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Are 29’ers and low bottom brackets faster? Part II

As threatened – here’s part two of the post I started the other week. You might well remember that I was slipping on the lab coat and pocket protector to work out if bigger wheels and lower bottom brackets were the key to unlocking extra speed…

Regarding wheel size the answer was – 29’s might well be faster but having a chubby 27.5 plus out the back did give more comfort! 

The next stage was to start rolling the eccentric bottom bracket round to switch between the two drops available and see what effect it might have on handling. But what were we expecting to see?

Traditional thinking has it that the lower the bottom bracket the better – it lowers the centre of gravity and increases stability at speed. 

But then I got talking to my friend Matt. Matt likes to ride hardtails fast. Very fast. Quicker than most people can manage on a full suspension machine. He likes doing things like this:

Which means I’m interested in what makes him so rapid. He’s a fan of big wheels too but likes a higher bottom bracket as he finds it makes it easier to lift the front wheel.

He also feels that it ‘hooks up’ less on square edged hits. I think I can picture the theory of why that might be – it’s to do with the angle at which forces are applied and how the bike is trying to either ‘pull’ the rider backwards or ‘push’ them up.

The higher the bottom bracket the lower proportion of the force directed straight back slowing the rider down. The bike may be pitched upwards more but the overall braking effect will be lessened. Obviously there are compromises to be made here too, just making the bottom bracket six feet off the ground isn’t going to get us very far!

Tight fit baby…

First a word about Press Fit 30 shells and how I got on with the eccentric bottom bracket. If I was looking for a word I think I’d chose ‘fiddly’! The whole press fit idea works on everyone agreeing on the same set of tolerances to get a smooth interference fit. It’s interesting to note that many manufactures like SRAM use a nylon (plastic) shell as it will readily deform and cope with a wider range of tolerances than a metal shell will. It will also creak less than a metal to metal interface!

I chose a Wheels Manufacturing eccentric bottom bracket for my build as I could find one for a good price (that’s why it’s red!). The designers had a tough time with this one as the fit needs to be snug but still able to rotate to give you the adjustment. 

Unfortunately everyone erred on the side of caution which meant the cups were nowhere near ready to go in straight out of the box. Cue much filing and offering up to get the whole thing together.

You’ll need to keep it well greased to avoid the dreaded creaking too. And… I’ve a feeling I’m close to stripping one of the bolt threads that clamp the two halves together too!

It’ll work for what I’m looking to do but I’m not sold on PF30 as a platform for an EBB. If I was doing this again I’d be thinking about using a Bushnell system instead, the expanding wedges look like they’d do the job nicely – although it does require a larger bottom bracket shell.

Spin me right round, baby! With a bit of weld porn thrown in!

Just get the thing cranking!

I know, less waffle more riding. By about now you’re probably wondering how the thing rode. Just to remind you I was doing back to back runs swapping between a 29’er and 27.5+ rear wheel for both the 45mm and 57mm bottom bracket drops.

And the result was…

I couldn’t tell the difference between the two heights! OK, well that’s not strictly true. Running in the ‘high’ position meant putting the saddle height up 12mm too, that felt a bit perched when pedalling sat down. It made the drop to the bars feel a bit long as well.

When stood up though… Nope, not much in it from a riding point of view. Interestingly I did some of my faster runs of the day in the 45mm position, isn’t that contrary to the popular view that lower bottom brackets give you more stability and speed?

Or is this back to Matt’s point that the higher bottom bracket’s ‘instability’ actually helps you change direction quicker getting you down the track faster?

Perhaps the latest crop of extreme long, low and slack hardtails have so much inherent stability from monster slack head angles and long wheelbases that they’re not as sensitive to changes in the bottom bracket height?

Either way it looks like the change in drop would need to be more extreme to be noticeable, at least for me.

Moar testing needed!

Full 29’er and higher bottom bracket – you could tell that, right?

You’ve not read all that guff have you? Here’s what you need to know…

About which wheel size to run:

29’er Wide Trail wheels feel faster than 27.5 Plus. Putting a wagon wheel front and back gives you great rollover combined with good traction and less drag than the smaller diameter plus tyre.

Running a fatter 27.5+ wheel at the back (with the 29’er up front) gets you back the excellent bump smoothing that plus tyres offer which is a big benefit on a hardtail. You also get monster climbing traction and a better ability to ‘pump’ the bike for speed.

Ultimate choice? I’m not sure yet. I might even end up running a plus tyre on the back most of the time, for comfort, and swapping it out for the 29er on race days when I need more speed!

About which bottom bracket drop to run:

On a long, low and slack machine it might be worthwhile having a higher ‘less stable’ bottom bracket to make changing direction easier.

At the end of the day though if we’ve learned anything it’s… Moar testing needed! I’ll get the t-shirts printed up 😉

Watch this space for further developments…