Tuesday, 21 January 2014

Improving BSA bantam heavyweight forks

My first full bike restoration was a 1970 BSA bantam, which my poor apprentice self restored in Dads shed. New to motorcycling, I soon fell in love with the BSA Gold star racers of the 50's, but being a poor appo I couldn't afford one. Still, the bantam does have BSA on the logbook...so my cafe racer project was born.

4 years after finishing it, and 18 months since I last rode it, it's in bits again. There were a few areas I wasn't too happy with, either a result of crap parts, no money, no tools or lack of knowledge. I meant to get it going again last year, got it insured, and found the rear swing arm was wobbly. So now I'm going through it with a precision engineered fine tooth comb, ironing out the flaws to hopefully create the ultimate road going bantam. I hope.

So, on to my first area of improvement: The forks. The 1970 B175 models were fitted with some kind of shortened version of the forks used on various other BSA/Triumph group bikes. I've no idea which, although I think Tiger and bantam cubs got them around that period too. They are pretty beefy for a 175cc bike, with external springs and I forget how big stanchions. They are also flawed in both design and execution.

First up, the seal holder/spring. The seal holder is a deep cup, with an oil seal and a dust seal at the bottom, with the springs resting on top. A rubber gaiter then goes over the spring, supposedly keeping water off. Obviously, it's a british bike, so it doesn't. The top of the gaiter fits over the headlamp ears, which have huge gaps for rain, spray and damp to drop in. The seal then fills with water, and for even greater thrills the spring rubs on the bare ground steel of the stanchion. The first set of stanchions I bought rusted before I even got it on the road, when I fitted this set I filled the seal holder with grease and yet it still rotted. You can see the grotty bit of the stanchion end above too.

For now, I've smoothed down the stanchions. If that doesn't work I'll send them to Philpotts for hard chroming, like they damn well should have been in the first place. If you're restoring a set of these forks, I'd recommend it. New stanchions may well be half the price, but they rot in seconds. Also, I've seen some utterly horrific stanchions for these forks, with a rough turned finish and the taper that locates in the top yoke eccentric to the rest of it by a few mm!
To stop the springs rubbing, I've made some aluminium sleeves. These are a close sliding fit on the stanchion, which will hopefully keep the worst of any dust away. The bore is stepped to accomodate the dust seal lip. Photo below shows the assembly.

 Some people have suggested drilling a drain hole in the seal holders, but I'm unsure. They screw on, so if you rebuild them again the hole at the back may end up at the front. Then you'll get greasy, oily water pouring out (springs still need greasing remember!), which won't look great, particularly at MOT. Plus, it just seems wrong. I might try and seal the gaiters up properly, we'll see.

Next problem is the fork bushes. These look like some kind of oilite material, which is rather like a metal sponge. I'm not sure if the pattern ones I got are too big, or the stanchions ground undersize, but they had a good 8-9 thou clearance, nearly 0.25mm! Which hopefully explains the front end sloppiness the bike had. I've made some new bushes in brass, with a tight fit on the non sliding surfaces and about 2-3 thou (0.05-0.08mm) clearance. You can feel stiff spots at either end of the stroke, I'll probably hone the legs out a bit to even up the wear. Either way, they feel loads better.

I'll post all the drawings for the parts I make once I make them all. Next up: How to fit lovely, better and cheaper metric taper roller bearings in your steering head. Will contain blood, unlike my poor thumb...

No comments:

Post a Comment