The "NO BOB GIRVIN"

[Duxuk]

My son is home for the weekend so we've had a look together. My front wheel is in line with the frame but the rear is not! It leans to the left when observed from behind. First thing to try will be to undo the skewer and pop the wheel up straight to see what happens. At least this isn't my fault

 

[Duxuk]

It’s painted, lubricated, assembled and ready to rock. I’ve cured the tendency to pull to the left and it’s smooth and silent. The head angle has eased off a tiny bit to 70.5 degrees, in as far as I can calculate it. It weighs quite heavy but since the bike started at a svelte 28.5 lbs, I don’t see that as a problem. I’ll walk the dog and take it off road.

 

[Duxuk]

I've ridden it
I took the Proflex for 3 laps of Healey Nab to the east of Chorley. I ride there a lot since it's only a 15 minute ride. There have always been trails on the hill but around 2010 a series of red and black graded trails were built, initially by well known designer Rowan Sorrell. The surface is almost always worth riding and after a good few days of dry weather was going to be fairly dry. It's typical man made trail riding. Virtually every corner is a berm and there are plenty of jumps, a few rockier sections and no massively steep climbs. I decided to be cautious and only ride the red trail. It's a circuit of 1.3 miles starting with a short climb to the trail head then a downhill of just under 2 minutes. The action continues with a section of 8 jumps followed by an undulating section with plenty of interest. The main climb to the trail head is only interrupted with a short, sharp downhill and has varying gradients. A fast, flowing downhill returns you to the start. Before any of this I'd set a second and a third fastest time on 2 road section on my way to the Nab. It may just represent my enthusiasm to ride a bike with a home made fork but it may also suggest that the narrow Velociraptor at nearly 40psi count for more than the extra bulk I've added. I noticed that over the stoney climb to the trails there was not a hint of bobbing. That's one aim ticked off. I'd decided to run the shock at 90psi with 50psi in the other chamber. There's then a rather stiff bump stop for the last 20% of travel. It's a DNM which cost me £65 but was sitting in a drawer unloved until now. The bigger rocks were rather more intrusive than on any of my 3 other bikes. The rear is more like a soft tail than a full sus. but was easily outperforming the RS Quadra 5 on my only other off road ride. 3 inches of travel at the front was never going to glide over rocks the size of half bricks, so a bit of shifting of body weight helped.
Since I arrived at the back gate I thought I'd go into a full red route lap and not go too mad. On early corners things just felt a little unusual. It was taking a bit of force to get it into the corners and on a fast kink where the gradient goes from a gentle but fast down to rising, bermed right hander I found my self running wide. As I continued the lap I realised that I needed to push the handlebars down into the turn and give it more force than I'm used to. The 560mm bars can't be helping here but at least they're original. After a short climb there's a new and fabulous section of jumps and turns. Down and turning left I lined up for the first big hump and forced myself to take a bit of speed off since I didn't know how it would jump. I always stand up on the approach and absorb the first one to keep it on the ground for the table top which follows straight after. I still was going quick enough to get some air. The series of twists which follow are sublime and with no noise, flex or feeling of danger I caressed the limit of grip. Nice. I'm think the back will let go first in this kind of terrain.
As I completed the 3 laps I kept making sure I steered in the "modern" way, leaning the bike but with my upper body staying fairly upright. This seemed to get the bike round without it wanting to run wide. I'm imagining that there's probably a tad too much offset in the fork after recent adjustments but with a little practice I have to say that the handling is safe enough. I can now confirm that the front now outperforms the back suspension. A lower pressure might make things better over the smallest of ripples but nothing needs fixing. I broke the chain on lap 3 but looking at the sprockets they're like new so a new chain should sort it out. I took the damaged links out, pinned it back together and finished the ride. I dropped in at the Bike Cabin to show the owner, Benji, the fruits of my labour and ask about a 7 speed chain.
A really good ride which I'm sure I'll remember forever. I'm delighted with the finished article and will ride it occasionally. As I get more used to it I think it will be very entertaining to use and the anti bob feature is something few other forks can boast.

 

[Duxuk]

I've written a slightly enhanced version of my ride report in my blog........https://wordpress.com/read/feeds/949356 ... 2466207979

 

[legrandefromage]

V brakes, please! They will stop you so much better than <squints at picture> a faff of cantilevers and some old Mag 21 brace!

 

[Retro Spud]

Great project and a nice read,

Can we have more pictures it’s the only thing that lets the thread down especially the finished product..... the solitary pic is a tad fuzzy

 

[Duxuk]

I took the picture after my chain break on a 20 quid smart phone. I just bought it for Strava and if it gets broken it's not the end of the world. It's a nice, sunny day so I'll get some better photos today.

 

[Duxuk]

V brakes, please! They will stop you so much better than <squints at picture> a faff of cantilevers and some old Mag 21 brace!

I've gone with the original cantis and they're colour matched! Anyway isn't braking the enemy of speed?
Seriously it's likely to be a dry weather bike and a well set up canti works OK (at best) when the rims are dry. The bridge is taken from the original Quadra 5s and doesn't act as a brace but only functions as a brake mount.

 

[Retro Spud]

Many thanks, look forward to seeing the pic’s

 

[Duxuk]

I'm having a bit of trouble getting the pictures onto my Wordpress page so I only have this one ATM.

It shows the different angles of the top and bottom linkages. This is important. When the fork compresses the sloping top link causes the axle to need to move slightly backwards in order to move up. This means that when weight is applied to the handlebars, such as when you're pedaling hard, the force tries to squeeze the wheel forwards, thus counteracting bobbing. When you hit even a tiny bump the wheel will be pushed back, then up and with low friction in the rotating bushes it will respond well to small bumps and ripples. That's the theory at least. Putting the front brake on and squidging the forks feels very much like doing the same thing with my Whyte PRST and JW. Without the brake applied, nothing moves. Put the brake on an it compresses.
The bottom of the steerer tube and the clamp above are made from 50mm X 25mm box section. Yes, they'd look a lot better if they were CNC'd but with a budget of £25 plus a tenner for a tin of Hammerite smooth that was never going to happen. I made 2 28.6mm holes in the top link and they fitted well over the steerer tube and clamp firmly. This clamp raised the handlebars by an inch and the space above the wheel is a further inch, so the bars are a much improved 2 inches higher. It was very low at the front previously. A one inch rise of the steering head has made a tiny difference to the head angle. As far as I can work it out it's still an old school 70.5 degrees.
The air valve of the shock is obscured by the top link so changing the pressure needs a partial disassembly. This might not be true of other shocks. If I could make one change I would like to reduce the offset to less than the current 50+ mm, though it does reduce on compression. Wider bars might help with control but this is still a retro bike so I don't want to change too much. I'm very pleased with my work and think it's a real improvement over the Quadra 5s. The anti bob feature is something a telescopic fork can never achieve without flicking a switch, at least.

I've managed to get another photo up. A top view shows that things are pretty snug. The zip ties show that on my ride I got most of the compression. The last bit is so stiff that I never seemed to get it all when I used the shock on my Whyte JW4. The gear cable had to go through the top link for alignment.