Comet suspension

[greg brillus]

That's excellent David, thanks very much for posting these pictures.....people can now see what is happening from one type of fork combination to the other.....you can see why the stock Girdraulics is the odd one out, and the cause of the bad handling issues. Amazing how a relatively minor change, can make such a difference.....Cheers for now........Greg.

 

[davidd]

People spent a lot of money, To make Tele' Forks, (Anti dive ), We have it for Free !! Cheers Bill.

I was intrigued at the time all those incredibly complex anti-dive mechanisms that became popular in the 1970's. Hydraulic brakes were plumbed into the forks to allow fluid to be pumped into the legs to counter-act the dive due to rapid weight transfer. Interestingly, designers eventially found all of that was unnecessary. All they did was lower the center of gravity on the bikes and it minimized the weight transfer.

I guess that technically the Girdraulics are "anti-dive," but, I think I have said this before, they are really "pro-extension." It would be one think if you grabbed the brake and the fork and it did not dive, but with the Girdraulic every little bump you go over ratchets the front end higher until it is at full extension. This happens automatically if you use the brake while crossing railroad tracks, for example. And it does come at a little bit of a cost. The bad behavior causes the lower link quite a bit of damage. I suspect Maughan's has made a bit of money over the years repairing the "anti-dive" mechanism on Vincents.

David

 

[Chris Launders]

David by stock concentric do you mean standard FF2 with the eccentrics in the solo postion or do you mean with concentrics instead of the eccentrics.
Sorry I haven't been following the technicalities very closely.
Chris.

 

[davidd]

David by stock concentric do you mean standard FF2 with the eccentrics in the solo postion or do you mean with concentrics instead of the eccentrics.
Sorry I haven't been following the technicalities very closely.
Chris.

Thanks for pointing that out. So, in the first graph that plots the three axle paths I used the term Stock to refer to the use of the stock hole positions on the fork (and stock FF2). I plotted all three paths with the concentric. However, the use of the concentric or the eccentric does not change the shape of the path only the wheelbase (and therefore trail also). I plotted the three paths with a solo eccentric, concentric and then side car and they were all identical and in a row with the solo being the shortest, then the concentric and then side car as the longest.

David

 

[Chris Launders]

Thanks David I followed that ok,
Do you have any data giving the relative trails with the various FF2s with and without the eccentrics ?
Chris.

 

[Oldhaven]

Since I have been working on my Brampton front end and following this very interesting conversation at the same time, I noticed that the distance between the rear spindles for Brampton and Girdraulics varies considerably, if I measure correctly, and using the G's dimension in Hadronuk's spreadsheet. He has G's rear spindle distance as 8.21 and I measure 8.68 for B spindles. Almost half an inch. Of course other differences like link lengths may mean this is not significant. Another thing I thought about is that for those who fitted the Patzke taper roller conversion (I am in the middle of this), the distance for B's and (I think) G's will be almost 3mm greater. Is this enough to change the steering geometry by much?

 

[davidd]

Thanks David I followed that ok,
Do you have any data giving the relative trails with the various FF2s with and without the eccentrics ?
Chris.

The simple answer is "no." At the time I was trying to figure this out there were several old MPH articles that attributed the Girdraulic problems to the variation in trail. I made an assumption that trail did not matter. This was based on Tony Foale's experiments with rake and trail on the BMW. His conclusion was that trail did not matter on a solo bike, as long as there was some. Assuming this to be true, I eliminated trail variations as a cause.

I did a quick calculation at the time on the three paths in the first graph. From left to right at 1/4" above full droop the black line trail is 1-1/8", the green line is 1-13/16" and the red line is 1-7/8." Due to the variation in trail throughout the range of motion, it is difficult to say exactly what the trail is. I think it is best to use the numbers only relative to each other.

