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Why stuff does stuff

Brant is the On-One (and Planet-X and Titus) designer, and gets his knickers in a twist quite a bit about things that don't bother other people.

This week (as this is going to be a regular slot) he has a bit of a scribble about "frame compliance" and why it's NOT the rear triangle of a bike that makes the ride comfortable (or not).

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If we consider a frame to be loaded at the points as shown, let’s look at what happens to the various tubes.
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This is what people who think rear triangle compliance has an effect on frames happens. I think this is bollocks.
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This is what happens. The front fork tries to rip the front of the bike off, and splays the top tube and downtube under simple bending, resulting in the wheels spreading apart under load. This is what happens.
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The rear stays do splay a little if left unsupported. But they bow outwards. There is FEA done to show this. They don’t bend the other way.
S bend stays make people think they are springy as they look a bit like a spring, but a spring is actually a coiled up torsion bar. It’s not curved like that for compliance as such.

First seen at shedfire’s dumping ground

2 April 2012

Comments

  • GuyP

    What affect does moving the load down to the bb have on your force diagram?

  • assaf

    can you let us know what loads you are design for? at road/XC/AM please

  • steve

    anyone fancy a pint?

  • Frankie

    And then there's the Scalpel from Cannondale....

  • BBB

    Most of people simply need to loose 5-10 PSI from their tyres.

  • Alan Burr

    Having built my own frames I have have tried fitting strain gauges to a frame, the results that I found was that was there was more strain on the frame from pedaling than riding down a flight of stairs.

  • NickSea

    With all this frame flex, don't forget that static rider load on the frame is greater on the rear axle than the front. This changes whilst riding due to acceleration; braking & gradient particularly; steep hills, & that wheel & tire choice & tire pressure also significantly affect the overall "give & feel" of the final bike.

  • AndyL

    I was just thinking the other day about strain gauging a frame and going for a ride with a datalogger in my backpack as I wanted to get more of an idea about what is going on.

  • BrantR (On-One Team)

    S Bend stays were originally designed to put cantilever bosses in a better position for the rear brake. Make of that what you will.

  • Steff

    I'd always taken it that S-bend stays were intended to give a longer piece of material of a given bendiness (Young's Modulus, I suppose) to give greater bending under a given applied force while maintaining the overall geometry.

  • Johnni

    Though the thought of your frame flexing in such a manner may seen scary, it's why some frames are more comfortable than others. I'd go as far to say that there is no such thing as a conventional diamond frame anymore, as tube are custom drawn to the particular frame use and size. Thinking about rear stay compliance, there was only one bike which had designed flex in the rear stay, the Curly Tail Hetchins.

  • Kev P

    Interesting concept which makes sense to a non-engineer. Brant are you going to do an article on frame angles? I own a 456c & noticed that the seat tube has a slacker seat tube angle than most bikes. Some people say that this makes climbing harder. From experience it only makes steep climbing a little trickier. Sitting on the saddle nose & using low gears fixes the problem. Is there any reason for these slacker angles, apart from allowing us to climb slower & enjoying the experience more? An article / rant on this theme would be most interesting. Keep these articles. It's good to know other ideas of why things are, other than what the big company's & dare I say the main stream MTB mags.

  • JJ

    As a mechanical engineer and avid mountain biker your analysis of the interaction of forces is spot-on...I've always tired of that rear triangle compliance myth and am pleased to see someone set the record straight!! Nice job!!

  • epicyclo

    This can be seen working on a bike with an American cruiser springer fork. There's negligible vertical movement,but the result is more comfortable than one would expect.