Although an SBDU frame may be best known for the oversize seat stay cap design, it was the SBDU fast back that landed with a bang towards the end of 1982 that almost took the top slot. These are my thoughts, backed up with some data, about the SBDU fast back.
Contents…
- Background
- Definition
- New Reynolds Range
- Double Taper Seat Stay Design
- Seat Lug Design
- Prugnat S4
- Prugnat 62D
- Fast Back Evolution – My Theory
- First Appearance of the SBDU Fast Back
- SBDU Documentation
- 1983
- 1984
- 1985
- Earliest SBDU Fast Back Frame
- What Does the Data Show
- The Earliest Known SBDU Fast Back?
- The Fast Back Was Nothing New…
- Restrictions on the SBDU Fast Back
- SBDU Explanation
- Services des Courses Frame Design
- Mudguards and Panniers
- Severe Service
Background
Definition
Before going further, I want to start with the term “fast back”. Is it a ‘fast back’ or is it ‘shot in stays’ or is it something else? Within the walls of the Specialist Bicycle Development Unit, it was documented as a fast back. So that is how I always reference it when I write about this style of SBDU seat stay. More of that documentation later…
New Reynolds Range
To understand the fast back, there is a little background required about Reynolds tubing. TI-Reynolds updated their range of tubes in 1982. I’m not 100% sure at which point of the year that was but certainly during that year, new tubesets and features appeared. I’ve shared this video before from the Veteran Cycle Club archive, but it is worth another share here. This is a classic period Peter Purves informational video at it’s best.
At approx 6 minutes into that video, he explains the new tubes within the 753T tubeset… “a double taper pattern 24 gauge seat stay”. It is that “double taper” which is important.
Double Taper Seat Stay Design
A double taper seat stay does exactly what it says on the tin… it starts narrow at the seat lug, then it tapers for the first time as it widens at the brake bridge, before starting to taper once again back down to a narrow section at the frame end. It has two tapers, a ‘Double’ taper. The easiest way is to show you is through the power of video.
Seat Lug Design
Prugnat S4
The S4 lug, that one with the small triangular window cut out in the side, was used by the SBDU on their Reynolds 753 range of frames before becoming the norm across the range for a short while. The seat lug chosen for this range had ears on the back to locate a double ended seat bolt. However, for most frames built by the SBDU, they would modify this and add a reinforcement, brazed into place and then tapped to accept the usual 8mm threaded steel hex head seat bolt.
Prugnat 62D
The 62D lug that was being used when the fast back was introduced. The type of seat lug the SBDU used did not have the seat binder ears. Instead, the lug was simply a seat tube and top tube socket with no seat binder arrangement formed on the rear. A steel seat binder bolt housing could then be attached. (Picture for explanation only – the SBDU seat lug had longer points).
Fast Back Evolution – My Theory
As my video explains, it is my theory (and it is only my theory – I was not there!) that the combination of lug and seat stay brought about this design. The ongoing use by the SBDU of the 62D and the emergence of the double taper seat stay from Reynolds came together beautifully. It wasn’t long before the well established Prugnat S4 lug disappeared from SBDU frames.
First Appearance of the SBDU Fast Back
SBDU Documentation
1983
Built in the appropriate gauge of 753 for the rider and programme of races, normally this will be the new 753R tube. The new double taper 753 seat stays are fitted to the seat bolt housing.
Team Raleigh 753 Road
As far as I know that is the first appearance describing the new tubes and new seat stay fastening. The design was still without a definition, it was just a simple description of how they are attached.
1984
These models are built to road or time trials specification with a wider range of options to suit more general racing. They can be built to suit most components and may have conventional or ‘fast back’ seat stays.
753 Road and Time Trials
There is that phrase for the first time… ‘fast back’ seat stays.
1985
1985 continued with descriptions like this…
Earliest SBDU Fast Back Frame
What Does the Data Show
I have images of over 700 SBDU frames, and from these images, I’ve collated a huge amount of data. The data can clearly show a change in the type of seat stays on SBDU frames. I’ve included a few frame numbers in this small sample of data to provide a reference for dating.
I love data! Sometimes data presented like this just jumps out of the page at you. I still have hundreds of frames waiting to be logged but even this small sample can tell a few stories.
You can clearly see several bits of information…
- At the end of 1982/ start of 1983, there was a small period of time when some 753 frames were built with Metric 753 and some were built with Imperial 753R. Imperial took over as the main option but Metric was still offered.
- By the end of 1982, most frames (531 and 753) were moving away from the Prugnat window lug, the S4, and the scroll type of the 62D became the norm.*
- You can see in the ‘Seat Stay Design’ and ‘Seat Stay Taper’ columns that there is a definite switch over point – up to a point, all frames were the single taper stay with oversize caps, but this switched to the double taper with the fast back design.*
- Conventional side attached seat stays with the over size caps could be used on both S4 and 62D lugs, but the fast back only appeared with 62D.*
I’ve marked most of the items in the list with an ‘*’ as these are important in the development of the SBDU fast back and hopefully they are points that I can explain in more detail further through this post.
The Earliest Known SBDU Fast Back?
As the data shows, SB5464 is currently the earliest SBDU fast back that I am aware of. Of all the bikes I have recorded, and all the bikes still waiting to be recorded, SB5464 is the earliest example of an SB stamped frame with a fast back design using the double taper seat stay attached to the seat bolt housing. And it is one of mine!
SB5464 dates to a point towards the end of 1982. It has Metric tubing with an RGF BB shell, Cinelli SC fork crown and Prugnat 62D lugs – it is a beautiful example of an original Raleigh SBDU frame.
The Fast Back Was Nothing New…
Yep, there was nothing new about this design. Lots of frame designs and builders had used something similar, and had used it earlier. In fact, the SBDU did have earlier fast back frames, earlier than SB5464. My Dynaflite is an example, SB4409. But I haven’t included frames like this as the “earliest” – these weren’t what I would call mainstream design frames, they were specials, almost one-offs, built with very different and specialist tubing.
So in respect of a frame design that was part of the standard production process, the Dynaflite cannot be considered.
Restrictions on the SBDU Fast Back
SBDU Explanation
Services des Courses Frame Design
The Services des Courses (SDC) design was almost a throwback to the Prugnat S4 seat lug. The seat binder bolt ears are part of the lug, different to the 62D style used with the fast back. The SDC used the double taper seat stay design in either Reynolds 531c or 753R, but always used the side attached seat stays arrangement.
Mudguards and Panniers
Clearance is the issue. The fast back design is narrow, and it restricts space between the stays. This could make fitting a problem. I have three Randonneur models and all of these use double taper stays, Prugnat 62D lugs and the side attached stays.
Severe Service
Frames can fail.
Frame tubes and chain stays can crack or snap. The seat tube at the back of the seat lug can split. A fast back seat stay can break away from the seat bolt housing. I’ve seen this seat bolt housing problem with both the Raleigh Lightweight Unit frames and the SBDU fast back.
In terms of seat stay attachment, the fast back isn’t considered to be the strongest. Other types of fitting such as the side attached stays can still fail, but they are less likely to. The seat stay/seat bolt housing on a fast back frame is definately something to inspect when buying an old frame.
And there you have it; my short summary of the SBDU fast back design, when it appeared and what design changes helped to make it happen.
