Hi,

Does anyone know the spring rate for the front suspension spring? I've read in the service manual that 300lbs should compress the spring 2". I suppose the rate is linear and so 150lbs/inch. This seems quite soft to me?

Also, how much suspension travel is there (i.e. inches of movement at the front wheel) and if the suspension was to 'bottom out' what would hit against what? (Is there a stop?)

Thanks!

Ben

email (option): mad_king_soup@hotmail.com

Hi Ben..300lbs applied, resulting in a spring compression of 2", is the condemnation limit for the spring...it would be 'binned' at that point.
The 'acceptable' tolerance is slightly higher at 320-330 lbs. The spring is 'evenly' wound so I guess its performance is linear, though I'm no expert on the characteristics of springs under load.
Too firm a spring would not react to minor irregularities I imagine so its specification is probably a compromise based across the potential load range.(like most mechanical designs)
One of the problems with girder forks is the limited range of movement. This is governed by the arc of movement (or indirectly length) of the side links combined with practical compression limits on the main spring. I've never checked it but around 3" is typical I think.
As the side links move through an arc this is mirrored in the movement of the wheel spindle with consequent effects on the rake and trail of the forks, so I wouldn't think too much movement would be desirable.
One big advantage of girder forks over (older) tele fork designs is their greater degree of lateral stiffness, so on a good road surface they perform well and are better for sidecar use...Ian

email (option): ian@wright52.plus.com

Thanks for the info Ian.

It's really useful to be able to get knowledgeable replies on here so quickly. (Not always the case with forums!)

My next question is; has anyone tried replacing the spring with a hydraulicly damped suspension unit like a Hagon or similar? I've seen several sources for suspension components with choice of spring rate, length & mounting type etc. Does anyone think the advantages (i.e. improved damping/smoother ride etc) would be noticeable or worth the effort?

Cheers!

Ben

email (option): mad_king_soup@hotmail.com

They would need to be narrow and have the correct mounting ends on them.I suppose tou could leave the centre spring and mount two small shockers one each side. You can be first to try it!

Hi Ben..I have seriously considered such a modification. Hagon don't do a 'standard' shock that fits the criteria dimensionally, though there are plenty of other sources to investigate. Modifications would be required to the upper and lower mounting points but are entirely 'doable'..As well as improved performance under compression there would be big benefits on the damping side as the original damper is pretty non effective.
If you are not worried about exact originality I think it is the way to go to improve the forks and yet still retain the basic 'girder look'.
A possible 'plan b' would be to look at ways of improving the damper mechanism whilst leaving the compression side to the standard spring.
If you go with the shock idea and source a suitable unit(s), I would be very interested in the source and your progress with the modifications...Ian

email (option): ian@wright52.plus.com

It's definitely something I would like to have a go at! I had thought that the best plan would be to machine some aluminium blocks to fit the shock's mounting points & then bolt these to the original mounts. As Douglas says it does look narrow in there. Perhaps the unit could be mounted upside-down to fit the available space better. (assuming it will still operate correctly this way)

If anything comes of it I'll let you know.

Cheers, Ben

email (option): mad_king_soup@hotmail.com

Ian, what about the shock absorbers from a mountain bike? The dampers would probably be ok (maybe even if you had to strip them to modify them slightly, at least they appear to be strippable) maybe the springs too as they are usually mounted very close to the swinging arm pivot so are subject to quite high loadings so they may do the job

email (option): davmax@ntlworld.com

My '46 Indian Chief had a girder with two parallel springs and an Armstrong damper unit in the center. The damper was finished, so I fitted a chromed Japanese rear suspension unit in its place. I was working at a Kawasaki dealership then, so it was likely one of theirs. The added spring made the frame ride a bit higher on the girder, causing it to top out occasionally, but the damping and overall performance were improved.

But that was over 40 years ago.

Jeff

email (option): jjbandoo@aol.com

what about the front spring off the harley davison classic with the girder type forks

You never see them secondhand and they're too expensive to buy new just to experiment with

email (option): davmax@ntlworld.com

I've thought about mountain bike suspension. Most of the spring rates seem to be very high (750lbs/inch) I imagine this is because of such a short travel, although Ian mentioned only about 3" travel on the M20 girder. Also, unless they are expensive models a lot of the ones I have seen on e-bay etc state that they aren't hydraulically damped but use an elastomer instead. The expensive ones also seem to use air in place of the spring which would allow for adjustments to suit the application. I wonder whether they would be rated to perform at much higher speeds & longer durations than would be expected on a mountain bike without the likelihood of overheating?? They'd be a good bet to fit into the space though wouldn't they?!

Ben

email (option): mad_king_soup@hotmail.com

I've seen an old add for a combined spring damper conversion unit that was marketed in the 1940's so it has been done.It wasn't a great success apparently.I think it was an Armstrong unit.

Hi Pat!

That sounds interesting, I don't suppose you've got any details?

Cheers

Ben

email (option): mad_king_soup@hotmail.com

Hi Ben, The conversion was Woodhead Monroe in the 1950's There is a photo of a coverted set of I think Norton forks in the December 2011 VMCC mag. You might get more info from the VMCC library (you don't need to be a member). I can send a scan copy if all else fails.

Cheers Pat

Cheers Pat

That's good of you! If I can't find it I might get in touch with you.

Thanks for the info.

Ben

email (option): mad_king_soup@hotmail.com

Hi Ben a better photo in January's Mag

Cheers Pat