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Modified Cub Rear Swing Arm


brucey
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I want to widen and lengthen the rear swing arm on my trials cub. I intend to do the cut and shut myself then take it to a local fabricators for welding.

Can anyone tell me where to cut and what to make. I have a pretty good workshop with a lathe and small milling machine.

I have 2 standard trials swing arms to play with.

I was thinking of removing the LH (brazed in?, cranked) arm from one unit and sticking it on the RHS of the other but this seems wasteful.

I have seen a sketch on another post but it doesn't give any sizes.

What do you do with the rear shocks when the swing arm is widened as the bottoms won't line up with the tops! Are both sides widened by the same amount?

As far as I can tell, about 25mm is added to the length. Where is this added and how?

Is the swing arm pivot mounting width affected (I intend to fit a Sammy Miller swing arm bush kit)

Any help would be appreciated. A couple of photo's would be great!

Bruce.

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Bruce,

I'm missing something. What do you gain by widening the swingarm? I put a stock Cub swingarm on my Bantam and the stock 4" x 18" Trials tire fits fine. Not sure what you gain by lengthening it either but the Cub guys can fill us in there. I would think your turning radius would be increased and it would not be as inclined to wheelie the front end (but I really can't picture that being a problem with a cub).

Interesting project.

Alan

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The way PJ the Outlane guru widens the swinging arm is by cutting vertically from the rh side along the centre piece. The cut runs from behind the mounting lug thereby not affecting the mounts or the width of the sub frame. The rh arm is the slid across by the required amount and welded back together. The resulting gaps are filled with suitable infill pieces. A new wheel spindle may be needed unless you bend it back to match your wheel. This also gets over the shockers alignment problem. Front and rear wheels still won't align though unless you move the engine across to the left!

It only needs to be widened to stop the tyre rubbing the inside of the arm and also allow the wheel to be built across giving more chain clearance past the tyre. I reckon 20mm is fine.

The only plus for lengthening the arm is to prevent the build up of mud against the swinging arm in heavy conditions. Might be important in the south.

Longer swing arms reduce wheel grip on climbs and don't help on corners.

Just my thoughts on the subject.

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Thanks Alan and Stick for your replies.

The last (and only other) Cub I built used a standard trials swing arm. I just scolloped a piece out on the RH side where the tyre got close.

Also with the standard heavyweight forks clonking away, pulling wheelies was never a confidence inspiring activity on my previous Cub!

Somehow, this time, I've ended up with 4 pairs of forks! 2 sets of Montesa Cota forks with yolks (I intend to make 1 good set to fit the Cub and possibly turn down the spare Montesa sliders to fit inside the cub sliders) , a set of Gas Gas yolks adapted to fit a Cub and a pair of Honda CM250 forks which I turned down to fit inside the original Cub sliders. God bless e bay!

If the turned down Honda sliders and Gas Gas yolks are any use to anyone, let me know before I stick them back on e bay.

The main problem with my previous Cub was that as soon as it got wet, mud used to build up at the front of the rear wheel on the swing arm and either side of the rear tyre. This could be a real pain with the sticky mud/clay we have in Kent.

I wouln't have thought a extra inch on the length of the swing arm would make that much difference to handling but it should give enough room for the mud to travel through!

Where are the tubes usually lengthened? Is a sleve added and brazed in somewhere?

Can I assume 20mm on the width and 25mm on the length about right?

I also intend to fit mounting points for a side stand (RHS) and chain tensioner (LHS) at the same time.

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Hi Guys'

Hi Brucey.

The best way to lengthen the swinging arm is to extend the plates the wheel sits in by one inch then fill the slot in by one inch to keep the strength.

If you take a look at my Scott Ellis frame , on the BSA Otter site you will see how it is done it s in the gallery. http://www.bsaotter.com/ PS, I widened my arm by heating the left hand casting to match the right and then heated the tube to match the same, but it is tricky ,and don't over heat the casting or you are in trouble.

Regards Charlie.

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hi all.

hi brucey . i have just done a swing arm lenghten it at the wheel plates as charlie says mine must have been a bantum swing arm because both sides are bent out it takes a 400 very well. and there is 1 on ebay

cheers kev

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Kev,

From my experience a Bantam swingarm is about 5/8" narrower than the Cub unit (it will not take a 4"x18" Trials tire). They do take the exact same pin and are the exact same size at the point where they connect to the frame. If you are trying to upgrade a Bantam it is a real easy swap.

Alan

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Hi Guy's.

