SB3505 1980 SBDU Ilkeston Reynolds 753 TI-Raleigh Team Pro – Geometry and Design

The SBDU seemed to stop stamping their H reference on frames towards the end of 1979, meaning the frames coming out of Ilkeston at the beginning of the 80s would only have the SB frame number stamp and nothing to identify a frame as somehow different. I guess from this point on, the only reference to a specific build or geometry would be shown on the paperwork and build sheets that followed the frame and fork through its production, eventually ending up in Ilkeston’s records. If your frame does have an H reference then you still really have no idea what the ‘H’ was all about unless you have original documentation;  that is why I try and measure, document and figure out all my frames.

Everyone has a different need for a frame, either a specific cycling discipline, a pedalling style, or the requirement to fit particular parts or maybe a need for geometry to suit body shape. Ilkeston were no different to other custom frame builders, and their customers had their own requirements, meaning that the SBDU frame you own was more than likely built to suit any number of specific customer needs.

I wonder how SB3505 will measure up? At first glance it is a standard Reynolds 753 Team Pro road frame; but the two stories I have of this frame are based around time trials. So will the frame geometry make things clearer? But first things first… frame weight. I have the weight of each of my SBDU frames documented, every bit of data adds to the knowledge base. I’m not a weight weenie but I like to have this data and use frame weights for comparisons to other frames of different sizes and tubing.

The fork weight is what I would expect. Although the fork weight seems very light, it is a small frame with a short column. Blades are often very similar in gauge, and forks will typically only vary in weight due to column length and choice of fork crown. The frame is a little bit heavier than I would have expected it to be if it was built with the lighter gauge of 753, so it is possibly the slightly heavier tube.

Measuring the frame and fork geometry just takes a few basic tools and a little bit of time to set things up properly. I use a range of different length metal rules,  various calipers and a level box for measuring angles. The Super Record headset together with the sprint rims and tubs I am using give the frame and fork the correct stance. In this case I am using Mavic GEL280 rims with 22mm tubs.

Before actually fitting the headset, the fork rake can be measured. Fork rake is the offset (distance) between the centre line of the fork column and the centre line of the front hub. I know that these forks are nice and straight so it is easy enough to put them on a flat surface, set the steerer level and parallel to that surface and take a couple of measurements.

To find the fork column centre line, measure from the top of the column down to the table surface and subtract the radius of the column. The centre of the hub can be measured from the table surface up over, I fit a dummy front axle to make the measurement easier.

So for SB3505, fork rake is worked out as…

• (height of column from surface minus column radius) minus distance of axle from surface
• (63.54 – (25.4/2)) – 13 = 37.84 mm

The known SBDU measurement for a Time Trial fork rake was 37 mm, so with my not so accurate methods, my 37.84 is very close to that. With fork rake sorted, the headset and wheels can be fitted.

This is the first chance I’ve had to see the frame correctly setup with the top tube level and standing on wheels. Everything looks good but it does look like the bottom bracket is quite high. You can judge this by looking at the angle of the chainstay, the chainstay doesn’t drop very much from the rear axle to the BB, so BB height is the first thing I checked…

Different frame builders will measure geometry using either “BB drop” or “BB height”. BB drop is the measurement from the centre line of the wheel axles down to the centre of the BB, and as you can imagine, BB height is measured from the surface up to the centre of the BB. The SBDU quote BB height in their documents, and that is quite easy to figure out.

With a 12″ rule on the table surface, you can either use your eye to gauge the measurement to the centre of the BB, or measure up to the bottom of the BB shell and add on the shell radius. The stock SBDU road frame BB height was 268 mm with 270 mm on Time Trial frames, so 275 mm on SB3505 is much higher. My initial hunch about a high BB was right!

While I’ve got the metal rules on the table, I can measure the other tubes…

• Top tube is 525 mm centre to centre
• Head tube is 102 mm
• Chain stays are 413 mm
• Front Centre is 578 mm
• Wheelbase is 982 mm

The frame is stamped as 52 cm – the SBDU measure frame size from the centre of the BB to the very top tip of the seat lug. I can demonstrate this by placing a rule on top of the BB shell and measuring up to the top of the seat lug. That measurement is 50 mm – now add on 20 mm (radius of BB shell) and the result is 52 cm.

Seat tube length is often the only measurement quoted when someone is looking for a frame, but there are at least three different ways to measure that tube.

