FF: Forks Modified Steering Stem

[timetraveller]

Greg, can I just check that I have understood you correctly. Your combination of a 50 lb/inch spring and a 33 lb/inch spring is equivalent to a pair of 41.5 lb/inch springs. Call that 42lb/inch. I confess that I had never considered what would be happening if one was doing a 'stoppy' when all the weight of both the rider and bike are on the front wheel. In an extreme case that could be 650 lbs and even with a 'seven stone weakling' it would be about 550 lbs. If the springs are exerting a force of 200 lbs with the bike stationary and perhaps 250 lbs with the rider on board they have to exert an additional force of 300 to 400 lbs. This over a movement of about 2". Recalling that there are two springs this leads to a required spring rate of about 75 lbs/inch to about 100 lbs/inch. If the travel allowed is increase to 2.5" then those springs rates decrease to 60 lbs/inch and 80 lbs/inch. Those are pretty serious springs and are not likely to be conducive to comfort. It seems to me therefore that either we have to accept the fact that under really severe braking the forks are going to bottom out, and I assume that under those conditions the last thing on ones mind is comfort, and we can have some comfort or we give up one the comfort and settle for a hard, barely moveable front end. The question is, are you satisfied that with the equivalent of two 42 lbs/inch springs and two inches of pre-load you have the correct compromise between topping out, bottoming out and comfort? There is no problem getting springs like that made but I am already out of pocket on springs and don't want to have to increase the price of the steering heads to try to recover that money. The orders is already in for 30 new steering heads to be machined in the new batch with some with your mod and some to use the Oilite bushes.

Additionally could I now ask those who want to use Greg's mod and want the lower links to be opened up here to now contact me and then send their lower links. I already have four and it makes sense to machine the whole lot in one batch. Thank you.

 

[Chris Launders]

Greg, I know what you mean and agree, My idea was to find the softest possible usable springs and then work back, that way we would have a lower limit to work from, I was somewhere around where I wanted to be until I fitted the needle rollers. I'm now trying to get back to that position.
I'm determined to try it with the two 36's and damper adjustment before increasing the spring rates, this will give me some idea of the dampers range of effects before altering anything else.
Chris.

 

[greg brillus]

Yes it is unfortunate that this "Experimenting" becomes expensive and almost wasteful, and when I had a small batch of springs made, when I first used them I thought what a waste of time and money that was. Sometimes if you have nothing to lose, as I did and decided to just cut the springs down to get some sort of useful result. As we know when you cut a spring down it becomes stronger, therefore when I cut down the 40 lb springs I had by 3 inches from my calculations this rounds out to 50 lb's. Similarly when I shortened one of Norman's 36 lb springs by 1 inch I figured that would be nearer to 38 lb's at least. This gives an average rate for two same springs of 44 to 45 lb's. In Neal's Comet the shorter 40 Lb springs we cut down by 2 inches off each spring which probably gives a similar spring rate, but the installed springs have about 3/4 of an inch of pre-load which you can nearly install by hand (similar to David's springs) whereas my springs on my twin have about 2 inches of installed pre-load. Because I have a new Thornton shock absorber up front, it is easy for me to check the visual bumper rubber on the shaft of the unit to see exactly what is going on, and this has been quite a useful tool in getting prompt results. I don't feel that high spring rates are necessary, thus returning the front end to the original stiff set up. But somewhere in between seems to work fine. I had my bike out four times over the weekend, twice with my wife on the rear, and the front end worked well over some pretty rough bits of road, and I don't believe it bottomed from what I could see of the front bumper rubber. But I did not ride too aggressively either, which is not nice on a pillion rider.

 

[timetraveller]

I had an interesting talk this afternoon with a chap who races modern bikes and it seems that they just take it for granted that different people need different springs. He was at a circuit where there was a suspension guru who looked at his bike and told him it was set up incorrectly although he, the rider, was happy with the handling. The expert charged him £50 to change just about all the settings, which only took minutes, and the handling was much improved. We might be on the limit of what we can do here!!

