Blog Featured Posts General Topic SBDU Frame Design SBDU Ilkeston Seat Stays

Specialist Bicycle Development Unit (SBDU Ilkeston) Seat Stay Design and Seat Lug Attachment

Specialist Bicycle Development Unit (SBDU) Frame Design Seat Stay and Seat Lug Attachment

You probably think seat stays are quite boring, why on earth would I devote a blog post to the two narrow tapering tubes on the back of a frame? Well as you hopefully all know by now, I do like my details, especially the small details! The various designs and subtle differences in seat stays can tell me a lot about an SBDU frame, even the frame age and tubing type.

Following the blog post I wrote recently on SBDU fork ends, I’ve decided to write a small series of individual posts to look in detail at the other areas of SBDU frame design, and the simple seat stay seemed to be an easy one to start with.

SB4059 1980 TI-Raleigh Reynolds 753 Team Pro Campagnolo Super Record Rear Brake Fitted
SB4059 1980 TI-Raleigh Reynolds 753 Team Pro Campagnolo Super Record Rear Brake Fitted

Single Taper or Double Taper Design

Single Taper (1974 – 1982)

Going back to the very start of the SBDU in 1974, the seat stay design from Reynolds was a ‘single taper’. A single taper means just that, the external diameter tapers once. The seat stay starts off with a large round diameter for a short length at the top near the seat lug and then gradually tapers down to a much smaller round diameter at the fork end.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design
Specialist Bicycle Development Unit (SBDU) Seat Stay Design

Of the diameter choices that were available in this period, the SBDU built with a single taper seat stay that had a 16mm top diameter tapering down to approx 10mm. This is SB3505, a 1980 frame with a single taper seat stay 16mm >> 10mm.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Single Taper Diameter
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Single Taper Diameter

The Reynolds info above also shows the length of standard diameter top section and the point at which the taper starts; most of the excess length would be trimmed from the top 16mm section.

Double Taper (1982 onward)

During 1982, Reynolds launched a new tubing range, and introduced tubing specs such as 531c and 753R among others. They also introduced new tubing profiles, one of these was a ‘double taper’ seat stay. A double taper seat stay is just that, instead of a single taper, these new stays had two tapers. This is SB6560, a 1984 frame showing the typical profile of the double taper stay, 12mm >> 16mm >> 11mm

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Double Taper Diameter
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Double Taper Diameter

The section around the brake bridge remained at 16mm, and from there, each end of the stay tapered to a smaller diameter 12mm at the seat lug and 11mm at the fork end.

The only exception to the switch to double taper seat stays that I’m aware of was the 1983 “531 Team Standard” frameset, built with 531DB. The SBDU ran this standard frameset with single taper stays side by side with the 531 Team ‘C’ (single taper was also retained as an option on the 531 Team ‘C’).

Before moving on to show how the SBDU finished the top end of the seat stays, let’s have a quick look at an area of the frame that really goes unnoticed. Maybe the SBDU recycled all those seat stay off cuts? Is it just coincidence that the chain stay bridge on frames built with a single taper seat stay between 1974 and 1982 have a 16mm bridge…

Specialist Bicycle Development Unit (SBDU) Seat Stay Design 16mm Chain Stay Bridge
Specialist Bicycle Development Unit (SBDU) Seat Stay Design 16mm Chain Stay Bridge

… and frames built after 1982 using a double taper seat stay have a 12mm bridge…

Specialist Bicycle Development Unit (SBDU) Seat Stay Design 12mm Chain Stay Bridge
Specialist Bicycle Development Unit (SBDU) Seat Stay Design 12mm Chain Stay Bridge

Before looking at the various seat stay finishes, here is my usual caveat – there are a couple of frames that might differ in their seat stay cap finish, the SBDU built custom frames and it is always possible to see slight variations to the standard finish.

Seat Stay Caps and Plugs

The First 50’ish SB Frames

The first 40 to 50’ish SB frames, built from Reynolds 531 double butted (531DB) tubing, are difficult to report on as they are few and far between, I’m only aware of possibly 6 frames that have survived with an SB number below SB50. These frames had a flat and flush fitting seat stay finish which was almost certainly a seat stay plug – I’ll explain more about seat stay plugs in a moment.