David

 

[Chris Launders]

Having spent the first 30 years or so of adult life building extremely long forked custom bikes and covering many tens of thousands of miles on them I have to basically agree with Tony Foale's conclusions, however trail does affect the "feel", too much and the steering becomes heavy and "flops" the more the bars are turned, too little and the steering is "nervous",for example a friend had a 1980 Ducati who kept dropping it in car parks as at slow speed when he turned the bars the forks tipped sideways , after a quick look I pointed out it had the wrong yokes, the legs and steering stem were practically in a straight line, these yokes were for the early models which had the axle mounted ahead of the fork leg whereas his with the axle below the fork leg should have had yokes with about a 2" offset, changed the yokes and it handled properly, he had had about 2" more trail than he should.
I have a Triumph that has no trail or even a negative value and it steers beautifully, this however is due to the fork length (around 60") and the steering head angle of about 55 degrees(yes I mean 55 )
I found my twin fairly heavy steering but have put this down to me having very broad shoulders and vincent bars being quite narrow so my arms were angled inwards, also this twisted my wrists when I used the brake or clutch. I have since fitted wider bars (30") and angled the ends back 10 degrees and it's great to ride, I have noticed the suspension locking up under braking and wondered if just using a modified FF2 without anything else would help.
Chris.

 

[davidd]

I found my twin fairly heavy steering but have put this down to me having very broad shoulders and vincent bars being quite narrow so my arms were angled inwards, also this twisted my wrists when I used the brake or clutch. I have since fitted wider bars (30") and angled the ends back 10 degrees and it's great to ride, I have noticed the suspension locking up under braking and wondered if just using a modified FF2 without anything else would help.
Chris.

My answer would be that just using the modified FF2 without anything else would help. However, you would probably end up wanting to use different springs also. There is not enough feed back on the best spring combinations with the FF2 Mod to give good advice. I think that will be remedied very soon, but I suspect you would be drawn into using new short springs because of the wide selection of spring rates. The FF2 all alone will make the wheelbase lengthening issue go away, which will in turn make the suspension seizing go away.

I think you have proved what Tony Foale was saying about rake and trail. Tony came to the conclusion that zero rake made the bike feel the best. Unfortunately, the tele forks did not work well under braking at zero degrees rake. His conclusion, if I remember correctly, was that rake angle did not matter at all and trail mattered only in that you needed some. I think he would agree about the "feel" being different with differing setups.

I had some incredibly bad experiences with non-stock handlebars. I have used non-stock handlebars in the past with no bad effects. I replaced a set of Clubman's, which I had always used successfully, with another set. This was during a long layoff from racing. When I returned to the track the bike was almost impossible to steer correctly. It had the strange effect of not wanting to change a line while in a corner. I thought it was me. After almost two seasons of this precarious handling I decided that I should give up racing, but I wanted to try one last crazy thing: changing the handlebars. The bike was transformed! In my first practice lap I cut 16 seconds off my lap time. We all know that all these Clubman bars are different. I can only speculate that by chance I stumbled onto a handlebar that was different enough to cause an issue. This still troubles me because I do not know exactly what was wrong. I am troubled by the fact that the change in bars did not just change the feel, but changed the entire handling characteristics of the bike. I did not think handlebars could do that. Since then I recommend stock bars to start with before any changes. Then you have a base line to judge against.

David

 

[Chris Launders]

I didn't alter the bars without trial, realizing first they were too narrow for me I bought a set of sidecar bars and rode with these for a while, they were much better but when in traffic for a while my wrists started aching badly, I realised that when operating the clutch and especially the brake my hands had to twist inwards sideways to line up with the bars. so I sat on a chair with my eyes closed and tried several sets of different bars gripping them tightly until I found a pair where I could do this with no stress, measured the angle and bent some 2nd hand bars to match. They have really transformed the whole bike for me, it steers better, no aching wrists, my arms are relaxed and for some reason less buffeting at speed, in fact I've done three runs out in the last couple of weeks 50/50 main roads and back roads each around 180 miles with no trouble.
As regards springs surely rider weight is a factor, I'm 18.5 stone so what would suit most wouldn't work for me.
Chris.