Lengthening A Swing Arm “It’s me or it ! ” .
That was the single minded ultimatum given to my mate Marty, by his long suffering kidneys. The ‘it’ in question being the hardtail on his XS650 chop. The same chop I’d grafted the ZX10 front end onto a while ago. Click here For that article Actually the hardtail in question was suffering from stress and old age. The frame was circa 20 yrs old and the spars had already been welded up once, but the bits of tubing were bored in their relationship and would soon part comany if left to their own devices. We had an extended GSX1100 swingarm lying about which was a likely candidate but the pivot was too wide for the frame. Shortening it and deepening the bearing surfaces was gonna be a bitch to do, so the decison to source an original XS650 swing arm seemed the better (easier) bet.
Having sourced a swing arm, 16″ alloy, disc and caliper from those helpful folk at ‘Halco’ they were offered up to the bike. It’s probably my memory loss, due to large quantities of intoxicating substances over the year, or more likely ‘auld age!’ but I’d forgotten just how short a standard XS was. And so the job, like ever other job in custom bike building, wasn’t as easy a prospect as was first envisaged. A bit of fettlin’ and precision measuring later we figured out that the old XS arm would have to be extended by the length of exactly one bit of a kelloggs Rice Crispies Box, which, once we’d bother to find the rule, tuned out to be about 4 inches or for the metric amongst us 900mm, until you turn the rule the right way up, then it’s 100mm.
Extending it by 900mm would’ve been interesting. But Marty talked me out of it. So, extend the bugger it was. The optional methods of doing this can last at least ten pints in a pub debate but, here are my meandering thoughts on my way. Four ‘slugs’ were machines out of engineering steel, two tubes of thick wall CDS were cut and ‘faced off’ to the same length.
It’s important that the finished job leaves a swing arm that’s just longer. Not all twisted out of shape so that it could be exhibited at the ‘Tate Modern’ and make the bike handle like a tea trolley.
Normally I would have used the frame jig to do this but it had been dissassembled and stored, and I couldn’t be arsed putting it all back together. A piece of threaded bar is passed through the wheel spindle mounts and tightened up to hold them in place whilst cutting. Measure fron the pivot back and cut both arms. Best if you cut at 90 degrees, any colder and your fingers will get numbs, no, a wonky cut makes it the rest of the job a pain and it’ll most likely end up with arms of two different lengths. At his point I made up the parts for the extension.
The Japs make the swingarm tube out of rolled tube, so it has a seam weld. Marty was despatched to the corner of the shed with a round file to do an impression of a ‘Pot Head Pixie’, old Gong reference, ah!, I remember the days…. shut up, no I don’t, do I?, anyway, he was at it with the file for a good while before he spotted the drill with the grinding stone in it 😉 Assemble the ‘slugs’ , “Slugs…. heyttennnnshun!” and the ‘tubes’, ah the seminal eighties band of one good song “White Punks on Dope” …. shut up and get on with it.
The key element here is to take your time and make sure that the arms are the same length. Don;t and you’ll make lining up the wheel a job for planks and bits of string, instead of the highly accurate marks the japs put on the spindle mounts, Ha!. Clamp it up to a solid bit of steel plate and use a level to make sure there isn’t any twist. The threaded bar will make sure that the spindle mounts remain parallel and perpendicular. Leave a gap between the tubing and weld it up. If you are aesthetically inclined, you can used a flap disc to take the weld back for a neat finish. Folk have argued long and hard and got very……very…..drunk about the possible loss of strength in removing the bead. You can decide yourself. Check the lengths in before welding the sencond asm. When it’s all welded up, Use a level to check for twist. If it is twisted, swear a lot, stamp your feet, then select your weapons and let battle commence. Heat it and use a hydraulinc bender, smash it with a lump hammer, whatever suits yer mood. In this case it was poker straight 😉 Phew. In the next step ye get to play with yer gussets, phwaoor !….. Quiet in the cheap seats !. Having added four inches to the swing arm there’s an increase in ‘wobbly’ factor. You need a good gusset to make it stiff……stop it!.
Deflection increases by a power of 3 in proportion to the length, the gusset reduces the length and hence the deflection.
Reasonably thick plate should be used here, in my case 2.6mm, as I had it lying around. The plate should be of a thinner material than the tubing. The gusset will form a box, making it even stiffer. The trick here is to nip a ‘wee triangle’ off all the corners and NOT to weld the corners at any point and leave the sides of the box open.
This reduces the stress concentration at the corners. You can seal the box up with a flexible sealer then paint over it or leave it open and drill a small drain hole in the bottom gusset to let any water build up run out, a nice breathable gusset. And there you have it, one extended swing arm that’ll make the lines of the bike look a whole lot better, for a softail. This adds a fair bit of unsprung wieght to the arm, but it’s not really an issue on a chop.
Extending an alloy swing arm is a whole different ball game and shouldn’t be tackled except by a specialist firm. As usual, this is just my thoughts, as they say in latin “beogus empor”, roughly translated means if you do this, do it at your own risk, and if your not really sure what your doing then don’t.
Word & Pics by Al