The Significance of the SBDU H Reference Frame Number

Realistically, there may not be many ‘original owners’ of SBDU frames – I.E. those people who still own an SBDU frame that they ordered and bought for themselves from Ilkeston. There may be even fewer original owners who have a frame with an ‘H’ reference.

I mentioned in my previous post on the 753 Time Trial Special about the H reference stamp it had on the bottom of the BB shell and how this signifies a custom SBDU build.

Of the 6 SBDU frames I have owned, only 1 has had an H reference; so they definitely are not the ‘norm’. Normal SB referenced frames were built to stock geometry based on the model of frame – so a stock 56cm road frame should match another 56cm stock road frame but would have different geometry to a stock 56cm time trial frame.

Why have a frame custom built? I can’t answer that quickly, people write entire books on this subject, but there are a couple of particular reasons. Generally, not everyone is a standard size, people have shorter or longer legs and bodies, some people carry weight differently. And not everyone has a standard style of riding or use of a bike.

I’m curious and I want to know what is different with my 753 time trial frame and what exactly the ‘H’ reference changed in the design – I’ll never know the reason(s) WHY it was built differently to a stock frame, only the original owner and the person at Ilkeston who agreed to the changes would know that, but I should be able to see HOW it was built differently.

I’m in a position to do this as I’m lucky enough to have 2 Time Trial frames and both in 56cm, one stock and one ‘H’ so if I can measure all the tubes and angles, I should get an answer!

One of the documents I’ve seen from Ilkeston via the Yahoo group is a summary of the stock time trial dimensions…

With a sturdy, flat and level workbench, a metal rule, digital calipers, a set of sprints, an EDCO headset and an App on my iPhone to measure angles, I set about measuring both frames.

Stock Time Trial Frame Dimensions…

The first to get measured was my 56cm 1982 531SL Time Trial frame – this is SB5464 with no ‘H’ ref, so it should be stock! The idea is to measure the frame and forks and compare it to the Ilkeston document. This is what I came up with…

I don’t have a very technical setup to allow for some of the tolerances required to measure a 10th of a degree and I don’t have precision digital gauges, but the phone App and metal rule were surprisingly good. The main thing with the standard SB frame is that everything matched the Ilkeston stock measurements.***

Custom Time Trial Dimensions (H Reference)

Next up was my 56cm 1978 753 Time Trial frame – this is SB2692 with H591 – so how far does this deviate from stock? With the same sprints and headset fitted to give this the same stance, I set about measuring it. This is what I got…

Almost everything is different – the dimensions which have remained the same are fork rake, chain stay length, down tube length and of course, seat tube length.

The BB height measures 268mm which is actually the height for an SBDU road frame. This appears to be a 73 parallel frame, so the seat tube and head tube are more ‘relaxed’ together with a 5mm increase in top tube length. Because it has a lower BB height, it naturally has a slightly lower stand over height. The front centre measurement (centre of BB to centre of front axle) is also 5mm longer, but as the chain stay is the same length as stock, it has only increased overall wheel base by 5mm. One of the significant differences you can see with the eye without the need for measuring, is the clearance at the front around the forks. The down tube and head tube angle, shorter head tube and longer fork blades have given more clearance between the front wheel and the frame and more clearance between the tyre and the fork crown.

What is the significance…

Getting back to my post title “The Significance of the SBDU H Reference Frame Number”

Anyone buying a 2nd hand SBDU frame can buy a stock SB frame and should know what the geometry will be. That means you should have a good idea if it is going to fit you and be suitable. However, if you are buying a 2nd hand SBDU frame and it happens to have an H reference, then unless you are buying from the original owner, you have little or no idea if the frame you are buying will fit you or be suitable for what you want to do with it. You cannot simply buy a frame because it has the correct seat tube length.

The second point is that frame geometry is about “frame & forks”. If you have an H reference frame, it is really important to have the original and corresponding forks. Mine are stamped with 591 which shows that these forks were designed and built specifically to match the geometry of this frame. They both go together to create the position of the final built bike. If I fitted my stock 531 SL time trial forks to the custom 753 frame, even though rake is the same, the geometry and handling would be completely wrong.

*** I have compared a 1978 & 1982 frame to dimensions that were current in 1980. I do not know if SBDU ever changed and updated what they considered to be stock geometry.

** I’ve written a more recent blog post about the ‘H’ reference number, you can read it here **