Sometimes my frames come to me without any signs of originality meaning that I can’t accurately identify them. There is seldom a known history to accompany a 2nd hand frame. SB7121 was a classic example of this. I bought it on a whim, based on a basic description which indicated that it was probably a Reynolds 753 frame. The frame number dated it to 1985 and it had a hand painted finish with ‘Wheel Craft’ frame transfers, but underneath the paint, it was 100% an SBDU. I wrote an initial blog post on the frame but always wanted to come back and dig deeper and take a proper look into what it was.
I’m currently writing another blog post looking into the stories told about Reynolds 753, and this frame features in that post, so I want to try and confirm what this is before I can move that post forward. The last time I looked at SB7121 was several months ago and that ended as usual with more questions to be answered. The main thing I found with it was that I could not fit a 27.2 seat pin into the seat tube – it came no where near fitting, it was just too big. After initially labelling this as a 753R frame, I now had second thoughts.
It will eventually be renovated which means I can dive in without worrying too much and get rid of some paint and investigate the raw tubes. Out came some tools and some heat and some emery paper! My thinking was that the diameter of the tubes would give me an indication of the tubing type. Measuring tube diameters on top of paint isn’t exact and can give poor results, and this frame has several coats with a thick brush painted top coat, so bare metal tube is always best. Starting with the top tube I removed about 4 inches of paint from the head lug to the first brake cable stop…
The digital calipers came in with a measurement of 26.03 mm – this is Metric diameter tubing, and that explains why a 27.2mm seat pin won’t fit and confirms that this isn’t Imperial size 753R or 531c. Imperial 753R/531c has a top tube diameter of 1″ or 25.4mm. If the top tube is 26.03mm then the seat tube should measure 28.00mm (Imperial would be 1 1/8″ or 28.6mm).
After exposing a few inches of the seat tube just above the BB shell, I measured 28.01mm – that confirms Metric tubing! A little bit more digging under the paint at the head lug revealed a beautiful thin line of Silver Solder, an indication of Reynolds 753.
So this is a 1985 frame using Metric 753 tubing, that must mean 753T.
The problem is that there aren’t many references to technical data about 753T tubing in any Reynolds documentation. Their own material focuses on 753R meaning 753T is a bit of a mystery tube. It’s something I’ll cover more in a future blog post.
People generally assume that 753T was only ever used on Track frames because the ‘T’, as defined by Reynolds, was ‘Track’, but it was actually listed as Pursuit or Time Trial. 753R was defined as ‘Race’, which was road racing and sprints. ‘T’ was thinner and therefore slightly lighter but meant it was less stiff – ‘R’ was slightly thicker and slightly heavier but it was stiffer.
The only dimension data I’ve ever found for Reynolds 753T is in the Paterek Manual for Bicycle Frame Builders and was dated 1984. It listed 753T as Metric tubing with the following tube spec…
Outer Diameter (Wall Thickness) Top Tube 26.0 (0.7/0.3/0.7) Seat Tube 28.0 (0.7/0.3) Down Tube 28.0 (0.8/0.5/0.8) Seat Stays 12.0/16.0/11.0 Double Tapered (0.5) Chain Stays 22.0/13.0 (0.6)
You will see that the data for 753T is very similar to the original 753 from 1975 to 1982. The only difference is the seat and chain stay design. The original spec 753 had a Single Taper seat stay with a thinner wall which also tapered to a smaller diameter (16.0/10.0) . Original 753 chain stays also tapered to a more narrow end (22.0/11.0). This means that for a similar size frame, a 753T is just a little bit heavier than an earlier 753. SB7121 weighs in at 1678 grams. If this was original 753 with the lighter stays, I would expect closer to 1600 grams, maybe sub 1600 grams.
Back in April last year, I only had 1984 and 1986 SBDU information, but since then I’ve seen some 1985 information and this is what it lists for SBDU 753 model frames…
Based on that list, SB7121 was looking like a TTS (Time Trial Special), with 753T tubing, Cinelli BB, Cinelli SC fork crown and Campag (road) fork ends. But the SBDU is never as clear cut as that. The description that accompanies the list is much more general.
After reading that paragraph, it means that this frame could be either a Road or TTS frame. The wording is non-specific and seems to show that ANY of the available options could be spec’d for either type of frame. Fast Back or Conventional stays were offered, 753R and T was offered and Vertical or Record fork ends were offered. ‘Record’ ends were conventional horizontal 1010B short road ends. This choice of ends is something that is often mis-understood because SBDU frames are often categorised incorrectly based simply on what ends the frame has. This clearly shows that a TTS frame can have Horizontal ends and a Road frame can have Vertical ends.
