To "rubber" or not

This is a subject that's been making me nuts since the first time I mounted a motor on a bike. I can argue for both sides but I honestly don't understand the logic behind solid mounting. I not saying it's wrong I just don't understand it; maybe I'm doing something wrong. I would really like someone to explain the benifit of it. With solid mounting the motor stayed planted but the vibes were unbearable. Nobody is going to convince me that high frequency vibration isn't going to destroy a motor and or bike over time with stress cracking etc.
I used inner tube and it was worthless. It reduced vibes for a short period of time but the motor moved around too much and the rubber disinergrated within a few days and then it vibrated worse than ever. The next thing I tried was leather strap which worked way better than tape or inner tube. It did disinergrate but it took all summer to do so. At first I had to tighten the mounts often until the motor settled in but after that no more issues. It still vibrated but it was much more dull.
I remember a guy on here that made mounts with polyurethane bushings. I wonder how that worked out? That seemed like the way to go. Also the vibration issue were worse on certain bikes. The cheapo 70's open road canterlever frame was the worst by far and my '50 Schwinn was a distant second. The smoothest was on a "Next" mountain bike. I needed to use the over size tubing plate on that bike, I wonder if that was the reason? I dunno.

Solid has always worked best for me. When I build a bike I don't want some raggedy old inner tube hanging out from my mount. I build quality bikes. I have never scratched the paint either. But I always use the custom billet motor mounts. Attention to detail always wins out.

ruppster said:

This is a subject that's been making me nuts since the first time I mounted a motor on a bike. I can argue for both sides but I honestly don't understand the logic behind solid mounting. I not saying it's wrong I just don't understand it; maybe I'm doing something wrong. I would really like someone to explain the benifit of it. With solid mounting the motor stayed planted but the vibes were unbearable. Nobody is going to convince me that high frequency vibration isn't going to destroy a motor and or bike over time with stress cracking etc.
I used inner tube and it was worthless. It reduced vibes for a short period of time but the motor moved around too much and the rubber disinergrated within a few days and then it vibrated worse than ever. The next thing I tried was leather strap which worked way better than tape or inner tube. It did disinergrate but it took all summer to do so. At first I had to tighten the mounts often until the motor settled in but after that no more issues. It still vibrated but it was much more dull.
I remember a guy on here that made mounts with polyurethane bushings. I wonder how that worked out? That seemed like the way to go. Also the vibration issue were worse on certain bikes. The cheapo 70's open road canterlever frame was the worst by far and my '50 Schwinn was a distant second. The smoothest was on a "Next" mountain bike. I needed to use the over size tubing plate on that bike, I wonder if that was the reason? I dunno.

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Ok from what Ive seen posted it goes something like this. The motor is making the vibration and by using rubber you end up focusing the vibrations at the motor mounts and that seems to break the mounts. When the motor is solid mounted the vibrations are spread thought out the frame saving the mounts. Atleast thats what Ive seen posted.

The way I see it ''some of'' this experimental rubber mounting produces lower bass frequencies. That can even crack a frame. This sorta thing takes the hardware with it. When it is mounted solid to the frame solid simply means no rattling. At this point it is no longer rattling it is solid. At which point putting some sort of vibration deaden-er like motorcycle bars get on real motorcycles. This takes care of the high frequency vibrations.

Bass comes from more movement and is more violent its simple physics 101. Watch a sub woofer move then go look for movement in a tweeter audio speaker. Which speaker rattles everything loose in the car?

I have some high compression rubber patches that look like inner tubing. Think thin strips. It's not the same as tire inner tubing. The only place it works correctly in my opinion is with nice meaty billet style mounts. I don't ever see it working with the stock china mounting.

I worked on and maintained 16 bowling machines for a while. I wish folks could see the work of physics there. When the bushings etc wore out on a shaker board every nut and bolt rattled out of a machine tears up everything.[ Move the pins to an elevator wheel for resetting]

I saw bolts that were not repaired or simply re-tightened which allows things to rattle. Every time something any thing rattled on these machines something broke. To the tune of rewedding and re-bracing the steel tubing on a machine. Funny thing is 98% of it was preventable. Those machines took a lot of preventive maintenance. A ton of oiling and inspecting. Not much different than these bikes lol. When they are slacked on it shows. I was the clean up man for a while then moved on.

It's funny, my Sportster is a solid mount, and I wouldn't want a new rubber mount one. But the reason is because they weight 100 lbs more than my bike, not because of the rubber motor mounts.

Sooner or later stuff will break no matter how you mount it because of the vibration. As long as you use heavy duty enough hardware you should be able to expect a reasonably long lifespan either way. When in doubt make it about 50 times stronger than it needs to be lol.

ruppster said:

This is a subject that's been making me nuts since the first time I mounted a motor on a bike. I can argue for both sides but I honestly don't understand the logic behind solid mounting...

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We have a reciprocating mass. Rubber mounts let the engine “vibrate” more, but you feel it less. You are isolating the vibration, not reducing it. If you bolt the engine solidly to a heavy mass, you actually reduce the amplitude. Even though it feels worse in your hands and butt, it is easier on the engine. The solid mounted engine will have less vibration induced problems, but the frame will have more. The heavier the frame, and the more solid the mounts, the less the engine vibrates.

