FF: Forks Brampton Coil Over Shocks

[ogrilp400]

G'day All,
I was intially quoting Alyns comment on Oldhavens comment " I have NEVER had the bottom link crash into the lower yoke" I was puzzeled by this as I could not see how the bottom link (sic) (links? there are two of them) could crash into the bottom yoke. I just asked how, just a question thats all.
Now Robert, thankyou thankyou finally some one else has seen this can happen. Some months and months ago I asked this and was told that it could not happen. I supplied photos and just recentlyasked again and supplied photos and again people were perplexed that it could happen. "Never seen that happen" "couldn't happen"On my B Bramptons the top cross tube of the girder assembly, the one that holds the speedo, crashes into the top of the spring. Can't happen I am told. Well mine does!! Yes, I have done all the measurements between the link centers and they are all the same.

Phelps.

 

[vibrac]

I know there was a spate of girder fork riders in vintage racing sporting a loose leather strap around the links not quite sure how it was arranged but I know that it was because someone had a bad day I can't remember what but I guess a spring shattered

 

[Albervin]

Ken, as Robert said "if the travel is not limited,". You are talking about bikes that have springs fitted which should (?) stop the clashing. As I posted to you, my new Ray Daniels forks do not clash when the spring is fitted. Maybe with a wheel attached and a full extension following a full compression it MAY but I am not going out there to test it.

 

[greg brillus]

The way I see it the forks have no upper or lower stops, only the spring.......So if the bike ran into a wall or into the back of a stationary car, the forks will extend further than the normal spring travel and this will cause the rear tubes to impact the lower stem housing on both sides.........I have seen and replaced forks with intents in these locations.

 

[Oldhaven]

From my real world bad experience, with no spring or coil over to stop it the two upper links come to rest against the handle bar and the steering damper puts a 1/2 inch deep dent in the front fender. If you are lucky the front brake cable doesn’t get trapped up there. The bike is rideable in this kneeling state and handles OK at low speeds, though I don’t recommend it. Full extension in this condition is highly unlikely. I have small dents in my fork tubes from over extension, but they were there before I did the coil over conversion, from who knows what happening since 1948.
With the standard Brampton spring constrained at both ends the fork travel depends on spring strength in compression and extension. With a coil over the fork travel depends on the limits in the internals of the damper, modified by the compression and rebound damping effect and the preload of the spring. My Works damper allowed full compression but has a 1/2 “ stop on the damper rod to limit the extension and protect the fork tubes. The way the internal rebound damping is valved with three flapper valves I doubt I ever saw full extension in normal riding though that is where it went while on the center stand. Full extension of the damper rod even with the limit stop still puts the lower links in a below horizontal angle and it’s possible this might make the forks more like unmodified Girdraulics during braking. That is speculation though. The damper rod and piston did occasionally bottom with major road bumps, and I could have increased the preload or changed the compression valving but the ride was comfortable as I had it. The spring is 120 lbs/in. Maybe a 130 or a bit more would be better, especially if riding two up or heavily loaded.
The pictures of the other coil over conversions even earlier Works conversions look like they have less travel than my Works conversion and users are happy with that. If I fix mine it will be even more limited than it's previous 1/2 inch reduction since that extra extension is not necessary and may be detrimental. I can see making the mounting arrangement at the cylinder end as short as possible whether it is up or down since it will allow more compression travel. I see no reason that the total allowable travel of a Brampton coil over should be more than the design limits of the original spring, and any conversion should probably follow that consideration going in. (personal opinion).

 

[ogrilp400]

This photo shows that at full extension the cross tube on My bramptons crashes into the top of the spring. So when people say this can't happen, then what is this?

 

[ogrilp400]

This photo shows a set of Bramptons I fitted to a customers bike. At full extension the rear tubes crash into the bottom bracket, thats why I have put rubber sleeves over the bottom bracket. This is not the first time, it happens on other bramptons I have fitted. Now I reiterate. I have accurately measured the spindle centers and they are exactly the same, obviously the diagonals will be different depending on where the forks are compressed to.

 

[Michael Vane-Hunt]

How long are your top links FT 213 center to center?

 

[greg brillus]

Obviously something wrong with the length or strength of the spring.

 

[ogrilp400]

3.375” center to center