Ride UK Tech Columns
#63 Frame Design
(This article first appeared in Ride
UK issue No. 63 and is reproduced here by kind permission of Ride UK.)
If you have a question, and you cant find
the answer on the site then you can mail me direct by clicking HERE
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'
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
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…
Copyright © 2003 G-Sport. All Rights Reserved.
The content of this website may not be reproduced or transmitted in any
form in whole or part without the written permission of the owner.