Britbike forum
Posted By: jimq Shortening conical hub forks - 02/10/19 9:20 pm
Hi all.

I am looking at shortening the forks for my BSA b50 road racer.

They are conical hub alloy slider forks as stock in b50's.

From my research so far i plan to put a home made spacer under the top out springs and use racetech springs with homemade preload spacers.

Could anyone here advise for or against my proposed plan?

Posted By: koncretekid Re: Shortening conical hub forks - 02/10/19 9:41 pm
The alternative is to find some custom top triples that allow the forks to be drawn up thru them. This lowers the front end for quicker steering and maintains full movement of the forks as long as the wheel doesn't hit the lowers.

If you shorten the stroke by inserting a spacer above the rebound spring, you probably won't need any preload spacer.

Posted By: DMadigan Re: Shortening conical hub forks - 02/10/19 10:53 pm
It is a good idea to have more overlap between sliders and stanchion.
Instead of putting a spacer under the top out spring you would be better off making a new damper tube shorter since you have to fix the damping anyway. Also you can use standard springs instead of buying shorter ones. You can make adjustable top nuts for preload.
Put the damper holes in the lower end of the tube with one set higher than the other so it gives a step in compression damping as the forks bottom. The slots along the side of the damper tube are the problem as they let the oil bypass the damper head once they move past the "seal" in the end of the stanchion.
You can make a damper head that uses shims like modern bikes or just put a series of holes at stepped heights around the damper top nut with a screw in the end which can be adjusted through the top nut to close off holes as required.
Posted By: koncretekid Re: Shortening conical hub forks - 02/12/19 12:05 am

You've probably already posted a drawing of what you propose, in which I would be interested, even though I no longer roadrace. The last set of B50 forks that I rebuilt were a mess with that bottom nut having what might have been a plastic seal, but was so badly worn as to offer little resistance to the oil just bypassing the slot and flowing around the seal. I made some new plastic seals but they aren't really a seal but more of a guide. The other thing that puzzled me was that the top nut was hollow, so any oil within the damper tube (which entered thru the slots) could simply flow out the top of the damper tube bypassing the stack valve altogether. In making a new damper tube, should we close the top of the tube so that the oil must flow thru the stack valve? How do we make that valve adjustable without disassembling?


Posted By: jimq Re: Shortening conical hub forks - 02/12/19 2:08 am
I too would like to see pictures or diagrams of what you propose to help me fully understand.

My biggest concern is my lack of understanding of what happens as the fork reaches full compression and what stops the fork from bottoming out if it ever gets that far,
Posted By: DMadigan Re: Shortening conical hub forks - 02/12/19 4:33 am
Here is a diagram of the fork and damper rod:
[Linked Image]
When extended the fork tube is all the way to the right with the end plug seal holder against the rebound spring. The space between the slider and damper tube is the compression volume and the space between the inside of the fork tube and outside of the damper tube is the rebound volume.
As the fork moves down the oil goes through the slots and two holes into the damper tube and up through the holes in the damper head nut then down through the damper into the rebound volume.
When the top of the end plug moves past the holes there is a drop in damping because some oil flows directly into the rebound volume bypassing the damper head.
When the end plug moves down so the slots open above the top of the end plug the damping almost goes to zero because there is very little restriction in the flow path.
When the end plug gets to the bottom of the slots it serves as a hydraulic bump stop except for the leakage past the end plug and damper tube.
On the way up all this reverses except the washer with the four holes under the damper head is pushed up against the damper head and increases the damping on rebound (I probably got this backward in my earlier post).
The damping curve looks like this:
[Linked Image]
I do not remember what viscosity that I used for the calculation.
My recommendation, instead of shortening the springs and adding a spacer between the topping spring and end plug was to make a shorter damper tube which would move the slider up the fork, keeping the distance to the fork top nut the same and lowering the front end.
Instead of the slots, put four holes near the bottom of the stroke on the damper tube, two at the top of the lower two. This will force the oil to go through the damper head both ways. When the first set of holes are covered by the end plug there will be an increase in damping and a hydraulic stop when the lower holes are covered.
You can replace the damper head with one like this:
[Linked Image]
The nut has a series of holes stepping down with a socket head screw inside. As the screw is moves up and down the overall damping can be changed externally through the top nut.
The shim stacks under the nut control the compression and rebound damping by diameter and thickness of the shims.
The only thing missing here is a "blow off" spring for the compression. You would have to add a wave spring under the lower set of shims holding them against the damper head but letting them move off on a hard hit.
Posted By: koncretekid Re: Shortening conical hub forks - 02/13/19 1:08 am

