In the beginning there was Metric Reynolds 753 tubing, and then later in 1982, a new Imperial Reynolds 753 tube was introduced. The new tube, called 753R, had different diameters, different gauges and was slightly heavier than the original Metric tube. And that’s how it was always known… Metric 753 before 1982 and Imperial 753 available after 1982. That seems quite straightforward doesn’t it?
But this is the Specialist Bicycle Development Unit (SBDU) and nothing is straightforward! Over the last few years during my research into the SBDU, where that research often wandered into the realms of Reynolds tubing, several unusual Carlton Capella lugged frames have appeared, all with 27.2 mm seat pins, all built in the first few years of the SBDU at Ilkeston (which was in the Metric 753 tube period), all boasting that recognisable early 753 frame transfer and all displaying period SBDU 753 frame features. So what is unusual about these frames?
If you are up to date with my posts, then you may know why they are unusual. If you aren’t sure then this is why…
They are unusual because Carlton Capella were an Imperial size frame lug, which fits in perfectly with the 27.2 mm seat pin size, but according to all the Reynolds documentation, Reynolds 753 at that time was Metric, so how can Metric 753 tubes fit into Imperial lugs?
Well what if they weren’t Metric tubes?
With Imperial lugs, a 27.2 mm seat pin and with 753 features, what tubing are these frames actually built from? That is what I’m hoping to find out.
You can see the difference in tubing diameters in the chart below. I wrote a blog post recently about some of the many stories about the SBDU and Reynolds 753, and this early Imperial story is the only one out of four stories that I couldn’t answer, I just had no data on these frames, I had never personally owned one or even held one; I offered a few educated theories, but I just couldn’t get that crucial answer.
Hopefully I can finally revisit that blog and provide that answer because I now have SB664 in my collection. SB664 is a Carlton Capella lugged Reynolds 753 frame built in the Metric 753 period, it accepts a 27.2 mm seat pin and displays the 753 frame transfer and period SBDU 753 frame features.
This is the part of the process that I like. There is nothing I like better than getting to know these frames, measuring and recording them and adding them to the data I hold.
Starting with the fork. I’m confident that these are Reynolds 753, but as always, I like to set out my thinking so I’ll go through a few things and explain. It helps to have a diverse range of different sets of forks to compare. The fork in the centre is SB664, my 1976 fork. The LH end is SB518, a 1976 531 fork. The orange fork is a Mercian 531 fork built in 1976 according to the Mercian number stamped on the steerer. The RH fork is SB2692, a 1979 753 Time Trial Special fork. The last fork is another 531 fork from 1972.
The significant difference between 753 and 531 fork blades in 1976 was the dimension of the oval blade. At that time 531 was still using the narrow oval blade. 753 and 531SL were introduced with the wide oval (new continental). 531 wouldn’t officially swap to the wide oval until 1977. This is a little comparison video I did with SB518 to show the difference in fork blade width.
But I know I’m probably not dealing with a production set of 753 tubes so can’t rely on known tubing documentation and dates so I’ll look further. Just by handling the forks, you can feel that there is a significant difference in weight between the SB664 fork and the other 531 forks. Two of the forks I’m using to compare weight have shorter steerer tubes so I’ve cut a 50 mm length of spare steerer to make up the difference and to compensate on the scales. With the additional 50 mm steerer added to the scale, the silver fork is slightly lighter than SB664 which I think is due to the difference in fork crown. The Haden crown on SB664 is a chunky crown compared to the other.
The 531 fork weights come in much heavier.
As you can see, the two 531 forks with the internal crown are much heavier. Even with a lighter fork crown, the Mercian 531 fork is still significantly heavier than SB664.
531SL also had a wide oval blade and would have a similar weight to 753. But there are two reasons why I don’t think the SB664 fork is 531SL. Reason one is that I’ve only seen the SBDU use Haden on 753 frames, there are only a few examples of these crowns in use, but all so far on 753. Reason two is that I already have a few 531SL frames and forks, and all the 531SL fork steerer tubes I have seen indicate the tubing type with a faint stamp marking of “Reynolds 531 Butted 16/13”. That may not always be the case, but SB664 has nothing except the ‘JH’ stamp.
