Time Trial frame? Road frame? Or a bit of both? What exactly is it?
SB7393 came with a story that it was built and used for time trials and ridden by a junior. I was also told by Rob, the previous owner, that it was a “lively” ride! It looks like a conventional SBDU road bike, but then again, SB time trial frames can look like conventional road bikes too; and with vertical and horizontal ends as options on both types, it is ultimately down to geometry to unravel the puzzle.
When I do this type of geometry and design blog post, I compare my findings to the information that was posted in the Yahoo TI-Raleigh group. There are tables of ‘Stock’ road and time trial data kindly uploaded by Mike Mullett, ex SBDU Workshop Manager. A lot of what I write will be open to interpretation as frame measurements could and would be altered to fit the rider and type of racing, but at least the stock data gives a starting point for comparisons to be made.
Starting as I normally do with fork rake. Rake or offset as it is sometimes called is the distance between parallel lines running down the centre of the fork column and the centre of the front hub.
Before you take these measurements, just be sure that the fork is straight, there is no point taking these measurements if the fork is twisted or bent. If your fork is true then it is simply a case of getting the column level with the work surface – set the digital angle gauge to zero on the bench and make sure it reads zero on the column.
I prefer to position my fork slightly differently to the image above, I turn it around and place the ends on the work bench. So my rake measurement will actually be B – A.
Distance A – Placing a dummy axle in the fork ends allows me to find the centre easier and then measure the distance against a metal rule. Using this method of fork placement will usually give me the same measurement no matter what SB fork I measure, but it is always best to check.
A = 13mm
Distance B – This distance is measured from the top of the fork down to the workbench and then subtracting the radius of the column.
This means that B = (71mm – (25.4/2)) = 58.3mm
Therefore, fork rake = B (58.3mm) – A (13mm) = 45.3mm
My first reaction to that measurement is that it doesn’t really fit in with either the Road or TT dimensions. The stock rake according to Mike’s data for a TT was 37mm and 42mm for a 57cm road frame. So I’ll leave that for now and come back to it when I know more about the frame.
Next up are the frame measurements, this means fitting the headset and wheels and getting SB7393 stood on the bench. I’m using the original Campagnolo Record headset and MA40 wheels with 700 x 23 tyres.
A quick measure of the brake drop is 45-46mm, which is what I expected.
Bottom bracket height is quite easy to measure. This measurement could possibly change depending on tyre size. The height is measured from the workbench up to the centre of the BB shell. Some builders will spec’ a BB drop which is maybe a more constant measurement. With BB drop, a line is drawn between the two wheel centres and a measurement is taken down over to the centre of the BB. However, the SBDU have BB height so that is what I’ll measure.
I measure up from the workbench to the bottom of the BB shell and then add on the radius of the shell – this total measurement gives the distance from workbench to centre of BB.
So BB height = (247.5mm + (41.18/2))
That gives a measurement of 268.09mm. There isn’t a great deal of difference between the SBDU stock BB height on their TT and Road frames, TT has 270mm and Road has 268mm; as I’m not using the most accurate methods of measuring, and my eyes aren’t what they used to be, I’ll round that figure to 268mm.
With a fork rake rounded to 45mm and a BB height of 268mm, I’m looking more and more at this maybe being a road frame, but what can the frame angles and tube lengths tell me…
Well that is interesting, with the digital angle gauge set to zero on the top tube, the seat angle is 73.9 and the head angle is 74.0. I wouldn’t normally advocate taking that head angle with the head badge fitted, I’d prefer to have the gauge flat on the tube, but for the sake of these measurements, this will do. Both of those angles match the TT dimensions for a 57cm frame.
The top tube is 560mm, the head tube is 146mm and the front centre is 590mm. This is how the geometry looks…
There are bits of both frame types represented by that drawing, but the angles and top tube length are matches for the SBDU TT dimensions. This is how they compare side by side.
There is a link in the fork rake and front centres of SB7393 and the stock TT measurements. My fork rake is 45mm which adds to the front centre dimension (centre of BB to centre of front hub). If you subtract the difference between my 45mm and the stock 37mm rake, you have 8mm – this is the difference between the two front centre measurements. If SB7393 had the std TT fork rake then the front centre would also match. So with most of the measurements matching the TT data, was the fork rake changed for a reason? Or was it the front centre that was changed, maybe to give more toe clearance?
The fork rake and head angle combined with wheel and tyre size give what is called the trail measurement. If I’ve used an accurate online trail calculator, then the trail on SB7393 is calculated at 50mm.
To keep things very simple so I don’t confuse myself, trail is the distance between the contact point on the ground of the tyre and the contact point of the steering axis, with the steering axis being a centre line through the head tube. Some say it as “how far the tyre contact point ‘trails‘ behind the line of the steering axis”. The amount of trail affects the steering and stability of the bike.
Frame builders will adjust head angles and rake to provide the trail they require. If you really want to bamboozle yourself with definitions and opinions on head angle, rake and trail and their relationship with each other then just google it and read through the reams of pages dedicated to this subject. Some recommend good trail between 50mm and 65mm, with the high 50s/low 60s often cited as an ideal . Therefore 50mm of trail is on the shorter side so the steering will be lighter and quicker. Rob may have been reacting to what some call more ‘twitchy’ steering, I’ve also had the word “nervous” used a few times.
