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

Mullet bike: business up front, party round the back…

You might have noticed Project S.Y.D. has reached version 2.0 (you can find out more over at www.screwyoudeath.com). One of the key additions to that frame was enough clearance to run either 27.5 plus or 29’er wheels – all in the name of finding out which size would offer the best combo of speed and cushioning.

But 29’er Wide Trail tyres are about 20mm larger in diameter than 27.5 plus – which would shift the bottom bracket height by 10mm. Would that mess with the handling too much?

My cunning plan was to whack in an eccentric bottom bracket that could be twiddled up and down to compensate for the difference in wheel size. Using a Press Fit 30 bottom bracket shell I could get an eccentric bottom bracket that would offer about 12mm in total height change, just what I was looking for!

Here it’s pointed North for a 45mm drop, when pointed South it’ll extend to 57mm, handy!

But what bottom bracket height to go for to start with? Actually, height might be the wrong word to use when bottom brackets are usually measured in terms of ‘drop’ from the centre line of the wheel axis.

Most aggressive 27.5″ bikes run 35 to 50mm of ‘drop’ with 29’ers going for 50 to 70mm, and sometimes more! The theory being you should go as low as possible to bring the center of gravity down which increases high speed stability. But you can’t go too low as you end up grounding out the pedals all over the place!

But…

Interestingly there’s a competing theory from some riders that says the lower bottom bracket can hamper the ability to lift the front end up and actually makes the bike harder to turn in. They also feel that running a higher BB makes the bike ‘hang up’ less on square edge hits – which makes the whole debate much more interesting!

What to do, what do to? I decided to take a best average and aimed for a centreline of 51mm bottom bracket drop. With the +/- of 6mm from the EBB that would work out as a 45mm drop in the upper position and 57mm in the lower – which should be spot on for the crazy experiments I had in mind!

What are we trying to do here?

On the day I was aiming to ride the same couple of trails, back to back, changing the wheel size and bottom bracket drop in between runs to see if I could feel a difference. Which would I prefer? Which would feel ‘right’? More importantly, which would be faster? 😉

I picked a couple of my favourite local trails that have a good mix of slow and medium speed corners, with plenty of rough ‘n’ rooty features and not much pedalling. I’d aim not to go as fast as I could, but to try and keep it consistent and instead concentrate on how the bike felt going down familiar trails.

I had originally thought I’d swap both front and back wheels out but… The front 27.5 plus tyre just has an annoying ‘DUNK, DUNK, DUNK’ resonance as it bounces down the trail. With a hefty pair of forks up front the extra volume of the plus tyre isn’t really needed and it’s potential for squirm means you have to be super careful with tyre pressure.

As you might have guessed by now, I feel that a 29’er Wide Trail up front just works better! It’s got better rollover, less drag, and I’ve not really noticed much difference in grip even though it’s slightly narrower.

This test then would concentrate on changing the back wheel for a 27.5 x 2.8 after the initial ‘reference’ run on the original 29 x 2.5WT wheels.

In full 29’er mode…

And with a 27.5+ wheel out the back, 29er up front.

Mullet bike!

With the chubby tyre out back I immediately felt more at home; it lowered the back end slightly which put the bars at a better height compared to the saddle. The familiar cushioned feeling was a welcome return too having spent the previous 6 months running chubby rubber. With a greater volume to play with the 27.5 plus tyres give more scope for ‘faux’ suspension which is perfect for the back of a hardtail. Even with a ghetto-core (pipe lagging!) insert in the 29’er wheel there’s still a limit to how low you can go with the air pressure before everything starts to bottom out.

The climbing grip is excellent too, the large volume tyre wraps itself around any roots and rocks to winch you up anything you’ve got the legs to climb!

So if it runs down the trail more smoothly it should be faster, right? Err…

Surprisingly I felt the 29’er rear wheel was faster, the larger diameter and higher pressure meant it rolled better! There was definitely more feedback over braking bumps but it seemed to gain speed more quickly. Indeed the quickest runs of the day were all done on the 29’er wheel. The smoother ride of the plus tyre felt less fatiguing though and over a longer run, or a bigger day out, the difference might well swing the other way!

So the ultimate wheel size would be…

29’er on race day and a 27.5 plus for the rest of the time? Perhaps even a 3 inch wide 29’er plus tyre?!

I think this will need some more testing over longer runs to really thrash out. I’m going to put a tyre insert in the 27.5 tyre for some ‘fit and forget’ protection and go hunt some more trails…

Hang about, you said something about bottom bracket heights?

I know, what a tease, right? I guess you’ll have to wait for Part II coming up soon… 

That’s all for now folks!