Scalloped Seat Stay End Design

Apart from a couple of exceptions, from approx SB48 onward, the SBDU eventually settled on what I call the ‘Scalloped’ seat stay end design. At this point in SB frame production, all frames were 531DB. This is SB447 demonstrating the scalloped design…

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Scalloped Seat Stay Ends
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Scalloped Seat Stay Ends

I’ve just mentioned about seat stay plugs, and this end design is an example. The 16mm top section of the single taper seat stay was cut straight across and a seat stay plug was inserted and brazed in place. The next image shows GH6175 (1975) and SB518 (1976) and you can see the faint line where the stay was cut and the plug starts…

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Scalloped Seat Stay Plugs
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Scalloped Seat Stay Plugs

You can see a subtle difference in the 2 designs above, the top images from SB518 have a slightly straighter centre line than the images below it. So although it was a pre-designed plug, there could have either been 2 slightly different designs or there could have been a little extra work done with the file on occasion. This design continued on SB 531DB frames until approx SB1350

Standard Flat Seat Stay Caps (Fitted Flush with the Seat Stay Diameter)

Reynolds 753 SB frames used a different design. This wasn’t because the SBDU wanted to make 753 frames look different to their existing 531DB model, they needed a seat stay end design that required very little heat. The existing scalloped plugs used on 531DB frames were relatively thick and heavy, so a different finish was introduced. Here are SB632 and SB664, both from approx Easter 1976…

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Flush Seat Stay Caps
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Flush Seat Stay Caps

This style of flat design is similar to those first 50’ish 531DB SB frames, the difference is that the early 531DB flat caps had a rounded tip, it wasn’t pointed in the same way as the examples above.

If you think about those chain stay bridges made out of simple round section tube, you will appreciate that the SBDU didn’t really do ‘fancy’ – yes, they did a few specials with nicely detailed lugs but the vast majority of their frames were plain and functional, their efforts really went into geometry design and tubing specification.

This design wasn’t a plug. The seat stay was a cut across at an angle and a flat piece of steel was brazed in place to ‘cap’ the tube . That steel plate was then finished flush with the outer stay diameter. You can see the line of the cut and plate on the next image of SB632 (this frame needs paint so I’ve removed some old paint from the seat stay cap to demonstrate the finish)…

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Flush Seat Stay Cap
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Flush Seat Stay Cap

You can clearly see how the stay has been cut at an angle before being capped by a plain piece of steel. You can also clearly see that it has been finished so that it is flush with the 16mm tube section.

Reynolds 753 SB frames kept this flat and flush design until approx SB1029

Flat and Oversize Seat Stay Caps

This is the design that the SBDU frames are known for. This is probably the most recognisable feature of an SB frame. Here is SB3505 again…

SB3505 1980 SBDU Ilkeston Reynolds 753 TI-Raleigh Team Pro Studio Frame Seat Stay Cap
SB3505 1980 SBDU Ilkeston Reynolds 753 TI-Raleigh Team Pro Studio Frame Seat Stay Cap

Although they are called ‘oversize’ they actually only extend slightly at the bottom edge of the cap, the edge overhanging the side of the seat stay. You can see in the image below that the top ‘pointed’ end of the cap is still flush with the section of the tube, it is only the round end of the cap that is left ‘oversize’.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Oversize Seat Stay Caps
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Oversize Seat Stay Caps

If you have kept up with me so far then you have probably figured out that the oversize seat stay caps must have changed size when the single taper seat stay was replaced by the double taper. The older single taper oversize seat stay caps were much bigger, the cap size reduced significantly with the double taper to fit in with the smaller diameter tube.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Oversize Seat Stay Caps Single Taper Size
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Oversize Seat Stay Caps Single Taper Size
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Oversize Seat Stay Caps Double Taper Size
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Flat and Oversize Seat Stay Caps Double Taper Size

The single taper oversize cap is approx 50mm x 16mm compared to the smaller double taper oversize cap which is approx 38mm x 12mm.

By approx SB1350, the scalloped design had disappeared. The Reynolds 753 and Reynolds 531DB frames were both now using the oversize style.

All the examples so far were classed by the SBDU as ‘conventional side attached’ seat stays.

