Author Topic: Eyeballing the stiffness of the non-diamond Thorn frames  (Read 3536 times)

Andre Jute

  • Hero Member
  • *****
  • Posts: 3385
Eyeballing the stiffness of the non-diamond Thorn frames
« on: May 06, 2013, 06:02:53 PM »
Jawine, in another thread, inspired the remarks below:
Mixte are quite stiff, originally used for racing for a bit. I guess frame stiffness is another one of those selling points.
I can't image with short stays etc. a lot of the frame stiffness talk is not just merely talk.

*unless may you are 100KG and a brute*

You learn something new every day. I didn't know about mixte having a racing history. But I can quite believe it. My Kranich, which is technically a crossframe, can easily be analyzed as a trapeze frame with mixte rails overlaid. It is as stiff between the axles (not just from the head tube to the rear axle) as the body of a large Rolls-Royce car weighing near enough three tons, designed by a German of course. Even with the extra "toptube" rail removed to turn it into a real mixte, the Kranich would still be stiffer than any diamond frame by some magnitudes. So I can see that people would race such a frame once it was thought up. After all, Pedersen brought the principle of triangulation to bicycle frames in the 1890s and the first Pedersens were aimed at racing and good at it. (Forget what you read about bridge trusses in relation to the Pedersen; a truss depends on mass of material. What is important is the triangulation, which depends on the disposition of much less material, clearly to be preferred for self-powered vehicles.) Whether the additional stiffness over a diamond frame is strictly necessary, given the modest sideways loads on a bicycle frame, is a different matter. You'd think that with the huge tyres I operate and the weight my bike carries, that there would be quite some twisting forces, but I've noticed, in a car park on a hillside where I ride intervals which involves taking two small radius turns at speed on each run, how close to a straight line body and bike remains even at extreme lean angles, with the body offset actually being on the opposite side to the pull of gravity, that is, relative to the vertical plane of the bike a cyclist does not lean into the corner but away from it. Keeping your balance on the bike is thus also a counterforce to twisting moments.

Bringing this back to Thorn, there are several different designs of low stepover Thorn touring bikes, specifically designed for smaller people or those who for some reason requires a lower stopover.

The S-T 390 one would have to test to see how much torque the key junction, on the downtube above the bottom bracket, is up to, and how much of the load it distributes to be resolved elsewhere than in the bottom bracket which, in this design, might otherwise be vulnerable to overloading. I expect that this design would match a diamond frame only with a weight penalty. In return it offers the lowest stopover.

But there's no question in my mind that an S-T 420 would be as a stiff as a diamond frame, or better, for a small or even no weight penalty in heavier materials to make up for the smaller enclosed area. Effectively the way to calculate the stiffness of the S-T 420 by analogy, rather than tedious finite element analysis, is to add the secondary constituent parts within the main notional diamond back onto the "top tube", these parts being the small trapezoid forward of the bottom bracket and the small triangle forward of the seat tube. If between these two parts that pull double their weight and the main trapezoid (framed by the head tube, down tube, top tube, forward seatpost stay) they make more area than a diamond frame of the same size, this low stepthrough could easily be stiffer than a diamond frame built of the same weight of material. (Adding the small triangle back might cause some structural engineers to frown. My reason is that it is within the notional "diamond frame area" already, effectively a doubling of the top tube, as on work bikes.)

I smiled when I read Andy Blance's (natural) worries about these two frames, back when Thorn first launched these designs. I have quite a bit of automobile chassis design experience (in racing, my field of expertise was suspensions which of necessity include a stiff frame to mount the suspension) and, short of plating the "hole" through instead of shooting triangulating tubes through it, which could be unpleasant for a laden tourer in a crosswind, I can't think of many better solutions than the ones Thorn arrived at in the 420. The Locomotief/Kranich crossframe, the mixte and the Pedersen are fundamentally superior designs because they add another dimension with obvious benefits to stiffness, and I think they're more elegant, but they may well be overkill from the parsimonious engineering viewpoint one expects from bicycle designers. In any event, I understand why designers of small-production touring bikes don't want to, or can't afford to, develop small-tube bikes; I can also understand why customers without applicable experience and analytical skills would instinctively shy away from small-tube bikes, so again it comes down to a commercial decision. If you can make a low-stepthrough bike as stiff as the 420 appears to be, even at a glance if you have the right experience, and it looks more like what your customers expect, and it works, why would you ever spend the money developing novelties?

Andre Jute
"The brain of an engineer is a delicate instrument which must be
protected against the unevenness of the ground." -- Wifredo-Pelayo
Ricart Medina