An in-depth look at Robin Mather’s Adjustable Geometry bike

When an artisan, an engineer and an inventor live inside one framebuilder, this is the result. Robin Mather’s adjustable geometry bike was undoubtedly the bike that drew the biggest crowds at Bespoked 2016, and won him The Steve Worland Innovation Award.

Robin wanted to try and determine the effects of head angle and trail on the way a bike handles by creating a bike with four head angles, adjustable trail and a three-channel analogue data recorder to take measurements of steering angle, lean angle and steering torque.

We spoke to Robin after the show to get a better understanding of his incredible creation.


Firstly, why did you build the adjustable geometry bike?

I was increasingly aware of the fact that I didn’t understand everything I wanted to understand about bicycle geometry and handling and so I started trying to read things. There’s some really difficult maths, and my maths isn’t great, so I thought I’d approach it from a different direction. There are some fairly well established rules about what works and what doesn’t work, but it’s not quite the same as really understanding why it works. So it was a kind of curiosity and feeling like there could be a more slightly more objective discussion about what head angle is best and what fork rake is best and what good handling actually means, and what bad handling means.

Tony [Corke] and I had talked about this subject quite a lot over at The Bicycle Academy. He’d built a bike quite like this with multiple head angles and ridden it around. That took him to a certain point, but then you ride it around and you think “yeah, that feels kind of weird” then you change it and you think “that feels better”, but what actually is different about it? What am I doing differently that makes this feel good or bad? Or, how am I responding to what the bike’s doing and altering my control input?

So it was definitely influenced by Tony and was kind of an extension of what Tony had done. The idea of trying to record what was actually happening was the logical next step from that I suppose.

How did you set about designing and building it?

I had a rough idea about which things I wanted to be adjustable. Specifically I only wanted to vary the head angle and the fork rake. I just followed my nose. I drew an overall schematic so I knew roughly the overall shape, and the riding position is just copied from one of my other bikes so I knew where the handlebars, saddle and bottom bracket had to go.

I chose a median position for the front wheel and just arranged everything around those fixed points.

The positions of the head tubes are arranged so that they all intersect at the same point on the ground, which is where the steering axis would normally intersect the ground, so that is independent of trail and changing the head angle doesn’t alter the position of the front wheel.

The way the dropouts bolt onto the fork allowed me to alter the trail and the head angle independently, then it was just sort of joining the dots up, I designed it as I went along.

Most of the bits on it have either been made a couple of times or have been brazed on in one place and then unbrazed and put somewhere else. There was a lot of making it up as I went along.


Were there any problems or difficulties along the way?

There were a few bits that didn’t work. The way the pens are held and move on the paper was bit fiddly to figure out. I made one system where they were held quite rigidly, but when I went round a tight corner the pens would fall off the side of the drum and then they wouldn’t climb back on so I’d end up bending the mechanism when I went in a straight line again. So I made a little spring-loaded thing that allowed them to hit the side of the drum and then pull themselves back into position. That was a bit fiddly.

I think the most difficult thing is the bit that I haven’t done yet, which is actually do the experiments and find out what it all means. All I’ve done so far is just make the equipment that allows me to do the experiments so it’s only half the investigation/job/experiment.


How long did it take to build?

Six or eight weeks, something like that. I was trying to get other stuff done at the same time, but I got completely distracted by it.

How does the data recorder work?

The drum is driven by the back wheel. It’s geared down so the drum rotates fairly slowly but it’s at a constant ratio with the back wheel. So for every metre that the bike travels the drum pulls through about 75mm of paper. There are three pens, which are fixed, and the drum rotates underneath the pens. One is linked up to record the position of the handlebars, the second one is recording the amount of torque – the force you apply to turn the handlebars – and the third one records the lean of the frame relative to the ground.

They seemed like three things that were fairly relevant, possibly not everything that’s relevant but the three things that I could measure reasonably easily. The idea is that by riding really systematically along simple courses of either straight lines or constant radius turns at a particular speed, and then changing the speed incrementally, or the radius of the turn, and then looking at how the steering angle and torque and lean change, it might be possible to build up a picture of what’s going on and then changing the head angle and changing the fork rake.

If I’m building a bike for someone there’s no equation that I can use that tells me what the head angle has to be so I’m more likely to rely on other bikes that I’ve made where I know that the head angle is about this and it seemed to work.

I’m not expecting to revolutionise bike design, I just thought it would be nice to better understand how it’s ended up the way it is, because I actually don’t think that’s particularly well understood, it’s just convention, custom and practise

What are you going to do with the bike?

I’d really like to find someone at a university who’s got an interest in this kind of thing, or someone from that perspective, and get some academic help with it. I’m hoping to go back to my old school in Chippenham where there’s a nice smooth sports hall where I can just start running some tests there and see what comes out of it. Do a few simple tests and analyse the results and follow my nose through it. I’ve got a rough idea of how to structure it. The key thing is to just change one thing at a time and go through it really methodically, but it could be time consuming. I was looking at the possibility of making it a bit less analogue and putting a digital data recorder on it, so that might happen at some point.

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