Rubber mounts that isolate the vibrations from the rider will make for a more pleasant ride. They do nothing for the engine.

i've mentioned this plenty of times, but one of the biggest causes of vibrations in my experience is the exhaust. most people just let the pipe hang down and don't strap it to the bike anywhere.

the exhaust can make a sort of "wave" of vibration (for lack of a better term) and as it shakes, it multiplies the vibes back into the engine, and back into the bike.

if you don't believe it, hold the exhaust with your shoe when your bike starts vibrating (a little burnt rubber never hurt anyone) and you can feel it going away.

it's like hitting a big rock with an aluminum bat. even though the rock hits one end, you really feel it in your hands...

Thanks guys. I never thought I'd be able to say this but it makes sense now. Basically the theory is is you spread the vibration over a large area(the frame) to unload the intensity of the vibes from the motor. I'll bet the reason my motor was so smooth it that mountain bike is because the frame sat much tighter in the mount and distributed the vibes over a larger contact point.
Now that I have a new perspective I have another question. Where the mounts are so large on the motors for an old bike wouldn't you still be better off to put some stiff material on the frame or the mount to take up the slop. With the theory of distributing vibration through the frame I feel that having a large contact point (motor mount to frame) would be even more important so it could transfer the vibration effectively. Otherwise the vibration would be transfered through a very small contact area which to me would cause a buzzing vibe and would be amplified by the frame "like hitting a rock with an aluminum bat" instead being distributed to the frame. I don't know if that made any sense.

I worked on and maintained 16 bowling machines for a while. I wish folks could see the work of physics there. When the bushings etc wore out on a shaker board every nut and bolt rattled out of a machine tears up everything.[ Move the pins to an elevator wheel for resetting]

I saw bolts that were not repaired or simply re-tightened which allows things to rattle. Every time something any thing rattled on these machines something broke. To the tune of rewedding and re-bracing the steel tubing on a machine. Funny thing is 98% of it was preventable. Those machines took a lot of preventive maintenance. A ton of oiling and inspecting. Not much different than these bikes lol. When they are slacked on it shows. I was the clean up man for a while then moved on.[/QUOTE]

That's cool you worked a bowling alley. My friend at work did the tour of duty for 16 years doing the same thing you did. He loved it,but he's about deaf!

ruppster said:

Now that I have a new perspective I have another question. Where the mounts are so large on the motors for an old bike wouldn't you still be better off to put some stiff material on the frame or the mount to take up the slop. With the theory of distributing vibration through the frame I feel that having a large contact point (motor mount to frame) would be even more important so it could transfer the vibration effectively. Otherwise the vibration would be transfered through a very small contact area which to me would cause a buzzing vibe and would be amplified by the frame "like hitting a rock with an aluminum bat" instead being distributed to the frame. I don't know if that made any sense.

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Ya you have to make it solid so youll need to shim it with something hard. Metal pipe split dow the middle seems to work good but I think even a hard plastic might do.

Here's a pic of the shims that I made because the engine mounts were a little larger than the frame tube. I cut them from electrical conduit.
They can also be used on the front mount. They really help make the engine mounting nice & solid!
I use a larger shim on the engine side to to help distribute the engine vibrations over a larger area of the frame.

heres a pic of what i do for over sized frames like the cranebrook, for frames that the motor has a good fit i bolt them up tight with no rubber. thats sway bar end links from auto zone.

camlifter said:

heres a pic of what i do for over sized frames like the cranebrook, for frames that the motor has a good fit i bolt them up tight with no rubber. thats sway bar end links from auto zone.

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That really gives me ideas.

Venice I'll be taking that tip for my next Schwinn build. 2door I was planing to make that front mount on my Higgins. I knew I saw it on the forum before but I couldn't remember where or who built it. Wouldn't it be safe to say that the reason people rubber mount because the have slop in their mounts causing excess vibes? I guess I no longer have a argument for rubber mounting, I'm a believer. Camlifter wouldn't that front mount be considered a rubber mount? I know poly is hard but it seems like the rear studs would take a beating with that being the only place for the vibration to exit the motor.

the bushings are very hard, the guy i sold that bike to is local to me, he's ran it for over a year and has never had to tighten the mounting bolts, the rear mount bolts where retaped to 1/4-20.
the only times i've every had vibration or motor mount bolt troubles is when using the grubee motors, they seem to vibrate a lot. the raw or jet motors in my experience are much smoother.

The Grubee's vibrate MORE? It was my understanding that the Grubee's were the best kit. I wanted to try a Raw kit on my next build anyways so that's one more reason to give it a shot. By the way camlifter I forgot to complement your work in my previous post, looks like you put out some super high quality work. With that said I noticed a alot of you often have a car in the making out in the background. Very interesting! Brian

don't want to open a can of worms here, but, i hate the grubee engines, i have 2 sitting here now that i put on bikes and was unable to sell the bikes because the engines made so much rattle. people thought they were going to blow up. had to swap them out for raw motors to sell the bikes. just going to use them for parts. people seem to think that just because the grubee has a better finish on the castings that there better motors.
i stoped buying from kings and gasbike because no matter what engine you order they send you the grubee and call it an up grade. imo any off the off brand engines have preformed better and had less vibration than the grubee.