Moving onto the frame. First up is seat pin size. Metric 753 took either a 27.0 mm or 26.8 mm seat pin depending on the gauge. SB664 was reported to take a 27.2 mm pin, and the Carlton Capella lug supported that. So I took a known 27.2 mm seat pin (taken from SB8851 753R), and fitted it to SB664, it fitted perfectly. The same seat pin was a perfect fit for SB518, an Imperial 531 double butted frame.
The Dura-Ace seat pin is 27.0 mm and was taken from the 1979 753 Time Trial Special that I’m using as a comparison. It is a sloppy fit in SB664 and drops straight down.
Seat pin size is 100% 27.2 mm – that size matches the Imperial lug. What about the frame tubes? Looking at the table above, I’m expecting to get readings of greater than 28.6 mm for the seat tube and down tube and greater than 25.4 mm and probably less than 26.0 mm for the top tube.
Tubes are difficult to measure! They are rarely perfectly round along the length of the tube. To combat this, I take three measurements and use the average.
- The Seat Tube average was 28.83 mm
- The Down Tube average was 28.71 mm
- The Top Tube average was 25.54 mm
All three measurements fall into the Imperial range. I also did a quick measure of the chainstay at the BB and the chainstays on SB664 are larger than my Metric frame chainstays. The bottom bracket is still a mystery, I’m still researching this. The BB fitted to SB664 is made by ‘RGF’. This is the same that the SBDU used on their Metric tubed bikes – but how does a larger Imperial tube fit into a smaller Metric socket? Did the SBDU enlarge them or was there an Imperial RGF shell?
SB664 is on the left and my Metric tubed Reynolds 753 1979 Time Trial Special, SB2692 on the right.
I think I need a short summary…
- I’m confident that the fork is Reynolds 753
- I’m confident that the seat pin size is 27.2 (Imperial)
- I’m confident that the lugs and tubes are Imperial
- I’m confident that the BB is RGF, the same as SBDU’s Metric frames
- I’ve got a small question to answer on the BB socket size
That leaves frame weight. The standard Imperial tube in use at the SBDU in this period was Reynolds 531 Double Butted. It was a great tube, but it was significantly heavier than Reynolds 753. I’ve got frame weight data on every SB frame I have in the collection. Two examples of 531DB frames are SB1861 which is a 54 cm frame, and SB518 which is a 57 cm frame.
- SB1851 (54 cm) weighs 2022 grams
- SB518 (57 cm) weighs 2209 grams (see image below)
SB664 is a large frame, it is 61 cm – if it was 531DB then it would easily be in excess of the figure above. But it isn’t, it is light. You can tell it’s light even without needing to put it on the scales, you can tell when you handle it. So the reading from the scale was no surprise.
SB664 weighs 1888 grams – that is definately not made from 531DB tubing. This weight puts SB664 in the region of other Imperial 753R frames!
Time for another recap!
- SB664 has Imperial diameter tubing
- SB664 has Reynolds 753 fork blades
- SB664 has original paint and transfers
- SB664 has period SBDU 753 frame details
- SB664 is significantly lighter than a similar size 531DB frame
With all these checks and measurements done, I now think I have proof that my theory in the earlier blog post about the SBDU and their use of 753 was correct. The SBDU was in a unique position with their association via Tube Investments to the Reynolds Tube Company (which was to become TI-Reynolds). I think it is certain that Metric 753 was not the only 753 available within the walls of Ilkeston. There was definately Imperial size 753 tubing several years before Reynolds introduced it under the name of 753R. SB664 and the other handful of similar frames prove this.
I still have questions to answer over the RGF BB shell, but there is no denying that SB664 isn’t just a beautiful frame, it is also one of only a handful of examples of this age and type of tubing. I’ve seen a few 753 Capella examples from early 1976 and half a dozen examples built in the last part of 1977, including TI-Raleigh team frames.
There may be more of these early Imperial tube frames but built with more conventional Prugnat lugs. These frames may have been built to provide a heavy gauge 753, or it may have just been an exercise to use up a small stock of developmental tubing. They may have built this tube type with Capella lugs for easy identification.
We might never know why this tube exists or why these frames were built with the combination of tubing and lug; one thing I know for certain is that it is another SBDU oddity.