tomos sprocket and mb5 wheel alignment woes

so as you may know i currently have a magnum project with mb5 swingarm, tomos a55 motor, mb5 rims and a bigport mk80 kit in the works. i had to have a custom sprocket made for the mb5 wheel and the smallest i could go was a 32. as you know tomos works with much smaller rear gears than that however tomos use 16 inch rims and i am using 18 inch rims so at least i have that going for me.

now the problem i am runnning in to is that (assume we are looking at the bike from behind) the rear gear is pretty far to the left even if i change the spacing and push the wheel to the right about an inch which is as far as it can safely go without brake arm issues. i was planning on using a 31tooth front sprocket as this would help give the bike more top speed. well as it turns out the 31 tooth only seems to be available in the "flat style" and not the "offset style" i would assume this is because on an a55 with a55 ignition you could not go up to a 31 without hitting the external pickup. i was planning on converting to a35 ignition to solve this.

I should also mention that the tomos motor is thinner at the mount than a puch motor. so i am also pushing the motor as far to the left as i can instead of evenly spacing it out. so after pushing the motor to the left and the wheel to the right, it still will not align with the rear gear. there is maybe an inch difference =(

with an offset sprocket it at least seems close enough to run but the only offset sprocket i have is a 26. that would be 26x32 gearing which would be too much torque.

so here is the math. stock tomos sprint 08 a55s come with 26 front and 28 rear which is kinda torqie for a tomos. 16 inch rims with 2.5 tires have a diameter of about 21 inches so

21 inches X 26t front sprocket = 546 / 28 rear sprocket = 19.5 GEAR INCHES

my setup is currently 26 front and 32 rear with 18 inch rims whoch give a final diameter of about 24 inches. so...

24 inches X 26t front sprocket = 624 / 32 rear sprocket = 19.5 GEAR INCHES


you can understand why i am upset. i dont want a cannon that can only go 45mph. i need some creative options to either raise the front gear as i can not lower the rear.

So it is maybe possible to remove the rear sprocket studs and somehow (no clue) grind the rim down torwards the center evenly on all 3 spots that the sprocket mounts to effectively push the sprocket to the right which would allow me to run a 31 front. is there any way to do this safely? i assume even the slightest uneven surface will cause disaster for my chain.

The other option would be to somehow offset the flat front 31 tooth front sprocket? has anyone ever done this or thought about it? i havent even pulled a tomos front sprocket before so i am not sure exactly how they mount but i dont think this is something that can easily be done.

am i missing something? obviously i will go with the biggest rear tire i can but it wont make much of a difference.

damn thats alot of text.

Re: tomos sprocket and mb5 wheel alignment woes

here is the rear sprocket area. as you can see it looks like there is material that could be ground down assuming the threading goes in far enough.

here is the difference between a flat tomos sprocket and an offset one.

Re: tomos sprocket and mb5 wheel alignment woes

I say take the wheel to a machine shop and have them grind it down like your talking about. or if you do it just get if EXACT. I wouldn't even consider your other option.

Re: tomos sprocket and mb5 wheel alignment woes

How far do the threads go before they bottom out? I don't think you would need more than a half inch of thread on those sprocket bolts. If you can, I'd have that hub milled down. Joe

Re: tomos sprocket and mb5 wheel alignment woes

as long as the studs go in deep enough, i'd say you'de be fine in machining the hub down. i'd either do it on a lathe or with a mill to make sure it's as flat as possible

Re: tomos sprocket and mb5 wheel alignment woes

you realize the answer to all your problems is getting some customs spoked wheels made. WAY lighter. Can get front and rear disks. Way better looking (mags on cafe racers are dumb). Fix sprocket and alignment problem.

Pricey, and a lot of work, but....

Re: tomos sprocket and mb5 wheel alignment woes

hehe tommy, i might consider it if i hadnt just bought a pair of mbx wheels. the mbx has the exact same design so i am just using the mb5 rim as an example. they are very very light wheels and would suprise me if they weighed much more than spokes.

but thanks for the advice all. i guess it shouldnt be too expensive to have a machine shop do that. i just gotta get the studs out and replace them with allen bolts or something. if i can fit a 31 front it would be the equivalent of running 27x24 on regular tomos so that should at least get respectable top speed but it wont be setting any records.

i dont suppose there are alternative main gears from a different model? ahh thats a whole nother post i guess.

Re: tomos sprocket and mb5 wheel alignment woes

Cupermatt o /

dunno if you remember harold but you were looking at my bike at the creature rally. Silver magnum with the mb5 stuff.

Anyway what I did was take a ruler and an angle grinder to the 3 surfaces where the sprocket meets the wheel. Super precise I know, but it's held up fine for 2 rallies and alot moer blasting. The studs actually go pretty far into the wheel. Just rip them out and get some quality bolts to replace them.

I'll post some pics tomorrow with some measurments.

Re: tomos sprocket and mb5 wheel alignment woes

hahahaha well i thought of doing that too i just figured it wasnt very safe and if i fucked up i would be out one set of 150$ rims from europe. i guess it couldnt be that hard to do it myself.

Re: tomos sprocket and mb5 wheel alignment woes

Harold when I wanted to run a 31tooth on the front of my bike I ran into a similar problem with my street mate wheel. I made an off set 31 by welding a 31z sprocket to a regular tomos sprocket, using nuts to space it out. it worked grate. problem is, if you produce a bunch of power and run it threw a tall ass set of gears (I had 31/20 gearing) it will blow the main drive in the center of the tomos cases and break it in a bunch of pieces. I hit 61mph before this happened, the next time i took off from a stop it was all over.

Re: tomos sprocket and mb5 wheel alignment woes

thats not a bad idea either! i like that. do youhave pics of the sprocket after welding? i dont think i will run into that issue because my gearing will get no where near that tall.

Re: tomos sprocket and mb5 wheel alignment woes

I was thinking you could make a short intermediary jack shaft with two different size sprockets on it. This would solve the alignment issue and could change the gearing but after what Mr Eversoul had to say regarding some inner bearing failure it causes a young mopeder to think.

Re: tomos sprocket and mb5 wheel alignment woes

A jackshaft would work, give you an opportunity to change the gear ration again, but that's another chain and tensioner.

Having the sprocket mounting on the wheel turned down with a lathe would be a pretty good idea too.

To throw another one into the mix for you, how about an adapter to let you use a motorcycle sprocket on the Tomos engine. You would be able to get a bigger stronger chain, and depending what you choose a wide variety of sprockets.

I'm thinking of a slug of metal drilled and tapped so it would thread on to what you have, and a sprocket that would either bolt on to that , or cut some splines in. Or, use a junk motor's output shaft that has the splines already cut and weld that to the adapter.

and clean up your welds. Quit burning paint!

Re: tomos sprocket and mb5 wheel alignment woes

no one has really ever taught me to weld. i just saw paz do it one time. watched a youtube video and bought a welder. my school is get thqat shit hot as fuck, and grind off the excess later lol.

Re: tomos sprocket and mb5 wheel alignment woes

it's pretty easy to weld, you'll get the hang of it once you do it more often. you have a mig or stick?

Re: tomos sprocket and mb5 wheel alignment woes

if you do a jack shaft you should switch over to belt for the final drive.

nobody has that

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