SB664 1976 SBDU Ilkeston Reynolds 753 – Geometry and Design

After working out that SB664 was built from an early type of Imperial Reynolds 753 tubing, I wanted to use the calipers and metal rule again to try and figure out what type of use the frame was built for. Was the geometry designed specifically for Road or Time Trial; the outward appearance such as the frame transfers suggest that it is probably an SBDU Time Trial frame. SB664 is an early SB numbered 753 frame and was probably ordered in first part 1976 or even in late 1975. Reynolds 753 was brand new, it had only just been shown to the public in trade shows and it wasn’t widely available outside of the TI-Raleigh team. So what was this frame designed to be?

I’ve measured lots of SBDU frames and they never seem to 100% match the stock SB frame information, there are usually a couple of dimensions that just don’t tally, and I have a feeling that because of the unusual features of this frame, this may be the same. It is rare to have first hand information from original owners about the details of an order so in most cases, working out the intended purpose of a frame based on geometry is often an educated guesstimate.

My process for every frame documentation is the always the same; starting with fork rake.

Some refer to the fork rake as the curve of the blade, but it’s not, that is just a curve, even straight blade forks have a rake. Fork rake is the distance, or ‘offset’ between the centre of the steering line and the centre of the hub. You don’t need a fork jig to measure this, a few simple items will work to enable a good measurement; I wouldn’t call it an accurate measurement because I’m not working off a flat surface table, but it is often good enough. I set up my fork so that it is level in two planes using a couple of levels. The first ensures the steerer is level to the workbench and the second level ensures that the axle is also parallel to the workbench. The dummy front axle helps to locate the centre point to measure up to. These measurements all rely on the fork being straight and true.

• My ‘A’ measurement was 91 mm
• My ‘B’ measurement was 53 mm

Therefore, fork rake for SB664 is 91-53 = 38 mm

With that measurement stored, I can fit the headset, fit some wheels and get the stance of the frame and fork ready to be measured. The headset is Campagnolo Super Record and that lower stack height is perfect for setting the frame angles; the combination of wheel, tyre and headset will put the fork crown and head tube in the right place. The wheels are Mavic GEL sprint rims with 22 mm tubs so the axle line will be in the correct (or very close) position.

The head angle and seat angle are both relative to the horizontal line. I always use the top tube to mark that horizontal point. My workbench isn’t a replacement for a frame builder’s surface table, so setting the level box to zero (0.0 degrees) on the top tube will mean that the other measurements will be made relative to that.

• Head angle = 74.5 degree
• Seat angle = 74.1 degree

Bottom bracket height is next. Without an accurate centre point of the BB to measure to, I measure up from the workbench to the bottom of the BB shell then add on the radius of the shell.

• Radius of BB = 20.36 mm
• Height from work bench to BB = 242 mm

Therefore BB height is 242+20.36 = 262.36 mm

That is a low BB height. You will see later how low it is when it is compared to known stock measurements for SBDU Road and Time Trial frames. Here are the other tube measurements of SB664…

• Seat tube = 610 mm centre to top of seat lug (595 mm centre to centre)
• Top tube = 575 mm centre to centre
• Head tube = 174 mm

SB644 appears to have standard Campagnolo Piccolo short brake clearances. Front and back clearances measure approx 45 mm

Using these figures, the basic geometry of SB664 looks like this.

A few measurements that jump out for me are the low bottom bracket height, short top tube, short fork rake and steep seat tube angle. Those angles, rake and tube lengths are similar to what I’d except of an SBDU Time Trial frame, but that low BB height is unusual, even for a standard Road frame. When you compare the data for SB664 against the stock SBDU geometry for Road and Time Trial frames of the same size, you can spot the similarities and gaps much easier. I’ve indicated the closest matches to SB664 in green shading and most measurements fit closely to the Time Trial geometry.

Bottom bracket height influences frame geometry and handling in a few different ways; one way it does this is lowering the centre of gravity, this makes a more stable bike. However, that stability from a lower BB height can also mean less clearance for pedals through cornering. If the BB height was designed like this then the pedal clearance issue must have been known but not thought to have been a problem for the intended rider or use.

The problem is that the SBDU stock data I’m using for comparisons is from a little later in SBDU production, the Time Trial data I’m comparing my frame with is from 1980, but my frame is 1976. I don’t know if the SBDU were continually developing their frame geometries or if a 1976 Time Trial frame would be similar to a 1980 Time Trial frame. Does that mean that design and geometry was changing and evolving over the years? Is my BB height normal for 1976 or is it lower for a specific reason – I don’t think I’ll ever know.

Another interesting feature of SB664 (or missing feature) are water bottle bosses. This frame doesn’t have any.

Bottle bosses were a feature on SBDU frames from the very start. They refined the style of these bosses over the first few hundred SB frames, but whatever the design, they were definitely fitted to the down tube as standard. In all my SBDU frame data, with the exception of Track frames, I’ve only seen one other Road based frame that doesn’t have bottle bosses. Does this mean that the original owner wanted to use an existing clip type bottle cage or did they just not expect the frame to travel the distances that would require the need for a bottle and cage, and therefore specified the frame without them?

Does the low BB height and lack of bottle cage mounting mean that the intention for this frame was short and fairly straight roads? Did they specify this type of Imperial Reynolds 753 tubing, frame geometry and low BB height to give them a very stable, lightweight and stiff short distance frame?

Based on the greater amount of similarities to the Time Trial data (with the obvious exception of BB height), I’m going to categorise this frame as a Time Trial frame. Whatever the original intended use was, it is still a beautiful addition to my small sub collection of SBDU Time Trial frames…

• Top left – 1981 SB4409 Reynolds 753 Dynaflite Ovoid Tubing
• Bottom left – 1979 SB2692 Reynolds 753
• Bottom middle – 1983 SB5794 Reynolds 531c (Metric)
• Bottom right – 1982 SB5464 Reynolds 531SL
• Top right – 1976 SB664 Reynolds 753 (Imperial)