I'm impressed by your conversion.

Here are some photos of the last damper tubes I rebuilt (stock). I'm not sure of the origin of this forks.
[Linked Image]
Unassembled parts. Note different type of top nut with no port.
[Linked Image]

So on compression, the only damping is due to the restriction of the size of the slot at the bottom of the damper tube as the stanchion is forced down into the oil filled slider beneath. This oil is free to exit the top of the damper tube into the upper part of the fork slider. But on extension, or rebound, the oil that is trapped above and within the area between the damper tube and the inside of the stanchion is forced out thru either the two small holes with very little dampening or thru the top nut "valve" with the 8 small holes. So by simply welding those two small holes shut, would we get increased rebound damping, as all the oil would then be forced up thru the "valve?"

Posted By: DMadigan Re: Shortening conical hub forks - 02/13/19 2:06 am
Closing off the two holes will make the oil go through the damper head instead of bypassing it. The single slot makes the hydraulic stop more progressive.
The port hole in the side of the damper tube nut keeps the oil down near the damper head. A hole out the top is going to shoot oil up to the top nut then drain down. If there is too much oil in the top of the fork tube there will not be any around the damper head so it will suck air on the rebound.
Posted By: jimq Re: Shortening conical hub forks - 02/13/19 3:16 am
This is all excellent information.

I am beginning to understand.

My own abilities to do machining are fairly limited so i think i will have to stick to a stock damper rod at this stage. I had thought in the future i would try to incorporate an emulator into some kind of modified damper.

One thing i think i can identify is that the spacer under the topout spring would have to be incorporated in such a way that it didn't obscure the two holes in the damper rod. I am thinking that would be achieved by fitting the spacer above the spring.
Posted By: koncretekid Re: Shortening conical hub forks - 02/13/19 4:32 pm
I assume you adjust your shim stack with an allen wrench, but with that lock nut, you must have a long extension for a socket to lock and unlock it. I have never taken any modern adjustable forks apart, but I can see a way you could make yours externally adjustable by broaching a hex hole thru the adjustable top nut on the damper tube and using a long hex rod attached to an allen head bolt which could be threaded into the nut on the top of the fork tube. Rotate the top bolt and lock it at the top. As long as the bolt that does the actual shim adjustment is broached thru, the long hex rod would just slide thru it as the forks compressed.

Maybe you have already done so or maybe just too much trouble.

Posted By: jimq Re: Shortening conical hub forks - 02/14/19 8:04 am
There is an alternative to the racetech gold valve known as the sports motorcycles sports valve that is adjustable by a tool that is inserted through the top of the fork leg.
Posted By: gavin eisler Re: Shortening conical hub forks - 03/02/19 3:44 pm
I have done the mod you propose, Kommando supplied me with stock length modified damper rods for my 71 A65. One leg does compression, the other rebound. See the next post for a proper explanation of how it works
i shortened the stroke by 50 mm, fitted thin tube spacers under the rebound springs.
Stock spring rate is approx 25 lb/inch
First try I cut down the stock springs, this made the forks way too stiff. Ended up about 28 lbs/inch.
Second try i cut down some lighter springs to fit, used an online calc for rider bike weight and spring rate. Ended up with 22.5lbs / inch. That worked , but I wasnt happy with the cut spring ends.
3 rd try custom made springs closed and ground ends , 23 lbs / inch.

