BritBike Forum logo
BritBike Sponsor BritBike Sponsor
BritBike Sponsor BritBike Sponsor
BritBike Sponsor BritBike Sponsor
BritBike Sponsor BritBike Sponsor
The Bonneville Shop BritBike Sponsor
Home | Sponsors, Newsletter | Regalia | Calendar | Bike Project | BritBike Museum | Spiders Cartoons, | DVD- Manuals & Parts books
Upgrade to: Premium Membership | Premium Life Membership | Vendor Membership | Site Sponsor Membership
Support Your #1 Forum, buy us a coffee :)
Still here since 1996.. Serving British motorcycle enthusiast's on BritBike Forum!
Member Spotlight
WingNut
WingNut
Atlanta, GA
Posts: 39
Joined: August 2006
Show All Member Profiles 
Shout Box
Search eBay for motorcycle parts in following countries
Australia, Canada, France, Holland, Italy, United Kingdom, USA
Random Gallery photo
Who's Online Now
66 registered members (AML), 392 guests, and 535 spiders.
Key: Admin, Global Mod, Mod
Newest Members
LanceH, 911speedster, cbowers, Ratbag, Kelly David
10567 Registered Users
Top Posters(30 Days)
franko 97
NickL 70
Popular Topics(Views)
807,369 mail-order LSR
Forum Statistics
Forums34
Topics67,440
Posts668,797
Members10,567
Most Online3,995
Feb 13th, 2017
Previous Thread
Next Thread
Print Thread
Rate Thread
Shortening conical hub forks #765076
02/10/19 9:20 pm
02/10/19 9:20 pm
Joined: May 2012
Posts: 25
new zealand
J
jimq Offline OP
BritBike Forum member
jimq  Offline OP
BritBike Forum member
J

Joined: May 2012
Posts: 25
new zealand
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?

Cheers.
Jim.

Support Your #1 BritBike Forum!

Check out British motorcycles for sale:
British Motorcycles on e-Bay UK, British motorcycles on e-Bay North America
Check out British motorcycles for sale:
British Motorcycles on e-Bay UK, British motorcycles on e-Bay North America
Re: Shortening conical hub forks [Re: jimq] #765078
02/10/19 9:41 pm
02/10/19 9:41 pm
Joined: Nov 2011
Posts: 213
Nova Scotia, Canada
K
koncretekid Offline
BritBike Forum member
koncretekid  Offline
BritBike Forum member
K

Joined: Nov 2011
Posts: 213
Nova Scotia, Canada
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.

Tom


Life's uncertain - go fast now!
Re: Shortening conical hub forks [Re: jimq] #765084
02/10/19 10:53 pm
02/10/19 10:53 pm
Joined: Jan 2004
Posts: 3,972
ca, us
D
DMadigan Online content
BritBike Forum member
DMadigan  Online Content
BritBike Forum member
D

Joined: Jan 2004
Posts: 3,972
ca, us
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.

Re: Shortening conical hub forks [Re: DMadigan] #765164
02/12/19 12:05 am
02/12/19 12:05 am
Joined: Nov 2011
Posts: 213
Nova Scotia, Canada
K
koncretekid Offline
BritBike Forum member
koncretekid  Offline
BritBike Forum member
K

Joined: Nov 2011
Posts: 213
Nova Scotia, Canada
Dave,

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?

Tom


Last edited by koncretekid; 02/12/19 12:08 am.

Life's uncertain - go fast now!
Re: Shortening conical hub forks [Re: jimq] #765170
02/12/19 2:08 am
02/12/19 2:08 am
Joined: May 2012
Posts: 25
new zealand
J
jimq Offline OP
BritBike Forum member
jimq  Offline OP
BritBike Forum member
J

Joined: May 2012
Posts: 25
new zealand
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,

Re: Shortening conical hub forks [Re: jimq] #765178
02/12/19 4:33 am
02/12/19 4:33 am
Joined: Jan 2004
Posts: 3,972
ca, us
D
DMadigan Online content
BritBike Forum member
DMadigan  Online Content
BritBike Forum member
D

Joined: Jan 2004
Posts: 3,972
ca, us
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.

Re: Shortening conical hub forks [Re: DMadigan] #765252
02/13/19 1:08 am
02/13/19 1:08 am
Joined: Nov 2011
Posts: 213
Nova Scotia, Canada
K
koncretekid Offline
BritBike Forum member
koncretekid  Offline
BritBike Forum member
K

Joined: Nov 2011
Posts: 213
Nova Scotia, Canada
Thanks,

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]
Assembled

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?"

Tom

Last edited by koncretekid; 02/13/19 1:29 am.

Life's uncertain - go fast now!
Re: Shortening conical hub forks [Re: jimq] #765257
02/13/19 2:06 am
02/13/19 2:06 am
Joined: Jan 2004
Posts: 3,972
ca, us
D
DMadigan Online content
BritBike Forum member
DMadigan  Online Content
BritBike Forum member
D

Joined: Jan 2004
Posts: 3,972
ca, us
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.

Re: Shortening conical hub forks [Re: jimq] #765268
02/13/19 3:16 am
02/13/19 3:16 am
Joined: May 2012
Posts: 25
new zealand
J
jimq Offline OP
BritBike Forum member
jimq  Offline OP
BritBike Forum member
J

Joined: May 2012
Posts: 25
new zealand
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.

Re: Shortening conical hub forks [Re: jimq] #765299
02/13/19 4:32 pm
02/13/19 4:32 pm
Joined: Nov 2011
Posts: 213
Nova Scotia, Canada
K
koncretekid Offline
BritBike Forum member
koncretekid  Offline
BritBike Forum member
K

Joined: Nov 2011
Posts: 213
Nova Scotia, Canada
Dave,
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.

Tom


Life's uncertain - go fast now!
Re: Shortening conical hub forks [Re: jimq] #765350
02/14/19 8:04 am
02/14/19 8:04 am
Joined: May 2012
Posts: 25
new zealand
J
jimq Offline OP
BritBike Forum member
jimq  Offline OP
BritBike Forum member
J

Joined: May 2012
Posts: 25
new zealand
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.


Home | Sponsors | Newsletter | Regalia | Calendar | Bike Project | BritBike Museum | Spiders Cartoons | DVD- Manuals & Parts books
Upgrade to: Premium Membership | Premium Life Membership | Vendor Membership | Site Sponsor Membership
Powered by UBB.threads™ PHP Forum Software 7.6.2