I’ve added some lines to the image below to show where trail is measured. One line comes straight down through the hub perpendicular to the ground and the second line, the steering axis, comes through the centre line of the head tube. The distance between the contact points of both lines on the ground is the trail. SB7393’s head angle, rake and wheel size give a trail measurement of 50mm – the contact point of the tyre trails 50mm behind the steering axis.
If you keep the head angle the same and therefore keep the steering axis point of contact to the ground the same but increase fork rake (move the vertical line through the hub centre away from the head tube) you will decrease the trail measurement. Likewise, decreasing fork rake for the same head angle (moving the vertical line through the hub centre towards the head tube) will increase trail.
If this bike did have a fork rake of 37mm which was stock on the SBDU TT frames (achieved by moving the vertical line through the hub centre towards the head tube), and the head angle and tyre remained the same, the trail measurement would increase and stability would be better; the trail would be calculated at 58mm and would be in that range that is often said to be ideal.
The original discussions and design stage at Ilkeston will always be a mystery, but the head angle and rake and subsequent trail will give a ‘lively’ ride, or fast steering as Rob indicated. I think both stories tally as the majority of measurements match the TT profile and the steering geometry matches Rob’s description of handling.
SB7393 is an Imperial tube frame meaning that the seat and down tube have a diameter of 28.6mm (1 1/8″) and the top tube is 25.4mm (1″). The seat pin diameter is 27.2mm. I’m busy researching and writing another post on SB frame weights. I find it interesting to record frame weight but I’m increasingly finding that interpreting the weight and trying to correlate that to tubing type isn’t exact.
You will probably see a different Reynolds tube ‘set weight’ depending on which technical data document you read. I’ve seen weights for 753R ranging from 1650g to 1800g and 531P ranging from 1700g to 1900g (531c can range from 1800g to 2050g).
Weight is very much dependent on the tube lengths used in the frame (different geometry will mean that tubes can be different lengths for the same size frame). There are also hidden factors such as tube gauge (wall thickness) that can easily alter a frame weight by 100g. This is the tube data from a Reynolds publication called ‘Reynolds Tube Talk’; this covers tubesets from 1982 to 1989 so SB7393 falls into this range.
If you look under the fork heading in the table you will see that the thickness of fork blades for 753R, 531P and 531c is the same – this is why I don’t normally weigh a fork, they are often similar. The only difference on fork weight will come from choice of crown, length of column determined by frame size, length of blade determined by clearance and if the column is plain gauge or single butted.
Most Metric 753 and Imperial 753R up to 1989 will have a plain gauge column. 531SL had a single butted column so it was thicker at the fork crown, but 531P according to the data above used plain gauge just like 753R. The easiest way to determine if you have a plain gauge column is to see if a stem fits in the bottom. Because a plain gauge column will be the same thickness top and bottom, a std 22.2mm stem should fit in both ends. You could also just use a caliper to measure…
The data sheet also gives the tube wall thickness in millimetres; notice how the seat tube on 753R, 531P and 531c all have a value of 0.55 at the non butted seat pin end – this is why all three Imperial tubesets will accept a 27.2mm pin (some datasheets list it as 0.5).
NOTE: Metric tubesets with a external seat tube diameter of 28.0mm consisting of 753, 531SL and 531c all had different non butted ends on their std seat tube of 0.3, 0.4 and 0.5 meaning they had seat pin sizes of 27.0mm, 26.8mm or 26.6mm respectively.
Staying with that data sheet, you will see that 753R and 531P share the same tube wall thickness on each individual tube, but they have a different overall set weight. This difference in tube set weight, I imagine, is made up from different length butted sections. As 753R is heat treated, and has a much higher ultimate tensile strength (UTS), the non heat treated and therefore less strong 531P may have a longer butt to maintain some strength and stiffness and therefore have an overall heavier tube? That’s just my rambling and thoughts, I really need to back it up with data at some point!
Anyway, I’ve wandered off the topic of SB7393 far too much.
Based on the weight, tube dimensions and frame fittings, I’m confident that this frame is built with Reynolds 531 Professional. The Reynolds frame transfer is original and I have no reason to doubt it or assume that this is mis-badged. Other 753R frames in my collection and in a similar size all come in with a lower frame weight. This frame also has a Haden bottom bracket shell, a 753R SB frame in this period of production would be spec’d with a Cinelli shell.
I can also discount Reynolds 531c. Although a standard 531c frame in this period came equipped with the same Haden BB shell, this frame has a plain gauge fork column, 531c was single butted. SB7393 also has a Cinelli SCA fork crown, the SBDU documents for this period only list 531c frames with either a Cinelli CCA, CC or SC. This is how 531 Pro frames are represented in the 1985 SBDU frame set document…
The (All*) next to ‘Chrome’ is a full chrome frame with Team finish and transfers. SB7393 is the fourth frame in the list with Nil chrome. My other 1985 531 Pro, SB7219 is second in the list with chrome forks and stays.
Some of the measurements do stray slightly from ‘stock’ geometry, but that is no surprise due to the custom nature of the builds offered by the SBDU. Tube lengths and frame angles match with the Time Trial geometry supplied by Mike and the steering geometry provides for a quick steering bike.
Frame geometry is a fascinating and often confusing area of bike design but I think I’ll definitely categorise this bike under the TT section on the blog. It is a great example of how you can’t just look at an SBDU rear fork end and say horizontal = Road and vertical = TT, it doesn’t work like that.