Ks dampers have a 7 mm threaded variable needle valve operated by a telescoping square rod / box tube about 5 mm square, this is attached to a modified CBR 600 fork top adjuster, screw in middle does damping, hex does pre load.
This fiddly to put together, getting the square rods to unite after removing the top nut is tricky.
I set the RHS comp damping about 2 turns out , aliitle less for the LHS rebound.

It works very well. Having more of the slider over the tube stiffens up the fork in a good way.
I havent noticed any increased twitchiness from dropping the front end.
There is much less initial dive on brake application.
Works very well.
There is a slight catch, there is no effective top out rebound and the fork make a noise on my seriously bumpy drive as they top out. This is undetectable on the road.

A couple of days ago I had the bike in the air a lot over a bit of road that is very humpy, no nasty noises or surprises.

I havent fiddled with the high speed shims ( beyond my pay grade) , the low speed settings are what K reccomended and worked right off the bat. I have fiddlesd with the settings on my other modern bike, and i know what putting too much on will do, skips over cats eyes . The K set up had no nasty surprises.

I plan to fit locknuts on the low speed needles and remove the top nut adjustment. Once its set I dont feel like fiddling with it ( the bars are in the way ), with clip ons this would maybe be more convenient.

I spent time getting the pre load and static sag set correctly. The top nut adjusters were handy for that.

Dave's shorter damper rod is a very good idea, saves unsprung weight ,added spacers and spring length change,
Posted By: kommando Re: Shortening conical hub forks - 03/02/19 5:37 pm
This is the write up on the Fork inserts I did for Gavins forks.

When looking at adapting modern cartridge dampers to our old clunkers I used the CBR600RR cartridges and down scaled them for the BSA 65 to 70 rod damper and the Triumph 63 to 70 forks. These are in prototype production and one of 3 sets is complete ready to fit. When I looked to carry these across to the OIF on the face of it this should have been much easier, the tubes are bigger so no need for down scaling. But the issue that came up was the method the forks use to hold them onto the bike. Pre OIF there are substantial fork seal holders which stop the forks from falling off over jumps, on the OIF it just has the 2 bottom bolts which retain the damper rods, lose these and then next time the front wheel leaves the deck the forks fall off. You can wheelie only for so long and then 2 stanchions hit the tarmac or earth.

You can see the problem here in this breakdown of the fork assembly.

As this bottom bolt needs to be drilled for access to the compression needle the bolt would be made weaker so another path was sought.

This is the solution which will be tested on Gavin's bike, the valves are increased in size so they run not in a cartridge but inside the stanchion itself, they are also separated so the compression is in one leg and compression in the other. This allows all the adjustment to be made from both top fork nuts so the retaining bolt stays exactly as OEM or stronger (12.9 grade allen bolt).

The first build is now complete and this is what it looks like.

[Linked Image from]

So how does it work. The oil flows in each leg have 2 circuits, a low speed one which flows through a hole drilled up through the centre of the valve rod, the rate of flow is controlled by a needle adjusted by the adjuster on the top of the fork nut. So the rider can control the damping he experiences when the forks are moving slowly eg on smooth roads. This is when you want higher damping forces and this includes when braking so it provides anti dive compared to the std setup.

[Linked Image from]

When the road surface becomes rougher and the forks start moving faster the mid and high speed circuit comes into play, here oil is forced through holes in the valve and this oil presses against a stack of thin steel shims lifting them at the edge to allow oil to flow. The faster the forks move the more oil flows and the more bent the shims become so the bigger bumps do not increase the damping exponentially as happens in fixed hole damping. This is called Digressive Damping and the OEM damping was fixed orifice damping.

Comparison of damping types

[Linked Image from]

Rebound valve

[Linked Image from]

Compression valve

[Linked Image from]

Valve without shims and reverse flow washer

[Linked Image from]

[Linked Image from]

There are 6 routes through the valve, 4 for the shim stack route and 2 for the reverse flow which is a simple lightly sprung washer so it does not create any damping effect in the reverse direction as it lifts immediately.