• centre of BB to centre of top tube
• centre of BB to top of top tube
• centre of BB to top of seat lug (the top of the seat lug is effectively the top of the seat tube).

A different BB height can also give you a different stand over height. You need to ensure that your idea of measuring the seat tube length matches a sellers idea of seat tube length. Depending on the method used, you could have a difference of between a 15 and 20 mm.

I find the topic of frame size so interesting. Seat tube length is just one small part of the whole picture, there are so many other aspects that are just as important or even more appropriate to frame size, and because of this, I’m planning another blog post specifically on that subject.

Seat tube length probably isn’t the best way to determine if a frame size is suitable, stand over height is much better. With a different metal rule, and measuring from the table surface, stand over height can be measured up to the top of the top tube.

Brake drop on the front and back is 45 mm centre to centre.

I measure frame angles from the horizontal top tube with a digital angle gauge. The table surface I’m using isn’t perfectly level so I ‘zero’ the gauge on the top tube and any angle I measure will be relative to that zero setting. It’s not a perfect situation to measure the head angle with the head badge still attached but it should do for this particular example.

While I’m taking all of these measurements I note them down on a blank frame drawing. I can then transfer them to my computer later.

The last item I need to measure to understand this frame is the seat pin size – this will tell me the type/gauge of Metric 753 that was used to build the frame. The common myth is that all small SBDU 753 frames were built with the lighter gauge and took a 27.0 mm seat pin and frames larger than maybe 58/59 cm took a 26.8 mm pin.

I’ve got different sets of dividers and calipers and use these to fit down inside the seat tube to get past any possible issues of a non-circular seat lug. The average of the measurements came out at 26.75 mm. so SB3505 takes a 26.8 mm pin, and that measurement yet again dispels the 753 seat pin size myth. I tested this size with 27.0 and 26.8 pins from other SBDU 753 frames and the 26.8 is a perfect fit. I’ve written about this commonly held seat pin assumption on a few occasions. Tube gauge seemed to be just one variable that was considered during a consultation.

The single butted seat tube on this frame will be 0.7/0.4 requiring a 26.8 mm seat pin. The lighter gauge of seat tube was 0.7/0.3 and took a 27.0 mm seat pin – this slight 0.1 increase in tube wall thickness adds a little to the weight but gives extra stiffness.

Taking all my measurements and notes, this is how SB3505 measures up…

Lots of these figures indicate a Time Trial specific geometry. The top tube length, fork rake and BB height are the main points.

There are loads of blogs and books online that give varying opinions on frame geometry and design, and I certainly don’t profess to be an expert, but in a nutshell, the effect of raising the BB height will influence a couple of frame features. It will shorten the chain stay and down tube length; it will also give more pedal to ground clearance. Shortening tubes will increase frame stiffness. The heavier gauge of 753 tubing will also give a slightly stiffer frame at the expense of a little bit of frame weight. Extra pedal clearance will allow the rider to lean more into corners while continuing to pedal. Everything about this frame seems shorter, more compact and stiffer.

When you compare the measurements from SB3505 against the stock road and time trial SBDU data things become clearer – the green shaded areas are the closest matches to SB3505.

The front centre is bang in the middle between road and TT. I’m about half a degree out for a road frame seat tube angle and one degree out on the TT frame (my digital angle gauge has a 0.1 error tolerance). Without any more knowledge of the build requirements and original consultation, I have to assume that the seat angle on SB3505 was selected by the customer.

This frame has the outward appearance of an SBDU road frame but with the hidden geometry of a Time Trial frame. The first story I have of this frame is a National 25 TT, the second story is an Eastern Counties 12 hour. Even though the frame came to me setup for a single ring using a Pista crank, and the picture I have of it being ridden shows the single ring, the frame also has double gear lever bosses and cable routing for a front derailleur. It has also been built with horizontal 1010/B rear ends (horizontal or vertical could have been spec’d on either road or TT frames). It looks like SB3505 was used equally for short and longer distance races and simply adapted through gearing to suit the course and type of racing.

The purpose of the blog post was to document the frame geometry, and that is done. But actually understanding the requirement behind this frame is a different thing and it’s still a bit of a mystery. Even with the insight I now have on the frame geometry of SB3505, I still can’t be 100% sure of its intended purpose. I’ll probably keep this categorised as a road frame, but it does highlight the fact that you cannot simply determine what an SBDU frame is based on frame features alone.