hard anodising.

Made from 7075T6 not steel.

The Hard anodising adds 2 weeks to the 1 week delivery time so I chose against it.

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Re: Drilling out a sprocket

Post by alexanderfoti » Sun Oct 18, 2015 9:53 am

Did some more testing today.

I have a radial run out of 2mm!

This is enough to cause the chain to go too slack at one point in the rotation. I believe the problem is made worse as the sprocket is so big. On a smaller sprocket it wouldn't be a problem I suspect.

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Drilling out a sprocket

Post by Stuart » Thu Oct 29, 2015 11:08 pm

I'm having to go big on the front of the Ruggerfield (35T) to spin the old 56T clutch up to give us a usable bike. 3800rpm up front gives us 2375rpm on the rear. Leaving the 29T up front is fine if you have the revs of a petrol :-)
34T up front might be better for an easier pull away but we have the torque to pull 35 so why not.
FYI: When it was belt driven it had a ratio of 1:1.59 with a 44 front & 70 rear clutch.
Stuart. M1030M1, Honda NC700S, Grom!, Toyota Corolla 1.4 Turbo Diesel. Favouring MPG over MPH.

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Re: Drilling out a sprocket

Post by alexanderfoti » Thu Oct 29, 2015 11:22 pm

My custom sprocket arrived. Turns out that the superdream uses a centering spacer and a circlip to keep the sprocket on rather than 4 bolts through the cush drive and spacer like most bikes. Tim had drillled through and bolted to the hub to retain the sprocket, forgoing the circlip.

I should have asked the custom maker to make a 65mm center. Not a problem, just waiting on a 70 to 65mm spacer.

It also means my first custom sprocket would have fit if I had cut a 65mm hole in the center :)

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Re: Drilling out a sprocket

Post by alexanderfoti » Thu Oct 29, 2015 11:24 pm

Stuart wrote:I'm having to go big on the front of the Ruggerfield (35T) to spin the old 56T clutch up to give us a usable bike. 3800rpm up front gives us 2375rpm on the rear. Leaving the 29T up front is fine if you have the revs of a petrol :-)
34T up front might be better for an easier pull away but we have the torque to pull 35 so why not.
FYI: When it was belt driven it had a ratio of 1:1.59 with a 44 front & 70 rear clutch.
The same problems as me then. Nice to have a play with final drive gearing on a cvt as it's something I haven't don't before. It's still out of action until my other bits arrive so will see how it changes the cvt engagement etcetc.

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Re: Drilling out a sprocket

Post by alexanderfoti » Sat Oct 31, 2015 8:30 pm

65 to 70mm spigot ring adapter arrived today and it sorted out the run-out on my custom made sprocket.

It would seem that my original sprocket would have likely worked with this spacer as well I think.

428 o ring chain fitted. Will see how long it lasts.

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