The outside of the valves are sealed against the ID of the stanchion by low friction PTFE sealing rings, this directs the oil through the 2 circuits instead of bleeding off around the valves. This should be an improvement of the OEM O ring which can give stiction.

To get the compression bump stop back the valve rod is tapered towards the bottom to trap oil on full compression to give a ramp up instead of metal to metal contact and the top out spring is retained. The spacer above the spring is optional and to limit fork movement if the builder prefers a more normal 5" of travel instead of the OIF 6.5".

The Fork Nuts have the slow speed adjuster screws in the top and I took the opportunity to add pre load adjustment as it was on the internals I used for the valves.

[Linked Image from]

As the adjuster needles for the slow circuit are fixed relative to the legs and not the fork nuts as is the normal method a telescopic adjuster rod was included to cope with the suspension movement.

[Linked Image from]

The concept of splitting the compression and rebound damping between 2 fork legs is not new, its been used successfully in the past. Also the use of a valve outside a cartridge and using the stanchion as the effective cartridge is not new either, it is was introduced onto the CBR1000RR in 2013 and called the Big Piston. What is new is fitting internals from 30mm cartridges directly into stanchions, so this is a much more speculative build than the scaled down CBR600RR cartridge. It will take most if not all of the summer to test, the shim stacks will need refining, oil viscosity and oil levels.
Posted By: koncretekid Re: Shortening conical hub forks - 03/06/19 1:53 am
Thank you for the explanation of your system. I understand the principles here but I'm not sure I agree with the direction arrows (center set of red arrows) which show the oil flow into or out of the annular space between the damper tube and the stanchions. In compression, this annular space becomes greater as the stanchion moves downward, so I don't see oil exiting this area, but instead oil must enter this area. The compressed oil can enter thru the bottom hole(s) in the damper tube, or bypass these holes by flowing past the plastic bushing in the bottom nut. Or is it that most of the oil enters by flowing past that plastic bushing and then is forced back into the upper holes in the damper tube? Of course there is more oil being displaced below the end of the stanchion than this annular space can absorb, so the excess is forced out the top thru the slow speed circuit or thru the shim stack. The opposite must occur on rebound.

It looks like a great system. If only I were a little younger and could take advantage of it.

Posted By: kevin Re: Shortening conical hub forks - 03/06/19 2:35 am
Originally Posted by koncretekid

It looks like a great system. If only I were a little younger and could take advantage of it.

you don't need any more advantages.

ed vallicutt laughs about your thumper making mincemeat of the 650s every time i talk to him, like today.
Posted By: kommando Re: Shortening conical hub forks - 03/06/19 10:41 am

As the PTFE bush has a gap like a piston ring there is leakage but ignore that as it should be constant and just means its get added to the low speed circuit which can then never be nil.

The PTFE is an effective demarcation line between 2 chambers, one above and one below. The old valve kept on the bottom of the stanchion has plenty of clearance to the damper rod, much greater than the 4 holes, it only comes into play as a compression bump stop as it nears the bottom of the damper tube and goes below the bottom 2 holes so until then under the PTFE is one chamber not two, I needed 4 holes under the PTFE, I could have put all 4 at the top or the bottom but thought 2 top and 2 bottom would reduce risk of the old valve coming into play if it was ever damaged and taking oil from the bottom and top would cause less risk of cavitation.

Posted By: Hillbilly bike Re: Shortening conical hub forks - 03/06/19 11:26 am
Yes, looks great....Does the whole affair slide up and down inside the tube or is the part with the split seal stationary and just the rod moves?
Posted By: kommando Re: Shortening conical hub forks - 03/06/19 11:46 am
The PTFE is stationary inside the fork lower and bears on the inside of the Stanchion which it slides inside providing the boundary between 2 chambers, the old damper tube fixes it in position relative to the fork lower. The slow speed damper adjuster rods have a telescopic section so they can cope with the up and down movement of the fork lower.
Posted By: kevin Re: Shortening conical hub forks - 03/06/19 3:04 pm
yo, this thread needs to be stickied.
Posted By: gavin eisler Re: Shortening conical hub forks - 03/07/19 1:50 pm
"yo, this thread needs to be stickied."

