This is post number two looking at new arrival SB632, and I’m measuring geometry. How will this frame measure up? What size is it? How long (or short) is it? What angles does it use? What is the bottom bracket height? Does the fork clearance affect the frame geometry and size? More importantly, will the geometry give me any clues to enable me to pinpoint what this frame is? Lots of questions needing lots of answers!
Oh, and this is also my 100th published TI-Raleigh SBDU blog post! A small milestone that I’m very proud of.
As usual, I’ve fitted wheels and a headset so that the frame and forks are setup ready for measuring. I don’t have a set of track wheels to hand at the moment so I’ve squeezed in a road set just for this task; they are the same diameter wheel so the stance should be the same. First up are the seat and head angles and after setting the gauge to zero on the top tube, these two angles will show a reading relative to the initial ‘zeroed’ measurement.
It appears to be a parallel frame, meaning the head and seat tube angles match – 74.4 degrees. After a few minutes and with the help of some metal rules, I have a full list of frame measurements recorded.
I don’t have any geometry data for SBDU track frames, so these measurements and how they relate to specific track disciplines are a bit of an unknown at the moment. In fact, I don’t think track frames had a stock geometry as each rider and each discipline would be different. This is how my measurements of SB632 match up with stock geometry for SBDU road and time trial frames.
The SBDU measurement for the seat tube of this frame is ’52’. That is 52 cm from the centre of the bottom bracket to the very top tip of the seat lug (20.47″). The seat tube measurement from centre to centre is 50 cm (19.68″). An SBDU road frame BB height would normally be 268 mm and a time trial BB would be 270 mm – SB632 has a higher BB which is what I was expecting, but not as high as maybe a sprint specific frame – the BB height of this frame is 272 mm. The top tube length of 530 mm is similar to an SBDU time trial frame for this size frame. So far, the geometry of SB632 is more akin to that of a time trial frame than a road frame.
Different track events have different requirements and therefore the geometry will differ depending on the chosen discipline. A track pursuit frame would be different to a sprint frame. A pursuit would take place on the black line and the rider would attempt to hold that line; a pursuit frame wouldn’t need to climb the banking or require sudden changes in speed or direction. Although this frame has a steep head angle, it is combined with a rake that gives what many would consider to be a perfect trail, giving the frame a stable feel but keeping it agile.
The head tube of SB632 is longer than a comparable SBDU road or time trial frame. I would expect this because the overall length of the fork is shorter. The head tube must extend further, and be longer to meet the shorter fork, while keeping the top tube level. A track fork is shorter because it is built with zero or ‘nil’ clearance, there is no need to attach a brake caliper, the fork just needs to be long enough to hold a wheel and have enough clearance for the tyre.
I’ve attempted to demonstrate this in the image above. The red fork on the right for SB632 measures approx 364 mm but the fork on the left built for short reach piccolo brakes measures approx 375 mm to the top of the fork crown. The fork on the right with brake clearance is longer so the corresponding head tube would be shorter. It’s a basic demonstration but hopefully it shows the difference… an 11 mm difference in fork length.
If SB632 was a 52 cm road frame it would look different, it would look and feel smaller. The stand over height (the height of the top tube above the ground) would be lower because the bottom bracket would be lower. The head tube would also need to be approx 11 mm shorter to accommodate a longer fork (built with brake clearance). A 96 mm head tube on a 52 cm SBDU road frame would have the effect of almost bringing the 2 head lugs together – the gap at the back of the head tube between the head lugs would disappear and the lugs would touch. Geometry has a big affect on bike size and bike fit. A 52 cm frame from one builder and designed for a specific purpose is not the same as a 52 cm frame from another builder, and it’s so important to remember that when buying a bike.
Frame geometry is definately something that deserves its own blog post. I find it a fascinating topic as there are so many factors in frame geometry that affect other areas of the frame geometry. So maybe I’ll write one soon!
The fork rake on SB632 comes in at 33 mm. As I always say, these measurements are only approximate because I don’t use accurate frame and fork fixtures to measure these dimensions. I’m always quite close though!
Remember, fork offset (which I think is a much better term than rake), is the difference between the centre line of the steering axis and the centre line of the front axle – it is the distance that the hub sits in front of the steering line – fork rake is not the curve of the blade. With the fork offset of 33 mm and a head angle of 74.4 degrees, the ‘Trail’ equates to 59 mm. Trail is something that I’ll cover in a frame geometry post, but it is trail that affects Steering. or even just holding a straight line.
One final point regarding the fork on SB632 is that it is very light, built with a lightweight and as yet unidentified fork crown, and oval blades, ideal for pursuits. A standard SBDU track fork would use a stiff Fischer fork crown and round stiff blades. The features of a standard track fork would produce a stiff, strong and heavy fork, strong enough to deal with the forces generated in track sprinting. SB632 just doesn’t need these super stiff features.
So I’m happy to label SB632 as a track pursuit frame, built with similar geometry and characteristics to a road time trial frame. It is a steep angled frame with a short wheelbase but retaining excellent steering and handling. It is built for pursuits using light gauge 753 tubing producing a frame that is stiff enough and light enough for the rider and purpose. This is the aim of every frame produced by the SBDU – matching frame material and design to a rider and their racing.
The modification to the rear track ends by adding a gear hanger also fits in with my theory that it started life as a track pursuit frame and ended as a dual purpose pursuit/time trial frame. This frame would probably suit Beryl for either discipline.
Frame size will always be a topic of discussion and it is usually the first thing someone will mention when trying to relate a bike to a rider. Just remember that perceived bike size can be misleading due to geometry and usage. From what I’ve seen, Beryl rode a progressively smaller frame over the years. Her frames appeared larger with the saddle almost sitting directly on the top tube with very little seat pin showing (bottom left image). This changed gradually with slightly smaller frames and a little bit more seat pin (top middle), until the early to mid 1980s when her frames were much smaller with lots more seat pin on show (bottom right).
Beryl Burton had a career that spanned several decades, starting in the 1950s, she won World titles on the road and also won five World championship pursuit titles on the track, with lots of silver and bronze in other years. She also had a long list of National titles and records. I have excellent provenance that SB632 came from Beryl, but sadly I don’t know how and when she used it. But the style and geometry of SB632 fits in perfectly with Beryl Burton’s pursuit and time trialling specialities.
Whatever this frame is or was, it’s association to Beryl is amazing, and I’m over the moon to have it in my collection. I’m looking forward to bringing this frame back to original SBDU spec as a track pursuit frame when I undo the modifications to the rear ends. I’ll use the current red paint as the base to a TI-Raleigh renovation and add the black head tube and black and yellow seat tube panels.
I am a little bit torn about removing the gear hanger because it is part of this frame’s story, but that is why I write these blog posts, they capture that history. This frame is just going full circle and completing it’s own story.