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Old 09-10-2009, 07:24 AM
Fabian Fabian is offline
Join Date: Sep 2009
Location: Australia - Melbourne
Posts: 168
Default Jackshaft Kit U-bolts


I have a Jackshaft kit installed on my bike - i love it; it works great and after having gears to play with, i'll never go back to a fixed drive system.

There is only one small problem to be addressed in the attachment of the Jackshaft kit to a bicycle frame and it's a saftey related issue.

Basically you can end up with very serious injuries if your down tube cracks and breaks open due to metal fatigue from a crush point/pinch point or just plain cracks from excessive pressure being applied on the U-bolt clamp.

When purchasing my 2-stroke engine kit, the manager advised that i use thick rubber under the U-bolts to prevent the frame cracking, particularly thin walled aluminium frames.
One of his customers over tightened the down tube U-bolt, cracking the frame and then rode the bike down the road.
A short time later the downtube seperated in half and the bike opened up beneath him, throwing him, head first into the bitumen.
He doesn't have a face left or any front teeth and is missing a good portion of his nose.

After hearing that story, i decided to create a simple fix for the problem so that scenario would not happen to me from a metal fatigue issue.
In the photos attached you will see that i have used 1mm sheet steel, formed around the lower downtube U-bolt and the face of the smaller seat tube U-bolt carrier.
The seat tube U-bolts can have the original plate steel Chinese engine mount modified so as to be a backing plate for the U-bolt, doing a similar job to forming sheet steel around the curved section of the U-bolt; saving you time.

For 1 & 7/8 U-bolt clamps, i used a sectional size of 60mm x 25mm on both the U-bolt and the backing plate.
You can't go all the way around and cover 180 degrees on both sides as the straight part of the U-bolt will then crush the plate steel into the frame tube on the sides.

For the smaller seat tube clamp i used a sectional size of 40mm x 25mm for the backing plate.

Once sheet steel is formed to the curvature off the U-bolts and backing plates, you need to secure them so they sit flat and square and evenly spaced along the radius.
I don't have a welder so i just used superglue to initally hold the plates in place (the surfaces being glued must be absolutely clean, spotlessly clean for the glue to hold).
Then i placed the clamps on the frame and tightened them up, so forcing the plates to make a firm connection with the U-bolts and backing plates.
Once the superglue dried (that stuff is incredibly stong) i removed the plates & U-bolts and applied 4 layers of gaffa tape to provide a level of smoothing for any subtle differences in the curvature and also to increase the friction level between frame surface and u-bolt assembly.
The glue is only holding the plates in place, it adds no structural integrity.

This method has big advantages as the chain tension clamps can easily be adjusted as the clamps don't dig into any tape wrapped around the frame tubes.
I found that prior to making the backing plate modification, the two sections of the plate simply forced their way through the gaffa tape wrapped around the frame and started cutting into the aluminium.
When trying to adjust the motor to get correct chain tension, the plates wanted to remain stuck in the groves they cut into the gaffa tape.

Someone with good welding skills should make up a batch of these U-bolts and approach manufacturers of Jackshaft kits to have them included in the package, on the basis of saftey.

Hope this post provides someone with (at very least) peace of mind when installing a jackshaft kit.

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Old 09-10-2009, 07:35 AM
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Pablo Pablo is offline
Master Bike Builder & Forum Sponsor
Join Date: Dec 2007
Location: Duvall, WA PNW
Posts: 3,649
Default Re: Jackshaft Kit U-bolts

Good ideas. There are many ways to spread the load of the u-bolt clamp. Just for clarification:

Our Shifter kit uses the stock front mount, and the guy who cracked his frame and lost his face was not using our product and was not our customer.
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Old 09-10-2009, 07:59 AM
Fabian Fabian is offline
Join Date: Sep 2009
Location: Australia - Melbourne
Posts: 168
Default Re: Jackshaft Kit U-bolts

That is correct.

The guy who took his face off, was just using the standard fixed drive, Chinese engine kit.

In my opinion the SickBikeParts kit actually adds saftey to motor bicycling because if you are in a situation where need to accelerate to get out of the way, you can just upshift a few gears and put yourself in a safe place.

In city traffic this has helped me many times and with the trailer it is an absolute necessity.
With fixed drive you wouldn't even get moving.

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Old 09-10-2009, 09:53 AM
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Ghost0 Ghost0 is offline
Motorized Bicycle Elite Member
Join Date: Mar 2008
Location: Bellingham, WA
Posts: 764
Default Re: Jackshaft Kit U-bolts

As usual you bring up some valid points. We do touch on this subject in the manual and especially caution when using aluminum frames. We do recommend using the stock strap from the engine kit but when it comes to safety you can never be too cautious.

The functional issue here is that when you increase the surface area of the clamps they no longer offer as much clamp force and can allow the engine to slip on the seat tube putting too much pressure on the chain adjuster. It is a fine line there. Typically what I do is increase the support of the lower clamp because it does not have the added support of the seat post but leave the upper clamp unmodified so that enough clamp force can be applied.
Design is easy, execution the hard part.
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Old 09-10-2009, 10:11 PM
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Newbie Newbie is offline
Motorized Bicycle Member
Join Date: Apr 2009
Location: Fountain Hills, Arizona
Posts: 41
Default Re: Jackshaft Kit U-bolts

anyone have a link to that story? It kinda makes me sick to my stomach, and it makes me want to wear my motorcycle helmet.
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Old 09-11-2009, 05:21 AM
Fabian Fabian is offline
Join Date: Sep 2009
Location: Australia - Melbourne
Posts: 168
Default Re: Jackshaft Kit U-bolts

There is no link to a newspaper article as it did not make it into the newspaper.
In the city of Melbourne (Australia) you only make it into the news paper if you are driving 5 miles over the speed limit, ok, not 5 miles, but 20 miles (30 kilometers) over the limit will get you in the newspaper especially if it's on a motorised bicycle - the roads authority wants a complete ban on liquid fuelled bicycles and any assisted bicycle that travells over 20 kilometers per hour (15 miles an hour).

The story was graphicly conveyed to me by the seller of the Chinese engine kits in my area.

I wear a full face motorbike helmet and motorbike jacket with armour and steel capped boots when on my motorised bicycle.
Those bicycle helmets are a complete joke and a waste of money - they don't protect your head in any real way as the straps allow the helmet to move about on your head with a decent impact; just rotating out of the way and your head takes all the force.

Yes, the motorbike clothing is heavy and uncomfortable but i value my safety and won't compromise on it.

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