Fast Back Seat Stays

If you browse the internet and bike forums you will no doubt see a couple of opinions on what to call this seat stay arrangement. This is SB5794, an early 1983 frame.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Fast Back
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Fast Back

I call it ‘Fast Back’ – I call it that because that is how Gerald O’Donovan referred to them in his yearly frameset documents. Here is SB5464, this frame dates to the 3rd quarter of 1982. It is the first SB numbered frame I have seen with the Fast Back style.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Fast Back Stays
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Fast Back Stays

So why did it take until 1982 for the SBDU to design a fast back arrangement like this? My theory is that it coincides with the introduction of the double taper seat stay. The smaller diameter of 12mm allowed the stays to be cut and attached to the seat lug boss. The older and larger 16mm single taper would be too big. This is an extract from the 1983 frameset document which is the first to describe the new double taper tube and seat stay attachment.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design 1983 Frame Documentation
Specialist Bicycle Development Unit (SBDU) Seat Stay Design 1983 Frame Documentation

And here is an extract from the 1984 document with a reference to the ‘fast back’ style.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design 1984 Frame Documentation
Specialist Bicycle Development Unit (SBDU) Seat Stay Design 1984 Frame Documentation

The 1983 extract has advice that the conventional side attached should be used for ‘severe service’ – side attached seat stays are considered the strongest while fast back are weaker in comparison.

Although I said that SB5464 is the earliest SB frame I’ve seen with the fast back style, there are earlier examples, but these only appear on some SBDU ‘Specials’ such as the 753 Dynaflite with Ovoid tubing. These frames had almost flat seat stays and were fitted to the back of the seat tube/lug. Here is SB4409 dating to 1981.

Specialist Bicycle Development Unit (SBDU) Seat Stay Design 753 Dynaflite
Specialist Bicycle Development Unit (SBDU) Seat Stay Design 753 Dynaflite

Some frame designs could not use the fast back style. For some frame designs the choice of seat stay attachment was governed by practicality. A touring or randonneur frame, or any frame that was spec’d for mudguards, was fitted with side attached seat stays to give the required clearance. Both of my randonneur frames have side attached seat stays (SB7657 & SB7660).

Raleigh SBDU Ilkeston TI-Raleigh SB7660 1985 Randonneur
Raleigh SBDU Ilkeston TI-Raleigh SB7660 1985 Randonneur

The Services des Courses (SDC) frame design was also spec’d with conventional side attached stays. Unfortunately the frameset documents don’t elaborate on why. So in the absence of any given SBDU reason, here are my thoughts…

This is my main theory; the Cinelli seat lug used on the SDC frame was very different in shape and style to the standard SB seat lug being used at the time. Here is the standard SB seat lug which could be fitted with either side attached or fast back stays, this is SB3800

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Standard SB Seat Lug
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Standard SB Seat Lug

If you note where the seat bolt bosses are set on the back of the lug in the images above and then look at the shape of the Cinelli lug from an SDC frame below, they are very different…

Specialist Bicycle Development Unit (SBDU) Seat Stay Design Cinelli Seat Lug
Specialist Bicycle Development Unit (SBDU) Seat Stay Design Cinelli Seat Lug

My second theory is based on the description given by the SBDU of what an SDC frame is designed to be.

SBDU Ilkeston Panasonic Raleigh Services des Courses Reynolds 531 1984 SB6560 Heading
SBDU Ilkeston Panasonic Raleigh Services des Courses Reynolds 531 1984 SB6560 Heading

An SDC frame is an “…out and out ‘Team Issue’ frameset embodying our current thinking on design and construction…”

Contrary to what you might see in some team launch publicity pictures, team frames did not use the fast back style, they always used the side attached seat stays. Therefore, for an SDC frame which followed the design principles of the team frames, they would also use side attached stays.

And that is how things remained during the life of the SBDU at Ilkeston.

Specialist Bicycle Development Unit (SBDU) Frame Design Seat Stay and Seat Lug Attachment
Specialist Bicycle Development Unit (SBDU) Frame Design Seat Stay and Seat Lug Attachment

I actually thought that would be a quick and simple blog post but there is much more to seat stays and seat stay end design than you might think!

 

 

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