 

[greg brillus]

The fine tuning of the suspension is very experimental, but not in a dangerous way, provided you don't try anything silly. Our aim was to firstly improve the safety factor by changing the fork geometry so it could function correctly even under braking, and to improve ride quality by softening the front end to more acceptable levels. I would say we have achieved that without a doubt. Though as I said, I personally feel that the comfort is mostly improved with the singles due to their less weight, whereas the twins are much improved, but going soft on springs past a certain point achieves nothing except the bikes increased ability to bottom under heavy braking, thus a loss in suspension function which returns the bike to a similar, if not opposite problem to the original issue. When I rode Neal's Comet late last year, I would say it is the best Vincent as far as comfort and compliance of the front end that I have ever ridden, the rest of his bike is a completely stock Comet. So the changes are definitely worth while, it just needs more riders out there to find out for themselves.

 

[roy the mechanic]

I'm quite convinced that the last book the Ohlins guy read was "the kings new clothes". There are only three results when alterations are made, better worse or just the same. The trouble comes when the rider is unable to give reliable feed back.

 

[Martyn Goodwin]

Before I embarked on this exercise the front end of my comet was very stiff, due in most part to crappy high friction setup of the Girduralics. I had replaced the springs in the bike when I got it (a set of 16 1/2 D springs) with a mixed set of David Dunfey springs in the hope of preventing a tank slapper. I used one each of Davids 14" springs, one 75 and the other 130 Lb/inch, no preload spacers.

Thanks to the super support of Greg Brillus, I transisioned my Comet to the new "John Emmanuel" Steering Head.

Dave, My Crash Test Rider, has just returned after another run on my Comet after the latest 'tweek'. On his first run, with a pair of 30 Lb/in, 16 1/2 long springs, dave who weighs in around the 75 Kg mark commented that the front end bottomed out 'with ease' .

Based on my observations to date I knew that the front suspension was in equlibrium with an upwards force of approx 210 Lb (thats where it settles on static sag). I also knew that the pair of 30 Lb/in springs provided an full deflection upward force of 345 Lb, while Davids set up gave 615 Lbs. So I set about trying for a sag point between 25 and 30% and full deflection force mid way between the two earlier settings - around 450 Lb

I changed the springs using what I could find in my 'parts bin' and ended up with the sag point at 25% of travel and a full deflection force of 446 Lb.

Dave today reckongs Its been nailed - spot on.

He tells me that today he hit a few solid bumps but at no time did it feel that the suspension bottomed out. On subsequent examination we could see that it did actually bottom onto the bump rubber but with insifficient force to press that rubber over the shaft securing nut. (Thornton front )

In the right leg spring box I now have a 75 Lb/in spring, 14 inches free length plus a 3/4" spacer. In the left leg spring box I have a 30 Lb/in, 16 1/2 ling spring and NO preload spacer.

I am busting to get clearance to get on the bike myself - the medico's tell me it should be in around 2 weeks time!

Martyn

 

[greg brillus]

Hi there Martyn, Sounds like a good result. If you do the sums, you will find the springs to be near to what I had come up with on mine less the weight difference between the singles verses the twins. The set up in mine has an installed loading of 180 lb's. One side has approximately 40 Lb spring with 2 inches of pre-load, and the other is 50 Lb's with 2 inches of pre-load as well. It is not hard "Topped out" at all and after I sat on the bike and got off, the shocker rubber was down about 15 mm, ride comfort is great, though not as good as yours......................Cheers, Greg.

 

[Harry Hulstone]

Just got back from my second ride of Martyn G's Comet equipped with modified steering head and bearing linkages installed by Greg Brillus.
In my opinion it is now as good as it gets. Martyn has fitted a different combination of springs, harder, and played around with preload i think, he'll explain later I'm sure. The ride is now very plush. The road I was on had re-surfacing step ups which I was confident hitting without slowing down, unlike last time. Using the Thornton dampers bump stop as a measure of travel used it showed a couple of mm remaining.
At 80 kg with my gear on this is a perfect set up for me.
Time to get mine done me thinks : )

 

[timetraveller]

Someone might like to check my figures. I have worked out the installed total spring force and the maximum spring force for both of Martyn's and Greg's bikes using the figures they have given above. I have assumed that the installed spring height is 13.5" and the fully compressed spring height is 10.5". For Martyn's Comet I make his set up the equivalent of two 42 lbs/inch springs installed with 2" of preload and for Greg's twin I make that the equivalent of two 46 lbs/inch springs with two inches of preload. Both bikes have Greg's ball race modification, Chris, who weighs more than they do, found that 36 lbs/inch spring with 3" of preload to be suitable when he used Oilite bushes and with needle roller bearings installed at the rear of the lower link this now needs either more damping or stronger springs. If these figures are correct then it seems that we are homing in an acceptable solution.