Comet suspension

[davidd]

I'm a bit confused (about normal for me) what you mean by "preloaded". I spent some time in the 70s sorting suspension on a couple of specials I built, unfortunately I've forgotten most of what I learned, but I'm sure the idea was to set the spring "preload" so that, with the rider on the bike, the suspension was about 25% compressed.
The theory being that bumps go down (potholes) as well as up, and the springs keep the tyre in contact as the road falls away. If this wasn't the case, the front end is likely to become light and light front end plus bumps seem to be the prerequisites for a tankslappper.
I'm happy with 1 and 2, it's point 3 that I don't understand, my knowledge of girders is about the same as my knowledge of the dark side of the moon, but surely the springs become preloaded when they take the weight of the bike.

H

Howard,

When the springs take the weight of the bike they simply become loaded. When you jack up your bike and compress the spring box to fit on the perches, that is pre load. Simply put for the Girdraulic, there is a downward force that you must exceed before the outer spring box moves, that is the pre load. Many owners have to use tie downs or spring compressors to fit the spring boxes, which means the pre loads are quite high. With the Thorntons it is 150 lbs. per spring or 300 total.

My racer weighs 285 and when you realize that only 47% of that weight is on the front (the rest is on the rear), then you subtract the unsprung weight, there is only about 100 lbs pushing down against the front springs as sprung weight. You have that 300 lbs of pre load that you have to overcome that number before the spring boxes move and the bike loads the spring around 100 lbs. If you weigh 200 lbs. with gear and assume 47% of that goes to the front suspension, which is still only 200 lbs. pushing down against the 300 lbs. of pre load pushing up, so the forks are still at full extension. Because the front end stayed at 0% compressed I thought that this needed investigation. I would note that Thornton first recommended 30 or 35% of the damper rod exposed. He then upped it to 50% exposed. I always thought it was an odd recommendation as there is no simple ride height adjustment on a Vincent.

David

 

[hadronuk]

A degressive system could be added to a standard Girdraulic “C” or "D" spring to give zero preload.

The internal springs cost £4 each. Total material cost should be less than £25.

Lathe and welding or brazing equipment needed for construction and assembly.

 

[vibrac]

So far graphs and all I will stick with my broom handles but I will check the strength...

 

[hadronuk]

So far graphs and all I will stick with my broom handles but I will check the strength...

Mk1 broomstick.

 

[Howard]

Ok Davidd and Hadronuk (you'll have to tell us your name I keep transposing the r and d when I read your nom de plume). Brain now totally addled, but I don't think we're all that far apart. I think I was talking suspension preload not spring preload, hence my confusion.

Let's say we have 4" of travel, Davidd's 200lb bike and rider weight on the front end, and the heaviest bump the springs take is 600 lb (just for calcs). By my memory the upward spring travel should be 75% of max, so the springs need to take 300 lb each in 3" ie 100lb/in to take full advantage of the travel available. Unfortunately, Girdraulics have no way to adjust the ride height (preload adjustment?) but you need to compress the springs 100 lb each (1") at 25% of the suspension travel ........... maybe we could use a piece of broom handle.

My lovely coil over damper system had a preload adjuster.

H

 

[hadronuk]

Ok Davidd and Hadronuk (you'll have to tell us your name I keep transposing the r and d when I read your nom de plume). Brain now totally addled, but I don't think we're all that far apart. I think I was talking suspension preload not spring preload, hence my confusion.

Let's say we have 4" of travel, Davidd's 200lb bike and rider weight on the front end, and the heaviest bump the springs take is 600 lb (just for calcs). By my memory the upward spring travel should be 75% of max, so the springs need to take 300 lb each in 3" ie 100lb/in to take full advantage of the travel available. Unfortunately, Girdraulics have no way to adjust the ride height (preload adjustment?) but you need to compress the springs 100 lb each (1") at 25% of the suspension travel ........... maybe we could use a piece of broom handle.

My lovely coil over damper system had a preload adjuster.

H

Hi Howard, I'm Rob. I'm a bit concerned by my wifes loud laughter at your comment.
Anyway, yes it's a pain having to remove the spring cases to add/remove spacers to adjust ride height. I have just bought some new spring cases and the "ends" are longer by a total of about 3/8 inch, i.e. less room for the springs, so the ride height is altered and I had to remove previously correct spacers. If I understand your example correctly, the suspension in theory should just bottom out. In practice of course, the hydraulic damper and the high friction from the heavily loaded bearings would halt the suspension before that. I think somewhere Phil Irving said it was bad practice to use stiff dampers to compensate for soft springs. I suppose it’s a question of degree, but Colin Chapman’s designs seem to work quite well. Perhaps because damper technology had improved by then?

 

[stumpy lord]

A word of warning when using a kawasaki type dampers. Always fit a washer larger than the eye end, below and above the eye end of the damper, as the eye end has been known to work its way of the rubber mount.
stumpy lord

 

[davidd]

A word of warning when using a kawasaki type dampers. Always fit a washer larger than the eye end, below and above the eye end of the damper, as the eye end has been known to work its way of the rubber mount.
stumpy lord

Good advice from Stumpy and I use one of these dampers that can be seen if post 107 on page 11.

David

 

[hadronuk]

A word of warning when using a kawasaki type dampers. Always fit a washer larger than the eye end, below and above the eye end of the damper, as the eye end has been known to work its way of the rubber mount.
stumpy lord

I didn't know that, thanks. Would you be willing post any pictures of your steering damper? After I had done mine, I saw ones where the damper sloped down steeply, so that it was possibly in a better position to give a more equal force in both directions, but without getting in the way.
(Edit) Just seen Davids post and picture. Does it fit a standard bike?

 

[vibrac]

And after years of racing the comet with a damper the most important hint
it must be loose not stiff on its mountings (able to wobble on the socket bearings) at the extremes of travel any stiffness and the seals will let go within a race or two
as I said earlier our dampers always go from a bracket at the rear RH head bolt to a eye bolt that uses the 5/16 hole in the eccentric fixing for the spring box. and its line just clears the tank bottom edge