Padded Cranks ???

[J1MS]

Is anyone else running home brew padding on their Lambretta crank.???

Ive put Balsa wood and Araldite onto one of mine to make it a sort of full circle crank... Is anyone else doing this.???

[sean brady scooters]

what crank is that jims..?

[J1MS]

Indian webs with a Yam 110 rod...

[sean brady scooters]

GP type then...?

[J1MS]

Sorry Sean, Yes its a GP crank...

[sean brady scooters]

any chance of some pics,as to how it was done/achieved........
ive never tried that myself on a gp crank.........done lots of the older /other types using wine bottle corks plus araldite tho..
with the gp crank ,its a different matter .....its not just plugging up round holes in webs,but actually making /forming a section to fit in to the 1/4 circle recesses.......
quite time consuming and tricky i would imagine.....
anyway some pics of this would be interesting.....

[J1MS]

Two more cranks to do, when they come back from Reedspeeds. I will try (if I can get the glue off my hands) to show how I went about it on these (with a few pics), when I get them back... The other crank looks a bit sad (not very tidy) as I used strips of Balsa wood, laid side by side, as it was all I could only get from the model shop... But you can cut it with a sharp Stanley blade...

[oioivespaboy]

Knowledge posted some fantastic pictures of his padded GP crank with TV rod on the massive sr190 thread on the LCGB boards. Very well explained and great photos.
Here's the thread
http://forums.lcgb.co.uk/phpBB2/viewtop ... ding+crank


Using balsa wood/cork is just asking for trouble. If you look at any of the japanese moto x bike cranks thesedays they are all padded with plastic. It's very easy to shape and will never fall apart. Good for high reving engines.

I was tossing up whether to pad my GP crank with 115 yamaha rod but it turned out to be far too complicated as I got the crank dynamically ballanced and that was going to drag the process out even more.
My tuner wanted to use a material called Malory which is a lightweight steel. Stronger than plastic and better for torquey engines.

Because I didn't pad my crank and they removed material to balance it I need some advice on an exhaust that will work well with my low primary compression (sorry to go off track a bit)

Cheers

[Knowledge]

I note that the pictures don't work any more, so I have re-drafted it (a little) with fresh links to the images.

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Here is my approach to padding a crankshaft. It might prove helpful, or provide a basis for discussion.

The GP crank needed a TV rod to suit the SR190/Suzuki TS185 piston conversion. The TV rod creates an additional 30cc of volume BELOW the piston, which reduced primary compression. Padding the crank using the method below increases primary compression by 40cc.

First I split the crank at Birtoni Scooters (01206 790867) and the roughly cut some 9.5mm thick soft ali plate into triangles to suit the recesses in the GP crank (note blank for two triangles sitting on the back of the vice)


The back of each triangle needs to be shaped and chamfered to allow it to sit correctly in the crankweb recess. Note how each triangle sits better in the photo below than on the above picture (and check out the b****** files in the background - excellent for ali)


Prior to taking the photo below, the crank and triangles were held together and drilled with a 4.2mm drill. Next remove the triangle and tap the hole in the crank webs to M5.


Re-drill the holes in the triangles to 5mm, and countersink them. Do a dry run, and when you are happy, mix up some araldite before glueing and screwing the pads into the crank.


Let it cure, and then put each half of the crank in a lathe and turn down the oversized alloy until it is the same diameter as the steel crank web (OK, you could do this with a b****** file, but I've gotta a lathe and I'm gonna use it).
Below is the finished item after Graham Best has squeezed it together and hit it with a hammer.


I was a little surprised that he doesn't use shims at the bottom end. I'll ask him about this, but then he has done hundreds of cranks.
I hope this has been informative.

Martin Leech

PS. Despite not following a balancing routine for this crank, it has proved to be very smooth, though this might be as much to do with the con-rod length and piston weight.

[oldbiker]

Yes I have used the balsa wood etc, I`m not sure but I think that Roy Bacon recomended this in his books as far back as the early 60`s.

The advantage of balsa wood is that if it does come lose it will be broken up and spat out of the exhaust without damaging anything else.

Having said that I never had any problems, slightly roughen up the steel and make sure it is clean, before you glue the balsa in place and make sure that the balsa is covered with araldite to prevent petrol being absored by the balsa