I have just been reading a bit about the "works" BSA Bantam, the first one, and Brian Martin used a Sports Cub Swinging arm on this. So if BSA comp- shop used this arm, that must be the one for us boys to use, if you can find one? Ia'm sure I have seen one of these on the Greystone web site, and as I have said before this is the S/A used on BOK228C. So If we can't find them we should make ours look the same?

Regards Charlie.

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Hi Guy's.

I have just been reading a bit about the "works" BSA Bantam, the first one, and Brian Martin used a Sports Cub Swinging arm on this. So if BSA comp- shop used this arm, that must be the one for us boys to use, if you can find one? Ia'm sure I have seen one of these on the Greystone web site, and as I have said before this is the S/A used on BOK228C. So If we can't find them we should make ours look the same?

Regards Charlie.

Charlie why should we make ours look the same? Why assume the BSA comp shop knew best? surely they would only use what was available from the parts bin not necessarily the best item for the job. Far from it. Surely we can do better nowadays ?

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Hi Guy's.

HI OTF.

Its this Pre 65 thing again , We could make swinging arms out of any metal known to man now, and to any shape , and we do. But if a guy wants to convert his cub into a trials bike, and give it more tyre (tire) room , and has a couple of cub swinging arms to modify, this would still be the way to do it. As for the comp-shop, yes they would have sorted through every arm that there was, and then picked the widest one that would fit the frame they were using, hence the Sports Cub one Quoted. (We have come a long way up the road from the sixties, but we still wish we were back there?) Quote.AWP.

Regards Charlie.

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Fair point Charlie. I suppose i've always been into " Motor sport is there to improve the breed" which as a tiny little wide eyed sprog i remember hearing somewhere and it's been indellably etched into my mind ever since :rotfl:

I remember way back in 1967 fitting Honda 50 hubs and alloy rims into my Greeves Scottish because i couldnt live with the weight of the standard ones. I always wondered why the factory kept fitting such ridiculously heavy things when better stuff was available. Oh well nothing changes eh?

Wasnt trying to poo poo the idea just personally couldnt see the point. Whatever.

I'm at the moment doing a similar exercise strangely on the Ossa.

I had to replace the swing arm as i wanted to improve the position of the lower shock pick up points to improve traction and steering. I'm using the std frame made of heavy weight gas pipe as a bit of a test bed :rotfl: then everything will be incorporated into the Gollner Ossa frame i have waiting. Anyway was astounded at the weight of the Ossa swing arm which i might sell the bloke next door as an anchour for his boat, not much use for owt else LOL. So the way forward i've decided is an aluminium swing arm which i'm in the process of making / modifying you know the score :rotfl:

Only recounted all that to illustrate my philosophy on trials bikes to try to explain my post earlier.

"One small step for a man. One giant leap for mankind" :P

Edited by Old trials fanatic
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Thanks for all your replies guys!

The latest on my Cub build is I have lengthened one of my std Cub Trials Swinging Arms by 1". I actually cut mine just in front of the wheel spindle slot (where it is kinked), chamfered all the edges and put a 1/4" thick x 1" wide plate in. Next job will be to widen it by 20mm. I will be cutting just to the rear of the RH pivot. I will cut to the middle of the swing arm, move it out 20mm then fabricate a pice of U section to fill the 20mm gap in the middle and take it to a local company to get it all welded up properly.

I contacted a guy on e bay when he didn't receive any bids for his Bantam (both arms kinked) swing arm and ended up buying it! Oh well, I feel a Mk 2 Cub/Bantam swing arm comming on!

One of the reasons I'm building this Cub is that I enjoy designing, modifying and making bits for it. It would be easy to spank

Edited by brucey
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Just a quick update.

I've attached a couple of photo's of the frame build to date. There is still a lot to do!

The rear swing arm went pretty well but I have to cut an extra piece out as the widened portion hits the rear subframe when it travels up. I'm not sure what I'm meant to do with the RH shock mounts as the bottom is now 20mm further out than the top one. (See photo)

I have fitted taper bearings and Montesa yolks and will try turning down my spare Montesa sliders so they fit inside the Cub ones. I may just use the Montesa set up as shown in the photo as it all seems to fit nicely. I haven't a clue what the Montesa wheel spindle looks like though!

Does anyone know what happens to the Montesa slider bottom bolts (are these used to hold them into the Cub sliders) and oil drain holes when they are turned down?

CubFrame001.jpg

CubFrame003.jpg

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  • 2 weeks later...

Hi Bruce, Like yourself I decided to do my own mods to my cub project. Have the same problem with the right hand shock and not sure if it is worth altering. Chain clearance to tyre looks like the next problem to sort. May rebuild the wheel with a bit more offset.

Keep us updated with your progress. Will try and post a couple of pictures of mine 90% completed 90% to go!

Dave M

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