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Motorized Bicycle General Discussion All topics regarding bicycles with engines.

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Old 12-29-2010, 11:22 AM
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2door 2door is offline
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Default Re: how much better is this?

I have a question regarding the 'shim' idea. If the builder takes the time and mounts the engine correctly, that being, making sure the inside diameter of the engine mounts fit the frame perfectly, something I stress by the way, how in heck are you going to 'shim' the rear of the engine and keep the front mount saddled against the down tube like we all suggest. Full contact between the engine and the frame is important but if you stick a shim/s between the rear mount you've effectively changed the angle that the engine would normally sit. That's what you do to shorten, adjust the chain tension, correct? Maybe you're relying on some misalignment in the front mount which will allow the engine to 'tilt' forward, but if that's the case then the front mount isn't setting how I feel it should for full contact throughout the inside surface.

The ideal method would be to have long drop-outs that allow for rear wheel position change, as do motorcycles or any industrial chain drive application. Unfortunately most bicycles do not have these and unless you're a fabricator capable of building this feature into a frame you're stuck with the dilemma of keeping two chains adjusted correctly. In the case of single speed bikes with no rear derailer or vertical drop-outs, this is where some means of tightening the engine drive chain becomes necessary.

I agree wholeheartedly that the kit supplied tensioner bracket/wheel is the weak link in the typical 2 stroke kits and coupled with the lack of mechanical skill we've all seen from numerous builders, it will remain the biggest stumbling block to a successful build, especially for the beginner or those with limited fabrication skills/equipment.

Going back to the dreaded 'drilling holes' discussion, Goatherder possibly missed my previous post when I said I will shun the idea of drilling holes to mount engines. That would be absolutely absurd, imo, and most others here.

Personally, I like the idea of having some means of adjusting the drive chain tension other than having to loosen engine mounts and 'shimming' them, or moving and realigning the rear wheel. That's one reason I like a chain tensioner but I have not used the kit supplied brackets or wheels on the last five bikes I've built. There are photos of the welded design I've incorporated into the my bikes and on one I actually used a part of the kit bracket and welded it to the chain stay. Tension adjustment then becomes a simple task of loosening one nut and sliding the wheel upward to tighten the chain.

There is just no easy way around this for some builders. They either have to utilize the kit bracket and hope for the best or take the advice given here by experienced builders who have lived through the initial bug-a-boo of keeping the chain tight and have found alternatives to the kit method.

This (tensioner) discussion will never go away and as long as we as builders/mechanics come up with ways to do things, there'll be differences of opinion on which way is best. This is where newbies need to devote the time to rear, read, read and make their decisions based on all the advice given here.

Like the man said. "Nobody's right, if everybody's wrong".

Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"
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Old 12-29-2010, 11:45 AM
msrfan msrfan is offline
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Default Re: how much better is this?

I'm in the process of trying to get hold of all the people I sold my motorbikes to, so I can reinforce the tensioner. The last three got 3/16" stainless pop rivets like the Giant in the photo. I'm going to try and not use the tensioner at all on my next bikes, but if I have to, I will span the frame with a custom mount eliminating the possibility of twisting. A stretched chain and or worn sprocket is just begging to jump teeth and bind. As we strive to mount our engines, sprockets and tensioners more solid, it's obvious that if/when there's a problem, the tensioner is the most likely weak link.

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Old 12-29-2010, 01:42 PM
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bairdco bairdco is offline
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Default Re: how much better is this?

the thing about the shim method that makes it work is the tiny amount you have to add to make a difference.

if your chain seems like you need to remove an inch of slack, an 1/8" shim will do the trick. (that's not an exact measurement, just saying that a little does a lot.)

by the way, i'm talikng about flat shims that go between the motor mount and the motor, not anything between the seat tube and the mount.

as far as it misaligning the front mount;

on a diamond frame where the motor requires no custom mounts and fits perfect, all the shim does is raise the engine slightly. by adding the shim, this would make the front mounted higher up on the downtube, but it's still at the same angle to the frame, so both mounts will seat into the tubes properly.

on a cruiser, 99% of them need a custom front mount anyway, so unless you've welded your own onto the frame and they have no adjustment, some minor modifications might have to be done, but as i said, a small shim will do a lot, so moving the front mount up or changing the angle very slightly can still allow it to seat properly.

if for some reason the shims don't work, the other option is changing the sprockets, as i mentioned. on a coasterbrake, switching up (or down) one tooth (from odd to even or even to odd) will usually take out just the right amount of slack.

if you go from an even numbered tooth to another even tooth, you'll end up adding links into both chains, and still have the exact same problem.

now, all of this is based on someone actually having a chain breaker in the first place. i think there's a lot of people using the tensioner because they don't have a chain tool, or don't know they exist so they just put the chain on, add the masterlink, then mount the tensioner to take up the slack.

i worked on a kid's bike who had never even seen a chain breaker. after taking out some links, his chains fit perfect with no messing around with shims or sprockets.

i also have never used a half-link. if i did, i'd put it on the pedal side. but the whole concept of a half link baffles me. i know it works, i've seen it work, but it's a concept that sends my brain reeling when i think about it.
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Old 12-29-2010, 03:18 PM
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wayne z wayne z is offline
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Default Re: how much better is this?

Originally Posted by wayne z View Post
Attachment 32244

Attachment 32245

here's a very easy way to stabilize the tensioner without altering the bike frame in any way.
I used a large crescent wrench below the adjustment slot to tweak the bracket to parrallel to the chain.

I was going to use the shim method instesd of the tensioner, but with the curved front downtube,the engine mounts only fits the frame right in just one place.

Wayne Z
This way is VERY easy, VERY cheap, VERY reliable
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Old 12-30-2010, 11:15 PM
Tacomancini Tacomancini is offline
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Default Re: how much better is this?

I had to use the chain tensioner, just cause of chainstay clearance. Its a tight fit even though I spread the frame. I didn't drill, but I agree that I need to find a better solution. Probably welding something permanent.

But I did one thing that I'd recommend to anyone when they first start out with the build. At the very least swap the two tensioner bolts and nuts for grade 8 stuff from home depot and tighten the hec out of it. Then check it to make sure its solid before every run. Never trust the stock hardware, it will strip, it will loosen and you can't tighten it enough. I would guess with folks' first build they just try everything stock. I don't think they should be shipping with such low grade tensioner hardware. I think thats the deal breaker right out of the box. Until a better tensioner ships with the kits, I'm afraid many folks will install the thing as is.
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