Chapter 5. For Forks Sake

I thought I would start with a photo of what the bike looked like yesterday, just so you can be confident my story is going somewhere. Although sadly but not unsurprisingly we heard that it looks like Le Puy de Notre Dame is not going ahead in 2020. Nevertheless, the possibility of getting Marge ready for the event has given the whole project impetus, and I feel I’m in touching distance of having a usable pre-war bike.

In last week’s article I described how I discovered the fundamental reason for the odd appearance of the BSA B31-2 and rectified it by adding 72mm spacer tubes into the rear subframe chain-stays. By the way, if anyone has a suggestion of which model the original subframe may have been fitted to I would be pleased to hear – a Sloper perhaps?

I didn’t complete the welding at this stage since I knew I would have to attach some lugs for the rear engine plate mounting and wanted to reinforce the stays with sleeves. Although I had a “normal” looking bike, and could push it around and sit there for hours admiring it, I had to hold the frame together with some straps. Otherwise as you can imagine the rear part of the frame could pivot around the bolt at the top of the seat tube and come away from the front section. I had some exciting moments as the two halves would separate and I was left shouting for help – which never comes – and struggling to pull it back together. 

This reminds me of an occasion many years ago when I was either taking the engine out of my Norton Commando, or putting it back, when my brother turned up at our house with his girlfriend and her very attractive friend. Presumably I was distracted since as they went indoors I found myself holding the engine in both hands, with my arms either side around the top frame tube. I couldn’t put the engine back in place or get it to the ground. The friend, who is now my wife Jane, can still remember me screaming for my brother to come back and help me. Anyway, I got the bike fixed and together; Jane, the Norton and I have had many happy years since.

As I said before the BSA forks were home-made, looked home-made and I had resolved that I would chuck them fairly soon and try something else. As we all know, buying a new set of girders from a UK fabricator is going to be expensive – several times what I had spent on the bike itself. The Indian replicas look OK in the photos, but I could not find anyone who had bought a set and could testify to their quality. If they were rubbish I can’t imagine it’s the easiest thing to send them back and ask for a refund. A couple of months searching on eBay threw up some possibilities, but all looked rusty and pitted, probably need re-tubing so you’re back to handing over your cheque book to Jake Robbins. This prompted me to have another look at the home-made items. 

My tried and trusted technique of measuring the trail, using a straight edge along the line of the head tube and a square edge – in this case a sheet of alloy – in-line with the fork spindle showed that I had 2-3/4” trail. Having crawled around the various bikes at Brooklands to see what can be done with the Douglas EW600, I had a good picture of what the trail should be for a pre-war girder fork bike. The best handling Brooklands machines do indeed have a trail similar to these forks. It is less than a modern motorcycle, and in fact closer to a race bike. By the way there’s a good book by Tony Foale about motorcycle frame geometry – but believe me you can read it and become more confused than when you started – best to crawl around some proven machinery with a tape measure.

From the photo above you can see that the top and bottoms links look at too great an included angle. They should be nearly parallel, with the pivot points slightly further apart at the front than at the yokes. This preserves the trail when the forks compress – very much like double wishbone suspension on a car. The bottom yoke looked fine but the top yoke was a work in progress, stacked up on the head tube with a random collection of spacers and nuts. I tried the yoke “upside down” without the spacers and indeed it made the geometry look just right.

At this point it’s worth saying that having considered fabricating a new set of girders from scratch I got to the point where the components I would have made, for example cutting lugs from solid steel billets, were similar to those on the existing set. The fabrication and welding quality was actually very good, the real issues being the very thick plates that made up the lugs and that ugly plate between the front and back girder members. I have thinned the lugs down as shown in the following photos – they’re still in the region of ¼” at the axle and top yoke and added a mudguard stay lug to the rear of the girder. For the heavy plate I have cut a radius top and bottom to reduce the profile and made up some slightly unusual – hopefully not too unusual – damper disc assemblies which largely cover the offending part as you will see in the top photo.

Rightly or wrongly rather than make a top yoke from scratch I machined a substantial boss and welded it into the flat plate, added some strengthening ribs and the handlebar risers. For holding the top yoke assembly together and providing the necessary adjustment of the head bearings I made a threaded sleeve to screw down onto the yoke. The sleeve has eight equally spaced tapped holes and the boss three corresponding plain holes that together will provide one twenty-fourth of a turn adjustment. By my calculation, with a 28 t.p.i. cycle thread this will give 0.0015” (one-and-a-half thou) adjustment, which should be enough. To complete the job I’ve made up some new spindles and a new axle (EN16T), and welded on a brake cable lug. 

Well there you are, lots of photos this week which are easier to follow than lots of words. But I very much doubt too many of you will be queuing up for a ride until I’ve done a few laps, without the front end coming apart. Next week a few words about the magneto – those of a nervous disposition look way.

All the best

Gareth

p.s. Jane has just handed me a photo of her Dad, on what looks like a pre-war Norton, so the passion still burns brightly after all these years.