FF: Forks Modified Steering Stem
- Thread starter greg brillus
- Start date
If I might intrude here, I think progressive springs are a very bad idea for standard geometry Girdraulics and questionable for modified forks. The inevitable increase in spring preload with progressive springs exacerbates the existing tendency for the standard forks to "jack" to full extension in some circumstances. This may have been a factor in Surtees '98 Goodwood tankslapper, as he had fitted progressive springs.
With the modified steering stem, because the links are now near parallel, any vertical force generated in one link is largely cancelled out by an opposite force from the other link. (It is not advisable to only consider the lower link, because under braking, tension forces in the top link are about 80% of the compression forces in the lower link.)
The benefits of the near parallel links may be sufficient to offset a largish spring preload, but I think there would be considerable problems in arriving at a suitable specification and source for progressive springs. Given the number of extra variables, think of the linear spring selection difficulties and then multiply a few times. A quick/cheap route might be to find an existing progressive spring S/H and hopefully chop bits off one end or the other. Springs for Triumphs might suit, but they are a bit small in diameter and around £80 a pop when new.
With the modified steering stem, because the links are now near parallel, any vertical force generated in one link is largely cancelled out by an opposite force from the other link. (It is not advisable to only consider the lower link, because under braking, tension forces in the top link are about 80% of the compression forces in the lower link.)
The benefits of the near parallel links may be sufficient to offset a largish spring preload, but I think there would be considerable problems in arriving at a suitable specification and source for progressive springs. Given the number of extra variables, think of the linear spring selection difficulties and then multiply a few times. A quick/cheap route might be to find an existing progressive spring S/H and hopefully chop bits off one end or the other. Springs for Triumphs might suit, but they are a bit small in diameter and around £80 a pop when new.
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I did think about progressive springs but did wonder whether, for test purposes, there might be another way. This would require another small diameter spring to be placed inside the springs provided, similar to the original Vincent design. The present springs all have a length of 16.5" (the 45 are 15.5") and are installed with 3" of pre-load, i.e. 13.5" long. The idea would be that an inner spring would have to have a length of, say 12.5" so it did nothing until the first inch of movement had occurred. After that it would increase the spring rate by an amount depending upon the inner spring rate. In the end the 45s proved so satisfactory when tested by Chris Launders (bearings in lower link, stiffer than standard AVO damper, twin with Chris with a weight of about 300 lbs, 134 kgs,) that there did not seem to be any need to go further. Chris posted a video of the movement of his forks over a ten mile run on south Yorkshire roads and the forks moved really easily. The Tilleys have just found the same, a really easy and controlled movement over a wide variety of road surfaces, from small ripples to sleeping policemen. This ability to deflect easily with small imperfections in the road surface suggests to me that this system is not over damped and the resistance to bottoming out (with an inch cut of the inner and outer spring boxes) also suggests that the spring rates are about right. breaking heavily while going down hill seems to me to be a good test but will cause bottoming out unless the spring boxes are shortened. The system is designed to allow 3" of movement and that takes the outer spring box very near/colliding with the bracket on the fork blade. Tracy tells me that they have also made a video of the fork movement so perhaps we will see that some time. Once again, the more feedback the better.
Not too sure about the benefits of reduced front wheel weight when loaded.
I have just been measuring wheel weights in an attempt to use the "3g bump" rule of thumb to help rear spring selection.
With my 166lb weight onboard, my Rapide front wheel weight is 294lb and the rear wheel weight is 360lb.
Rough estimate, passenger COG 6" forward of rear axle so about 10% of passenger weight goes to front. For 166lb passenger, this gives 311lbs on front wheel, 509lbs in rear wheel. Yes, any luggage behind the rear axle subtracts from front wheel weight, but I would have thought that sufficient to noticeably reduce front wheel weight would result in hairy handling.
Doh! Good idea, I had forgotten about that possibility. As only shortish spring travel is required, a cheap solution might be a short spring plus a spacer that could be adjusted in length if required.
Apologies if this has been mentioned, but how much leeway have you got before the springs are coil bound, and is this being checked for each application? Only when I was messing about with pulldown springs, I found there was a massive variation in front spring box "end lengths" to the extent that in the worst case a correct spec D spring (56 turns) could potentially become coil bound.
