Frame Design Details

*This article first appeared in Ride UK (#64) and is reproduced here by kind permission of Ride UK.

Over the years there have been some pretty wacky frame designs. Hutch, for example, used to make a frame called the Trickstar. This was loved by a lot of riders of the day, especially flatlanders, for its super steep head angle and abundance of scuffing room. The scuffing room came at the expense of strength at the weird head tube junction. Hutch obviously decided that it was still way too strong and went on to produce the Trickstar II, which had a specially weakened rear triangle to match. The rear triangle also helped to make the Mk 2 look like a wheel barrow (not just any wheel barrow mind, an UGLY wheel barrow) and that was that for Hutch.

S.E. Racing on the other hand came out with the Quadangle, although equally weird you could at least see what they were thinking. From a structural point of view it made a lot of sense but it was just too fussy for most peoples’ tastes. These days frames have pretty much settled down to a simple “diamond” shape which everybody seems to love and understand. Triangles are, as everybody knows, strong as hell. Look at any serious engineering structure and triangles are the order of the day. As we all remember from geometry in school if you fix the lengths of the sides of a triangle you fix the angles. If you fix the lengths of the sides of a four or more sided shape, the angles can still do whatever the hell they like. So in a triangular structure the sides lock out the corners and stop them flexing.

Yet frames break and bend with alarming regularity?!? The main reason is simple: the front triangle just isn’t. The front “triangle” on a bike is actually a “triangle-with-one-corner-trimmed-down-a-bit-and-an-extra-tube-welded-across-it- which-we-then-insert-a-huge-lever-into-and-rag-about-like-maniacs” (or twoctdabaaetwaiwwtiahliaralm). The “twoctdabaaetwaiwwtiahliaralm” is NOT generally recognised as a sound engineering structure and does not appear in serious engineering structures like bridges and aeroplanes. The reason that the “twoctdabaaetwaiwwtiahliaralm” doesn’t appear in other engineering structures is that its crap.

By using a nice short head-tube the top and down tubes have come very close together, so the front triangle is almost one, but ‘almost’ isn’t quite the same. Also you still have the situation where you are trying to rip that extra bit of tube off the corner of the triangle. This is the kind of problem that Hutch and SE Racing were trying to address with the Trickstar and the Quadangle, by dividing the frame up into proper triangles they hoped to stiffen everything up….

Having rejected this approach on the grounds of, “it looks wack” , we have added gussets and thickened tubes to cope with the stress in this problem area. But where should these gussets be? How long should they be? Should they be welded all the way round or left open ended? Everyone has a different view based on experience, personal preference and superstition, but what is hype and what is truly the best? Well unfortunately that’s a very very difficult question. Everyone rides differently, everyone reacts differently during a heavy landing, and this means that everyone loads the frame slightly differently.

We have all seen frames that have cracked and it is nearly always near the weld. When you weld a tube you do two things. You heat the metal up to its melting point and introduce new molten metal, in its molten state it is almost like going back to the foundry, impurities on the surface can get mixed into the actual weld and gasses in the atmosphere can get in there too. But also the magical alloying elements like the chromium and the molybdenum can migrate into or away from the weld, leaving a nearby area of weakened metal called the “heat affected zone”. The weld also acts as a stress concentrator, the shape of the weld creates a slight corner and you can imagine ‘lines of stress’ bunch up as they try to get round the corner.

The gussets also do two things: firstly they simply increase the amount of metal, but they also move the weld away from the highest stress area.There still is a weld of course but at the headtube its through a thick gusset (and hopefully the tube too) so the stress is lower. If we leave the gusset ‘open ended’ (ie. We don’t weld the far end) then we have eliminated the stress concentrator and heat affected zone. SO if we leave the gusset open ended it will be stronger but how can the gusset take any stress if it isn’t welded to the tube? Well it can but there is a problem. We do weld the gusset along the edges, this is down nearer to the center of the tube where the stresses are lowest; unfortunately this means that it takes some distance for the stress to transfer from the gusset to tube so open ended gussets need to be a little longer but are still the way to go…

Another problem with frame design is getting the back wheel in. Ideally we would all like the choice of nice short chainstays, masses of tyre clearance, no danger of the crank arm or sprocket hitting the stays, and big strong tubes. Unfortunately there is a narrow gap to get the stay through between the tyre and the sprocket. This limits most frames to 7/8″ or 1″ tubes at most and if you want room for a big tyre, a 44 tooth sprocket and short stays, something sometimes has to give. Some manufacturers put a dimple in the stays to clear the sprocket, this lets them leave a little more room for a tyre and reduces the chances of a sprocket hitting the stay. It may look like it weakens the stays a little but the reality is much much worse.

Unfortunately that little dimple is right up at the BB end where the stress is the absolute maximum for the tube. A little further forwards and the cross-brace would help out with the stresses and it wouldn’t be a problem, but its right by the tyre where it couldn’t be much worse. It reduces the strength of the tube AND it also acts as another stress concentrator. If you do harsh twists and tailwhips etc. a dimple is unacceptable. The chances are that you WILL bend the back end of the frame. If you have a frame with a dimple you might want to try this simple test to see if the frame is bent.. then again you might not… Tie a loop in a bit of string and slip it over the back of one dropout, walk the string up to the front of the bike round the headtube and back to the other dropout, pull it tight and hold it there. Now measure between the string and the seat-tube, both sides should be the same distance from the seat-tube, if they aren’t than the chances are your back end is bent sideways. When you go to buy your next frame reject any with dimples if you want to avoid this happening again…