Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board Damp squib!

Hi All, my first message here!
I have a few bikes, one of which is a C15 bought due to nostalgia (passed my test on one in '72). Due to form, the rose tinted specs are a bit foggy and the lack of speed and vibration I can just about live with but it's the non-existent damping in the forks that is the big bugbear, it bounces all over.
I've tried 20 grade(recommended) and 50 grade in 100cc volumes. I've got the forks in bits at the moment awaiting new seals and all the bits seem present acording to the spares list. Is there a cure, please, it's spoiling my enjoyment of reliving my dubious past!
Thanks in advance for any help. Dave.

Hi, and welcome to the forum.
My first real bike, sometime around 1968, was a 1961 C15S, (I'm not counting the prior step through Honda) and much like you I years later jumped on a project grade 65 model C15S, to you know, relive my youth. It's been a good little bike and I've gotten a lot of enjoyment from it.

On your lack of front damping, it's best not to expect too much but the added friction of tight new bushings can help. 100cc's is the correct amount for the road model forks. But just suppose, someone has fitted it with competition forks or B40 forks. those take 190cc's. I only mention that because you didn't get increased damping with the 100cc's of 50wt oil.

Post a picture if you can. Postimage is one of the easiest to use on this site.
All the best.
Stuart

I'm rebuilding the front forks on my 1962 C15 Scrambler at the moment. It would be interesting to see photos of your disassembled forks to compare them to mine. There aren't a whole lot of parts inside there; the fork geometry and the oil provides the damping, and there's not a lot to it ..... ?

Lannis

just how good do you expect a 60 year old 250 single rated at 11 HP (yeah right) to be ?

probably putting sae50 in the forks hasnt helped

do you weigh twice as much as you did back in the day ? I know I do so my M20 500 went about as well as my C10 did

Thanks for the quick replies! I'm thinking the forks may have been changed Stuart, to test that theory I think I'll fill 190 ccs and look to see if the head of the damper rod is immersed before I put the springs in. I don't think the oil I had in there touches anything and the forks bounce like a jelly. I don't want to head off through the hedges here!
Ahem, whereas I do indeed weigh a few pounds (stones) more than in my heyday, Ignoramus, I didn't expect the poor little bike to be a ball of fire and I'm quite content to rattle about at 45 mph on the little roads here where I live in Normandie, France. I think I've got the engine about as good as it'll get, crank straightened, good piston fit, new little end, needle roller timing side main bearing kit , AMAL carb repair kit and valves checked and reground. I need it to go where I point it now!
I'd love a B31 or M20/21 but the price of them here is amazing, you can buy a real bike for that price! (joke, please don't ostracise me).
I'll have to organise some pics, my computer skills are legendary so it may take a while.
Off to the garage now...... where's them Whitworths?

I agree with Stuart, it may be that the fork bushings and stanchions are worn out and by replacing these parts you may get some improvement.

Also worth checking the diameter and number of holes at the end of the stanchions and these control the oil flow and damping. I thnk the number of holes is 4, but I dont know the diameter. The only way to find out is buy a new pair from a reliable supplier such as Draganfly motorcycles.

Additionally the springs may have sagged over the years

I owned a C15SS back in the 70's and I don't remember the forks being bouncy, I think they were fine for the standard of the day.

Your forks should look similar to the pic below if they are the standard type, which take 100cc in each leg. The competition type has external springs and uses 190cc.

Have a look at the C15 & B40 service sheet which gives details of both types for forks (about half way through the doc), see This Link

C15 road bikes have an early central rod arrangement providing bump stops top and bottom with equal damping in both directions, damping is provided by holes in the stanchions restricting the flow of oil into and out of the space between top and bottom bushes and the rod which restricts flow top and bottom where it is thicker.. This gives damping equal in both directions except at the bump stops. As long as the parts are present and correct there are no moving parts so changing the oil viscosity is the only way to alter the damping.

The competition and B40 bikes use the common BSA heavyweight forks but again with damping equal in both directions as until 1965/66 the only damping was provided by the holes in the stanchions feeding oil into and out of the space between the bushes as the fork rise or fall. There is no need for the central rod as bump stops are integral to the fork lowers and the position of the holes in the stanchions. Again oil viscosity is the only way to effect damping.

So the C15/B40 plus competition never got got 2 way damping like the later rod damper B25/B44, these later 2 way damped forks are the same BSA heavyweight forks but with a rod damper with a valve on the end. The valve has two positions dictated by the direction of travel, when the forks are compressing you get a low damping rate, when they are extending they give a higher damping rate. In compression you are working with the spring as it resists so need a lower damping rate, when the forks are extending so are the springs so you need more damping.

You can fit later double damped forks to the comp/B40 bikes with care ie making sure wheels are identical plus mudguard stays etc but the C15 road bike with the 17" front wheel would necessitate a need for major surgery.

An M20 would be excellent in Normandy ( a girder fork version) that way you could go on a ride around the beaches on the 6th of June and say hello to any German tourists you encounter before heading for Berlin!

Dont know if they still do it but the WM20 owners club always used to have a run to Berlin every year and have a picnic outside the Reichstag EXCELLENT ! they would park their bikes on the grassy nole right on top of the site of the Fuhrer Bunker

was that of topic ?

Thought you were bitc*ing about the general performance of the little C15 in general ... but good on ya for keeping another BSA on the road, I guess thats why the forks are an issue to in fact keep it on the road ..sounds like an easy fix and you will get plenty of help here .

Thank you, I had no idea how the damping worked! Seems obvious now. The forks are, surprise, well worn so the damping probably is compromised with oil having no, or little resistance around the bushes. Looks like I need to dig deep (again) !

Yes worn bushes on worn stanchions will reduce damping due to oil leaking internally and worn springs will reduce the effective damping. New bushes, stanchions, springs and seals plus checking all the holes are.to size and clear will restore damping and then allow finer tuning with different viscosity oils.