I agree.
Posted By: gavin eisler Re: Shortening conical hub forks - 03/21/19 12:02 am
Posted By: gavin eisler Re: Shortening conical hub forks - 03/29/19 1:25 am
Posted By: Allan G Re: Shortening conical hub forks - 03/29/19 8:03 am
You would have to PM Morgan as there isn’t a moderator otherwise linked to this board. HTH
Posted By: harvey_mushman Re: Shortening conical hub forks - 03/29/19 7:05 pm
excellent i knew i had seen something somewhere, now all i need are some damper rods if anybody has some lying about they would like to sell?


Posted By: kommando Re: Shortening conical hub forks - 03/29/19 9:19 pm
Not going to make any more, have moved on. There is enough info in the pics and the posts for a machinist to make a set, plenty of donor BMW S1000RR inserts on eBay as there are replaced by upgraded cartridges, rest is turning and general engineering.

Posted By: jimq Re: Shortening conical hub forks - 04/04/20 9:28 am
Can anyone comment on the importance of the location of the two little holes. particularly in relation to whether they are above or below the stanchion end plug.

As stock at full extension the upper hole is inside the end plug and the lower is approximately 21mm below. If i run a 30mm lowering spacer under the top out spring the top hole is 21mm above the cap and the lower hole is inside the end plug and if i run a 50mm lowering spacer both the holes are above the end plug and the top hole will be partially obscured by the spacer.

Should i move one or both of the holes or perhaps replace them with one of an equivalent size?

My forks are stock except for emulators and 0.7kg springs.

Between 30-50 mm is the range of lowering i expect to be working in but i will not be sure what the final figure will be until i have tested and tuned the bike.

The bike is to be a road racer.

Posted By: kommando Re: Shortening conical hub forks - 04/04/20 3:45 pm
Read DMadigan's second post with the diagrams, in the text he explains the purpose of the holes.

When the top of the end plug moves past the holes there is a drop in damping because some oil flows directly into the rebound volume bypassing the damper head.
Posted By: jimq Re: Shortening conical hub forks - 04/05/20 1:46 am
So If I want a close to constant rate rebound damping I could move both the rebound holes so they are always open in the rebound Chambers then adjust the rebound volume with the oil viscosity. This would help me get around difficulties with wanting to experiment with fork lengths.
Posted By: jimq Re: Shortening conical hub forks - 04/05/20 2:17 am
Or leave all the damping up to the check valve
Posted By: Allan G Re: Shortening conical hub forks - 04/05/20 8:08 am
If your road racing wouldn’t a T140 disc front end be a better change? You could also pull the forks up through the top yoke to alter your ride height.
Posted By: DMadigan Re: Shortening conical hub forks - 04/05/20 4:21 pm
Only a guess but because these dampers were designed by test rider "feel", the small holes might have been put in to reduce the damping when under hard braking since they do not have the pop-off valve of modern dampers. I think you would be better off replacing the dampers with something more modern that can be tuned.
Vintage rules here allow up to 38mm forks. I would look at that route. Search on line for "motorcycle fork diameter list" and there are lots of 38mm forks to chose from. Pick a road bike used for racing made for a large range of years such as the FZR-600 89-97. Those would likely have good dampers already.
Modern forks typically have a separate damper tube for the head to slide in rather than the fork tube itself. This allows for a closer tolerance without having to hone the stanchion itself. It also is not affected by the slider/stanchion clearance or bending of the stanchion.
Posted By: marinatlas Re: Shortening conical hub forks - 04/06/20 6:35 pm

hope this help............
© Britbike forum