Nailing the Geometry PART 1 | Getting it wrong to get it right

You can throw all the high tech suspension, carbon bits and custom tuning at a mountain bike but if you don’t nail the geometry you’ll never go as fast or have as much fun as you should.

Starling Cycles team rider Peter Lloyd has been a huge force in helping us to experiment with progressive geometry and has spent the last year hammering trails, tweaking angles and reporting back what he’s found.

First - we’re talking about Chain Stay length. Why it matters, what happens when it works and what happens when it doesn’t.

Take it away Peter …

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INTRO

I’m a 20-year-old Enduro racer and mechanical engineering student (partly why this reads with all the charisma of a fruit bat). I’m 1.87m (6ft 2”), 80kg (12.5 stone) and do most of my riding on the steep, tight and technical trails of Innerleithen, Scotland. I previously spent 3 years riding for Whyte Bikes on their G-150 and then G-160s on their Enduro team.

I learned a lot from my first Starling Murmur and then applied that to my second frame which - in time - I’d like to turn into more improvements in the future.

Before I start, I better give Chris Porter at Mojo Suspension/Geometron bikes a shout out for his help during this experiment. No matter how progressive people might think I am being, I’ve realised that I’m actually just coming to the same realisations he did a long time ago.

The trouble with chain stays

Before I had the first Murmur made, my main concern was that chain stays (CS), or rear centres, were too short in relation to front centres. Over the past few years, reach measurements have been growing and head angles slackening, resulting in longer and longer front centres. This is a good thing. At the same time, manufacturers have been trying to keep chain stays short to make bikes easier to manual, while also selling the myth that short CS make tight corners easier. The opposite of this is true.

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My first Murmur

My first Starling Murmur frame was built around a 160mm fork and had a 510mm reach (15mm longer than my previous G-160), 64.5 degree head angle, 445mm CS (20mm longer) and 38mm BB drop for a BB height of 335mm. The wheelbase measured in at 1295mm, so very long by most people’s standards.

My initial impressions were that this was the closest a bike has ever come to fitting me.

I felt comfortable in the middle of it, with space to move, rather than restricted by the lack of space. Out on the trail it was very stable, feeling safe and easy to let off the brakes on fast sections of track. It also climbed well, despite the long front centre, due to the additional length in the CS, keeping me centred.

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But … corners

However, cornering took me longer to adjust to. I found that it was particularly sensitive to the correct bar height and I really had to force the bike into a turn by imagining slamming my inside hand into the ground. I could still make it corner well, but it did not feel as natural as I’d like. Quick changes of direction from turn to turn were also slower and more forced than I would like.

After some time on the bike I went back to the numbers and looked at what was going on with the bike. I spoke to Chris Porter. I spoke to Joe. I did some thinking, some sketching and some changes to my set up. Between us we worked out that the longer chain stay length had helped bring balance to the bike, but now other changes needed to be made to the steering geometry to help initiate cornering.

It’s all about the chain stays. But also the bottom bracket.

Consider a rider on the bike in the proper ‘attack’ position: stood up on the bike with all their weight through their feet and thus the bottom bracket (BB)- almost none through the hands. The closer the rear wheel is to the BB, the more force that will act upon it. The weight of the rider is constant, so therefore less of the force from the rider’s weight will be acting upon the front wheel, leading to less front wheel grip and the front tyre washing out in corners. If the CS is lengthened, then more force will act upon the ground through the front tyre, so you get more grip and can go around corners faster.

This has been simplified, because in reality the rider does put some force through the handlebar mid corner. When the rider compensates for the short CS by being further over the front wheel, in an unstable position with more weight through their hands, unexpected disturbances or slides can cause the rider to be pushed even further towards the front of the bike, arms potentially buckling under the force. In contrast, if the front wheel can be weighted from the stable ‘attack’ position then any surprises and stalling of the bike can be controlled by heels being dropped and the force acting through the BB.

Finally, on the cornering front, quick changes of direction from turn to turn were slower and more forced than I would like. This can be attributed to the low BB in conjunction with the long wheelbase. Both things add stability to cornering and help the bike ‘lock’ into a turn once leant over but together they made the bike so stable that it was hard to get out of a turn. If the BB was raised then the head angle and wheelbase would provide that stability, while a higher BB would help with direction changes and the small, but unimportant advantage of a bit more pedal clearance.

So ... what does this all mean?

Obviously mountain bike geometry is pretty subjective and is driven by rider preference.

That said, there are certain constants that we want Starling bikes to be built around. Chain stay length is one of those and isn't something that is customisable on the bikes. Joe has gone for 445mm stays which are generally a bit lengthier than you'd see on other bikes.

My findings here seem to agree with Joe - bigger stays make for a more comfortable, easier to ride bike that corners better than you might expect. 

Stay tuned for part 2 ... 

 

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