The "NO BOB GIRVIN"
- Thread starter Duxuk
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Duxuk
Retro Guru
I'm getting there with the fork. Although I only have bolts of the correct diameter to hold it together, it can now be squidged with very low air pressure in the shock.
I think the slope of the top link is probably too steep. It causes too big a change in the trail on compression. Still it's not disastrous and should provide interest when I ride it. I need to finalise some of the welding, fit the headset bottom race and then paint. Even with no nuts on the bolts it has a feeling of unflinching stability and strength. Pity it can't be lighter but that was never the aim. The aim was only that it would be rideable and trustworthy.
I think the slope of the top link is probably too steep. It causes too big a change in the trail on compression. Still it's not disastrous and should provide interest when I ride it. I need to finalise some of the welding, fit the headset bottom race and then paint. Even with no nuts on the bolts it has a feeling of unflinching stability and strength. Pity it can't be lighter but that was never the aim. The aim was only that it would be rideable and trustworthy.
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Assuming this works the way you planned, surely the next step is to chat with the resident fabricators & builders on here to get a small batch of these made...?
££CHING££
££CHING££
Duxuk
Retro Guru
I've been messing around with the half assembled fork and realised I can make a useful change to the geometry. This is one of the joys of a self build project, you give it a bit of thought and have a eureka moment. There's plenty of space between the bottom of the steerer tube and tyre so I can drop the bottom linkage mount on the fork leg by welding a bracket 30mm lower. This will correct the over steep top link and stop the big change in trail. I'm feeling much better about it now so I'll be off to Nuts of Chorley for some nuts. And bolts. Assemble with no lube for a trial ride. Take it to bits for paint. Life's good. I'm £14 in so far though I already had the shock and various bits of metal.
Duxuk
Retro Guru
Changing the geometry has reduced the offset of the axle. This may be contributing to the bike falling into slow corners. How do I know? I've ridden it!
It's a wet Friday so no riding today, except for a short test up the street but I've had the chance to do more work. I bought the appropriate socket cap bolts, nuts and washers. I removed the bottom linkage brackets on the fork legs, fitted the bolts and refitted the brackets lower down. This has left me with reduced offset but at least it doesn't change radically on compression. I fitted the fork to the Proflex with only 65psi in the shock. It only seemed to compress when something was blocking the front wheel, then it compressed easily. This is good news because it mimics the Whyte linkage so gave me the hope that the anti bob was working.
I fitted some flat pedals and took it out in the rain. No bobbing as I pedaled up the road and this will improve when I up the shock pressure. Running over a grid and I could tell the fork was absorbing the small ripples well. Bumping up and down kerbs was smooth and silent. The problem was a feeling of falling into slow corners and little desire to hold a straight line.
So rather than start to paint the fork I'm going to get a bit more offset. My original hope had been 50mm, uncompressed but I only have 25mm. This is probably due to moving the linkage brackets. What I'll do is make a shorter top link, which is the simpler one to make. This will pull the top of the fork backwards and pivot the axle out forwards to increase offset. It only needs a few millimeters of change and I'll calculate it.
It's a wet Friday so no riding today, except for a short test up the street but I've had the chance to do more work. I bought the appropriate socket cap bolts, nuts and washers. I removed the bottom linkage brackets on the fork legs, fitted the bolts and refitted the brackets lower down. This has left me with reduced offset but at least it doesn't change radically on compression. I fitted the fork to the Proflex with only 65psi in the shock. It only seemed to compress when something was blocking the front wheel, then it compressed easily. This is good news because it mimics the Whyte linkage so gave me the hope that the anti bob was working.
I fitted some flat pedals and took it out in the rain. No bobbing as I pedaled up the road and this will improve when I up the shock pressure. Running over a grid and I could tell the fork was absorbing the small ripples well. Bumping up and down kerbs was smooth and silent. The problem was a feeling of falling into slow corners and little desire to hold a straight line.
So rather than start to paint the fork I'm going to get a bit more offset. My original hope had been 50mm, uncompressed but I only have 25mm. This is probably due to moving the linkage brackets. What I'll do is make a shorter top link, which is the simpler one to make. This will pull the top of the fork backwards and pivot the axle out forwards to increase offset. It only needs a few millimeters of change and I'll calculate it.
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Such a great project, following each development closely
Duxuk
Retro Guru
I've shortened the top linkage by 10mm. Distance from bottom linkage pivot to axle was 15". From Bottom pivot to top was 6" so a 10mm shortening would give, by simple multiplication, a 25mm increase in offset. It seems to have worked a treat. The bike now feels good. The riding position has gone from extreme cross country to trail and suits me far better. I'm finding the steering is flopping to the left if I take my hands off. I don't suppose my alignment is quite right and I think the problem is crabbing. The 2 wheels are slightly misaligned so I'm moving the front to the left with spoke tension. It really is riding very well apart from this turning left thing. Fortunately it happens at a standstill so I can keep realigning the wheel until I'm happy. After a cup of Earl Grey (seriously) I'll be back on it.
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If it helps at all, there's a recent interview with Cy from Cotic in which he explains a lot about why his bikes are moving towards long and slack; long wheelbase and slack head angles. He talks a lot about trail, offset and their relationship with stem length. You might find it interesting, as he talks about exactly what you've described.
https://www.youtube.com/watch?v=IIyccZEPolU
https://www.youtube.com/watch?v=IIyccZEPolU
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