Hello my bike has knackered 11.9" shocks and everyone says to fit 340mm but when I open the gap to 340 my chain comes hard up against the swinging arm. Does anyone know whether my weight (14.5 stone) on the pegs will be enough to counter this? If not do you know where I can buy affordable 11.9" trials shocks please? Thanks John.

PS sprocket is 62 tooth. photos show 11.9 in place and 340 gap. Gap closes enough to lift chain free at 310mm by the way

Hi Chap, 2 excellent photos, showing swinging arm angularity, in relation to chain line.

Hi Chap, 2 excellent photos, showing swinging arm angularity, in relation to chain line.

So, is this good or bad ?

Couple of things,firstly you want a good amount of sag with your weight on the bike.Second,you can fit a slipper pad to the top of the swinging arm to allow the chain to run over it without damage.

If it were mine I'd have a chat to Robin at Falcon shocks and explain what you have,your weight etc.He is really helpful the advantage of Falcon shocks is that you can screw different length ends on them.He supplied a pair for the back of my HT5,about £130 from memory,they work very well.

So, is this good or bad ?

BSA got it about 'bang-on' for a design 1957ish. If in 2014 people decide to alter that geometry...so be it. Rear suspension works by mechanical advantage ( thats why the JJ Corba didn't work). But all the 'mechanical advantage' goes out the window if you have no grip (thats why good riders keep it rolling in greasy condition). No matter what is done, in this case (short of removing the rear chain) the vehicle will still move forward.

BSA got it about 'bang-on' for a design 1957ish. If in 2014 people decide to alter that geometry...so be it. Rear suspension works by mechanical advantage ( thats why the JJ Corba didn't work). But all the 'mechanical advantage' goes out the window if you have no grip (thats why good riders keep it rolling in greasy condition). No matter what is done, in this case (short of removing the rear chain) the vehicle will still move forward.

Well there’s your answer, couldn’t be simpler!

BSA got it about 'bang-on' for a design 1957ish. If in 2014 people decide to alter that geometry...so be it. Rear suspension works by mechanical advantage ( thats why the JJ Corba didn't work). But all the 'mechanical advantage' goes out the window if you have no grip (thats why good riders keep it rolling in greasy condition). No matter what is done, in this case (short of removing the rear chain) the vehicle will still move forward.

This link, Takes you the Tony Gorgot interview, and goes some way to explain why the JJ Cobas didn't work.........http://www.retrotrials.com/page-7-toni-gorgot-interview---exclusive.html

Edited November 21, 201411 yr by ask greeves

Thanks for the info as coincidentally, I have just had extensions made for rockshocks. Now making them 380mm, but as you say with a 50lbs spring there is a good 45 - 50mm sag. Easy to revert to original lengthy if necessary.

If at full compression of the rear suspension, the rear wheel comes within 1/2inch of rear mudguard, if 2 inch extensions are fitted to the shock absorbers, all it means the rear wheel will come within 2 and 1/2 iinches of the rear mudguard before it stops. On the other hand, the rear wheel starts its travel (at full extension of the shock absorbers) 2 inches lower. The down side of the extended shock absorbers,is the angle of the swinging arm, which starts its travel outside of the 'sweet spot', where power through the chain, helps to push the swinging arm downwards. The force from the chain trying to push the wheel/swinging arm is deminished even more at full compression, because the angle of the swing arm has been reduced by 2 inches by virtue of the shock extensions. The 2nd photo of BSA shows chain so low, that forces through the chain would pull the swinging arm upwards. Of course there's other forces working at the same 'moment in time', forward motion through wheel spindle and swinging arm pivot, which is helping to offset the bad chain angle.

Edited November 21, 201411 yr by ask greeves

Interesting, running rear shocks really soft so think might have been through "sweet spot" anyway. Easy to change back if it seems a negative move.

Also be interested to see what it does to the steering, equivalent to 0.5 to 1 degree pulled in approx.

have a chat with norman at njb shocks, he will sort out just what you need, at a very reasonable cost , he is ex girling man who knows as much as anybody about suspension units , he used to come out to events and make up units to fit the fantic trials outfits , taking my weight and other things into the calculations .i cant find his phone no but i am sure someone on the tc site will have it. will.

Hello - it's me again (original poster). Many thanks to everyone for all the advice and suggestions. I've spoken to Falcon - super-helpful and the advice is to get the gearing right first and then move on to the shocks (gearing = sprockets = chain clearance). They can provide shocks that can be adjusted later for length but ideally I'd try to get it right first time obviously. So I wondered whether anyone can advise:

- ideal sprocket sizes for this type of bike (with a view to swinging arm clearance)

- how I can assess the gearing inside my gearbox i.e. road or trials ratios? At present it feels like 4 X fairly close fairly low gears but 1st is not super low e.g. wants a bit of clutch at the bottom of a steep hill before a turn - which I'm told is a bad thing for a BSA....

Finally, is it possible to fit a B40 barrel on a C15 crankcase? And if so how would somebody know? Are there external differences? This bike feels much more powerful than 350cc. Any ideas?

Many thanks once again to everyone - I love this forum!

John

You should change the crankshaft pinion

http://www.trialsbits.co.uk/product_info.php?cPath=85_182_257&products_id=689

Re gearing, are you running 520 or 428 ?

Personally I wouldn't worry to much about the chain touching the swinging arm, it will wear a couple of small grooves and stop.

First thing you should do is whip the primary cover off and check what size engine sprocket it is. 23 teeth for the standard road bikes and late trials models. The early trials models had 18 teeth which is what most people seem to recommend these days. The only way to check what gearbox it is is (apart from taking it to bits) is to turn it over and count the turns of the gearbox sprocket for a given number of turns of the input shaft ie clutch. I suppose the best way is to take the primary chain off and turn the clutch drum by hand while counting the turns of the sprocket. Maybe there is a better way though?

Then you will have to look at the specs to see what gearbox you have from the ratios you find.

Mine is all in bits at the moment and I've been changing the gearing a bit and fitting new shocks. Mine are 340mm and the rear sprocket is now 62 having been 66 before. I've also gone smaller on the front down to 15 from 16 so I will be worse off from a chain to swinging arm clearance point of view. I'd not thought of that till I saw this post so I'll have to see how it goes! It's already a bit worn but nothing bad. I'll probably make nylon pad anyway.

By the way, mine is an original C15 T and has the 23t engine sprocket and with the gearing mentioned above is fine for me. It's not as low as some would have it but I've managed with it like that for years!

The Rupert Ratio book will tell you all the ratios, a good read if you've not already got one.