Ride UK Tech Columns

#83 Bent Frame Spotting (and another new BB size)

(This article first appeared in Ride UK issue No. 83 and is reproduced here by kind permission of Ride UK.)

When the ancient Egyptians were laying out the great pyramid at Giza over four and a half thousand years ago they had little more than bits of wood and string to help them lay it out. And yet they got all four sides bang on the four compass points and the lengths of the sides accurate to better than 0.1%. Time has taken its toll on the outer skin but it is still dead square and straight…
However in all that time they never once tried to 360 a set of stairs on it either…

Over my years of riding I have seen a lot of very talented riders do truly amazing things on bikes that were basically fucked. I have seen The Punisher manual a frame that I know to be severely bent for mile upon mile with no more evidence of a struggle than a little more pumping than usual.
Unfortunately we cant all be this gifted, and for the rest of us a twisted or bent frame can hold us back for as long as we own it. My own struggle with a bent frame resulted in not being able to manual in a straight line and turning the opposite direction for months after I finally replaced it with a straight one.

So the purpose of this months technical column is to show how you can check your frame for the less obvious damage like twisting and bending that might be at least partially responsible for your erratic riding style…

Most frame distortion takes place in the rear triangle of the frame. It is obviously much much easier to bend a three quarter inch chainstay tube than an inch and a half down tube. Although the tendency to stronger bigger rear axles has helped to brace the two sides of the rear triangle against each other they are still the weak point of any frame. Tricks like tailwhips and any twisting trick inevitably lead to the odd bodged landing. When you come down travelling any direction other than straight forward, or when the back end smashes sideways into the ground, then inevitably the side loads to the rear wheel get transmitted into the back end of your frame.

There are two main ways that your back-end can distort; bend and twist.

Checking for twist;
Checking for twist is pretty simple but requires a good eye. Simply sight along the frame from the headtube and look to see if the back wheel is in line with the seat tube. Ideally you should see the headtube, seattube and rear tyre all nicely lined up, indicating that everything is OK.
Obviously this requires you to have a reasonably true wheel to check against.

Checking for bend;
Bends are generally worse than a slight twist, not only do they throw your balance off but the chainline can be effected and in extreme cases the crank arm will start to hit the chainstay.
You can check for a bend easily enough with a piece of string and a ruler. Simply tie a piece of string to one dropout, loop it forward round the headtube and back to the other dropout. At the point where the string passes the seat-tube take a measurement (with the ruler) from the seat-tube to the string. It should be the same on both sides of the frame. Any inconsistency should be about the same as the amount the back end is bent. So if the string is 2mm nearer the seat tube on one side than the other then the dropouts are about 2mm off line.
A couple of millimetres isn’t a big deal but I have seen plenty of frames where it was well over 10mm, which was enough to make the frame FEEL very bent.

Other frame damage;
While you are inspecting your frame it is a good idea to look for cracks and ripples, any early warning of impending failure could be the difference between a summer spent riding a new frame or a summer at the dentists using that money to buy new teeth... Look all round every weld for signs of cracks on the weld or within a few millimetres in the “heat affected zone”. Ripples occasionally form in the top or down tubes as a result of head on crashes and soon get worse. It’s like the trick where you stand on a coke can and the slightest tap causes it to collapse. 

Forks nearly bend on the steerer tube before the legs bend, this means that the bend itself is inside the headtube of the frame where you cant easily inspect it for damage. However there is a tell-tale indicator. If the steerer tube of the fork bends then it nearly always bends just a little above the bottom headset race, so forcing the headset apart slightly. By looking closely at the bottom headset cup it is fairly easy to see instantly any problem with the fork steerer tube. If the bottom part of the headset (the cone that you hammer on to the forks) is perfectly parallel to the bottom of the cup itself then things are almost certainly fine. BUT if there is ANY misalignment visible then it’s a pretty sure bet that the fork steerer IS bent.
Forks can also bend sideways and/or twist. The way to check for these is simply to have a really good look at them, squint down from the top, check the headset again but from the front this time.
Forks are one of the most vulnerable and critical parts of your bike, people who snap their forks clean off usually hit the ground a lot harder and a lot faster than they like, so don’t take chances with them. Check the welds for signs of cracking too.

Bars usually bend down at the grip section, often just next to the lever or by the weld for the cross bar, but they can also bend down by the clamp bar or twist. If the bars are on the bike then just look down at them to see if the cross bar and clamp tube line up with each other, if they don’t then the bars are twisted. To check for bends then simply turn the bars sideways and measure from the tip of the seat to the end of the bar, then turn them the other way and compare it with the dimension on that side. Bear in mind that if you have mounted them on the piss or made an arse of cutting them down then that could distort the measurement too. If the bars are off the bike then just lay them on a table and test for “rock” to determine if they are twisted.


More Bottom Bracket News:

Another month has passed so as you would expect ANOTHER new BB size has been introduced.
This one is slightly special though since it is from Profile.
Profile have long been a dominant force in the 3 piece crank market but less well known is that they also make frames. I don’t know how many they sell, I cant remember ever seeing anyone riding one down my local park, but with their influence on the crank market anything they do is going to be closely watched by the industry.

The Mulville frame uses a bottom bracket that Profile call a “Hybrid” bottom bracket.

Like many of the new bottom bracket sizes it uses no cups with a standard cartridge bearing pressing straight into the shell. Unlike any of the other designs in use this bearing does not accept the crank axle directly. Instead there is a system of spacers to shim the various axle sizes up to the one-size-fits-all bearing. Details are still hazy at this point so this is just conjecture but it seems likely that this is done so that the system can use cheap readily available off-the-shelf bearings.

At first glance this may seem to be a really good idea. It should (hopefully) be able to accept all existing crank sizes with a single common bearing. The size they have chosen looks fairly large (somewhere between a normal USA BB bearing and the “Spanish” bottom bracket bearing) so it should be reasonably strong and long lasting and the shell looks fairly light and tidy.
BUT, there are factors to consider that may cause big problems.
With the normal arrangement of the crank axle fitting straight into the bearing and a cup on the outside, then if the bearing is a poor fit in the cup the tension in the chain always acts in the same direction relative to the bearing. So for example if the bearing is loose in the cup then it will be pulled to the back by the chain tension and down by your weight on the cranks, then it will stay there.
However if there is a slight gap between the crank axle and spacer, and/or a gap between the spacer and the bearing; then, because everything is constantly rotating they will be forced to “roll” around inside each other.

This isn’t a new problem to BMX’ers. If you have ever had a poor fitting spacer between your sprocket and crank axle then chances are you will have noticed it slowly get looser and looser and the chain tension get more and more un-even.
So the question-mark over this new “hybrid” bottom bracket has to be, “will Profile be able to get the spacers just right?”
My guess is that it will be almost impossible to make these spacers cope with all the slight inconsistencies in crank axle sizes well enough to avoid nasty creaking noises and excessive wear. Then again, as the dominant crank maker, if anyone can keep these tolerances between crank and bottom bracket under control it will be Profile….

Lets also hope they chose a good bearing….

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