I designed them to be one inch away from coil bound under maximum compression. That is the three inches of pre-load plus three inches of movement/compression. However, and this is an important point that you raise, if the ends of the spring boxes are so long that these measurements are compromised then that might explain why some people are finding different behaviour. I have just had a look at three spring boxes I have lying around and visually they appear the same. Distance from hole centre to the top of the tube on the outer is 30 mm. Distance from centre of hole to bottom of tube on inner, about 14 mm. Assuming that the thickness of the metal which blanks off the ends of the tubes is about 1 mm then that gives a total 'wasted' length of about 46 mm (1.81"). If there is a wide range in the lengths of these parts then that will have to be taken into account.
It's too hot to get leathered (lathered) up so decided to ckeck my front end.
SeriesD, Gregs bearings, 45 lbs springs ( I think), standard damper, long eye bolts, original rear monoshock.
I weigh 191 lbs (no gear). The deflection at the damper, measured from a position of both wheels off the ground to me sitting on the bike is 1ins. Will see how this changes when riding
Mac
Hope this helps
SeriesD, Gregs bearings, 45 lbs springs ( I think), standard damper, long eye bolts, original rear monoshock.
I weigh 191 lbs (no gear). The deflection at the damper, measured from a position of both wheels off the ground to me sitting on the bike is 1ins. Will see how this changes when riding
Mac
Hope this helps
Re coil bound springs etc, from my (fairly awful) memory, the biggest problem was with new spring boxes. This may have improved now, as I pointed out to VOCSC it caused several problems. I think the deceptive thing is the variable amount the end plugs protrude into the tubes. What David D usefully refers to as the "perch to perch" distance is quite awkward to calculate, so it is better measured directly using a bar of known length in place of the spring, then measuring hole centre distance.
From one of my old spreadsheets, I see I included a measurement I called "Total length of case ends" . I think this is the same as your "wasted length"? It was 2.706 inches for my new spring boxes and 2.125" for Big Eds original boxes. My original boxes measured 2.292". Quite a variation. Bugger innit?
From one of my old spreadsheets, I see I included a measurement I called "Total length of case ends" . I think this is the same as your "wasted length"? It was 2.706 inches for my new spring boxes and 2.125" for Big Eds original boxes. My original boxes measured 2.292". Quite a variation. Bugger innit?
A Bugger indeed. Given two 45 lbs/inch springs which equate to one 90 lbs/inch spring and then allowing a difference of 0.6" in available length gives a force available difference of 54 lbs. Equivalent to 1.2" on one 45 spring. Where is this malformed rubbish coming from? I like the Davidd's method of using a bar to measure the set up. I have noticed differences in the lengths of the springs I am getting made of about one eighth of an inch but I think that we can live with that.
I can't attach an AVI movie, so I'll post it to youtube.
Here's Chris' answers to the questions raised. He weighs 145LB dry!
He rode to the Severn Rally alone on the 45lb springs with the damper on but gradually reduced to completely off. Looking for potholes to make it work and watching the forks under heavy braking.
Then a Rally, Porlock, loaded as the photo, 2 up, I'm " cough" more than 170lb! Forks still a bit stiff on bumps. So, changed the springs to 36 lbs and tested locally one up.
So for the French kept the 36 lbs and one turn in on the damper.
Very happy, French roads, twistys, Andorra, Col de Chioula etc etc. When accelerating there is an obvious rise and then subsequent drop of the bars.
Here's Chris' answers to the questions raised. He weighs 145LB dry!
He rode to the Severn Rally alone on the 45lb springs with the damper on but gradually reduced to completely off. Looking for potholes to make it work and watching the forks under heavy braking.
Then a Rally, Porlock, loaded as the photo, 2 up, I'm " cough" more than 170lb! Forks still a bit stiff on bumps. So, changed the springs to 36 lbs and tested locally one up.
So for the French kept the 36 lbs and one turn in on the damper.
Very happy, French roads, twistys, Andorra, Col de Chioula etc etc. When accelerating there is an obvious rise and then subsequent drop of the bars.