That means I’m still no clearer; I know the tubing, but not the model definition, was it Road or Time Trial, or was it a mixed setup? To be honest, I don’t think I’ll ever know. The only people that would know the answer for the spec of this frame would be the customer that ordered it and the person at the SBDU who spec’d it. All I want to do now is see if I can look at frame geometry to see if that offers any clues for me to identify the design. Tube lengths, angles, BB height and fork rake may give up this frame’s secrets.
The SBDU had what they called a ‘stock’ geometry for both Road and TTS frames – this stock geometry is what I feel was always the starting point for custom frame design. It is relatively easy to measure a frame with a few basic rules and guides. I fit a set of sprints with some Clement tubs that would have been used for good road racing or time trials. I also fit a std head set – the bottom stack is generally about 14mm. The wheels and tubs together with the headset give the frame the correct ‘stance’ so that measurements can be taken.
Some things are easier to measure than others. The seat tube length is given to you by the SBDU, the top tube is simply centre seat tube to centre head tube, the head tube length is straight forward.
- My seat tube is 540mm
- My top tube is 540mm
- My head tube is 118mm
The BB height is more difficult to measure because there is nothing that is directly referencing the centre point of the BB – I don’t have this frame in any kind of fixture where the centre of the BB is a known fixed point, so I measure from the table surface up to the bottom surface of the BB shell and add this measurement to half the diameter (radius) of the BB shell.
- My BB height is 248mm (distance from table to shell) + 19.9mm (radius of 39.80mm shell) = 267.9mm
‘Front centres’ is the centre of the BB to the centre of the front axle.
- My front centre is 562mm (distance from front axle to edge of BB shell) + 19.9mm (radius of 39.80mm shell) = 581.9mm
Head tube and seat tube angles can be measured fairly accurately with a digital angle gauge. I set the gauge to ‘zero’ on the top tube – because your worktop or table may not be truly level, it is always best to zero the gauge at the top tube. Other measurements will then be relative to the top tube and that zero point.
My gauge is magnetic so will stick quite happily to steel tubes. With the gauge on the seat tube and then on the head tube, let it settle and note the reading.
- My seat tube angle is 74.6 degree
- My head tube angle is 73.1 degree
Fork rake is probably the most tricky. Frame builders have lots of guides and jigs that they can use to set up forks for building and these will allow them to measure rake. But if you don’t have these facilities then you need to ‘make do’ with a home made set up.
Fork rake is the distance between the centre line of the steerer and the centre line of the front axle. The ‘rake’ is also called the ‘offset’; the distance the front axle sits in front of the centre of the steerer. Setting up something like the diagram below is straightforward, you just need to get things level in 2 directions – the steerer needs to be level to the table and the front of the fork blades need to be level across each other.
My set up is like this. The digital level gauge, a spirit level, a seatpin to rest the fork blades on and an axle to help locate the centre of the dropout for measurements.
It’s possible to move and adjust the location of the seat pin to get both ‘up and down’ and ‘side to side’ level.
- My fork rake is 62mm (A) – 19.58 (B) = 42.42mm
After these figures have been rounded up/down, you can compare them to the SBDU ‘stock’ dimensions. The table below is a summary for a 54cm SBDU frame for both Road and TTS ‘stock’ geometry, together with my measurements for SB7121. The blue shaded areas are where SB7121 matches against the SBDU data.
So what would you make of that?
My frame dimensions match the SBDU TTS frame dimensions. My BB height matches the Road BB height. My fork rake doesn’t quite match the SBDU Road data for this size frame but it is close – the road data in the SBDU ‘stock’ chart, shows that the 54/55cm frame size switched fork rake. The 3mm less on the rake also decreases the Front Centre measurement by 3mm to 582mm.
In general, SB7121 looks like a match for a TTS frame, a more upright seat angle and shorter top tube but it has a little more stability by having a slightly lower BB and slightly longer rake/wheelbase. I guess that is the custom aspect that the SBDU offered.
After all this investigation and measuring, I’m going to categorise SB7121 as a TTS – a Time Trial Special. It is almost an exact match to all the data that I’ve seen from the SBDU. It matches the ‘8403 TTS’ model spec, it also matches the options listed for the 753 TTS models, and it matches almost all of the SBDU TTS geometry details.
The old saying is true, “Never Judge a Book by it’s Cover”!
