Suspension of Disbelief.

Suspension of disbelief is when you throw away logic and accept something surreal for the sake of enjoyment. Many might not believe it’s possible to build a part for a mountain bike which is as complex as a suspension fork. Some might fear it’s safety or think that it just can’t work. Maybe you think that technology is designed by wizards but it’s not. It’s designed by engineers, many of whom work in sheds in their gardens! I’m slightly better off than that since, my workshop is a garage at the side of the house which is also a laundry and place to keep a freezer. I’ve created 5 self designed and self built cars in the last 20 years and have always been very interested in the suspension systems. As I mention in my homepage I built my first suspension fork as a 15 year old at a time when mountain bikes hadn’t been invented as such. We just used any old bike for off road riding but only in Marin County, California did this quirky passion turn into a major, worldwide sport.

My recently acquired Proflex Attack LE came with a Rockshox Quadra 5 fork. It is a budget fork which uses an elastomer for both spring and damper. This is an elongated cylinder of squidgy, rubbery substance which compresses and springs back. The bushes on which the fork bottoms slide are notchy in action and the fork sags by almost half it’s travel when you sit your weight on the bike. This poor performance may have been a little better when the fork was new but not by much. On one of my rides I fell off, taking a bit of skin off my forearm so I resolved to improve matters. I also wanted to raise the handlebar height to make it less like a road bike and more like a modern trail bike.

You need to nail the design before you start to build such a project so the parts in the picture above were considered for a while before I started to make them. I wanted a fork which mimicked the performance of the linkage forks on my 2 classic, 2004 vintage Whyte bikes. These forks attach to the frame so I couldn’t copy them exactly. I decided to make my fork like a Girvin Vector, which was an option on some Proflex bikes in the 1990s. I changed the geometry to get the axle to move as it does with the Whyte forks. This J shaped path of axle travel prevents the fork from bobbing up and down as your weight tries to compress the fork when you pedal hard.

The Girvin Vector and Whyte forks.

Most forks are telescopic. The problems are twofold. Firstly, when you pedal they can bob up and down. This means that some of your pedaling energy ends up as heat in the fork, rather than propelling you forwards. Also telescopic fork have far more friction built in. The sliding bushes drag far more than the rotating bushes of a linkage fork. This particularly affects the performance over small bumps and the kind of rippled surfaces you find so often off road. Friday was a day of almost continuous rain so was a chance for me to get a lot more done on the project. Freddie, my dog, wasn’t happy at a day when he didn’t get a walk but I made some real progress. I bought the bolts I needed and finished the bottom linkage so it was time to fit it to the bike. I’d welded the lower bracket to the fork but it wasn’t right. The top linkage was too steep and it caused a big, negative change in the axle on compression. I dropped the brackets by 30mm and the axle now moved as I wanted.

Now the offset was too small. This is the distance that the axle lies forwards of the steering axis which turns the wheel. I test rode anyway and found the bike falling into slow corners and feeling unstable, not wanting to stick to a straight line. I knew what I needed to do. If I shortened the top linkage it would rotate the axle forwards, giving me the offset I needed. A 10mm shorter linkage gave me an increase of offset from 25mm to 50mm. The ride was transformed. It feels fantastic over small bumps. It’s very sensitive but when I pedal hard it feels stiff enough, refusing to bob unless I deliberately force it. It’s just what I hoped for and offers around 3 inches of travel. The Quadra 5 had 2 inches of travel but the initial sag accounted for a good proportion of this.

Not everything is perfect, though. If I let go of the handlebars the bike veers quickly to the left. There’s an alignment issue somewhere and I’m working on it. After that there’s just a bit of finishing and tidying to do, then I can paint it and have some stickers printed which will say “NO BOB GIRVIN”. Yes, like the Vector designed by Bob Girvin but with a greatly improved anti bob capability. Can’t wait for my first off road ride.