Experiment: GT Sanction 26 – 650b Conversion

Not possible people said, yeah whatever!

Several months ago, when all the wheel size hype was emerging and I realised I didn’t then (and still don’t) have the dosh for a new rig, there were rumours of people trying to shoe-horn in the bigger 27 1/2″ wheels into a 26″ wheeled bike, and judging by reviews on the internet, not without their problems. I read a few little articles where the so called knowledgable people said it was a bad idea and your angles were all wrong, I even believed the hype and dismissed it as a bad idea, until proved wrong by a friend of mine. The friend being Richy Lewis 2012 UK Gravity Enduro Veteran champion; he said he had it sussed, basically if you have a bike with certain types of drop outs, then a drop out spacer can be made to go in between the drop out and the frame. This spacer puts the back wheel up and back to compensate for the extra 1 1/2″ wheel.

OK so it’s obviously not that simple, as there are four other very important factors to consider, head angle, BB height, wheel base and fork brace clearance. Well lets look at them individually. The fork is the main factor straight away, I don’t know about the separate brand’s clearance, but when it comes to Fox it can only be achieved on the 36, as nothing less has the clearance below the brace. When Richy did his Intense Tracer he used Shwalbe 2.4 tyres and you could hardly fit a gnat’s dick between the tyre and the 36’s brace. I opted for the new and brilliant Maxxis High Roller 2’s, coming in at a nice low profile 2.3, I ended up with a good 20mm clearance. When I first put my conversion on, it was like riding a chopper, with a head angle of under 64 degrees! so this ( I thought) could be cured by taking the Burgtec off-set bushes and fitting them the opposite way round, sharpening the angle back up to around 65.5 degrees. The problem being that you then sent the BB height rocketing up to a thoroughly un-exceptable 362mm. 

The next job to have carried out was to reduce the fork travel from 160mm to 150mm to drop the front, as advised by our local nice guy, suspension servicer, James at J-Tech. Now there ain’t a lot this guy doesn’t know about fixing and servicing suspension, he informed me how, once this was done we could swap the off-set bushes back again to lower the BB height once more. James had already done a great £80 upgrade to my Fox shock, when I had the bike from new. This was to make it so the Pro Pedal lever made a much bigger difference when switched on, it’s a great upgrade, really stiffening up the suspension for those nasty climbs, so much so, that when you’re sat on the bike in Pro Pedal and switch it back to the open position, you actually feel the bike drop as it sits down into DH mode! James spent the afternoon on the bike and very kindly re-serviced the rear shock too. He told us of the massive differences between Rockshox and Fox when it comes to after-service, how Rockshox offer downloadable instructions on any after service technique, but Fox are very tight on letting any suspension gurus even order after parts for main bodies of shocks. He told me how the bike, with the now lower BB will sit into the corners lots better than before too. He said the Burtec off-set bushes make 5mm difference per bush in BB height and the difference of reducing the fork travel one inch would change the rake by up to 1 degree. To reduce the fork travel James takes the lower legs off, removes the air assembly and puts in a 10mm spacer on the top side of the negative spring.

Next test ride and the bike still didn’t feel right, the suspension was lovely, plusher than a kings bed, but the front was still too slack, running around just over 64 degrees. I tried to pin it at my local trail centre, only to have the front end washing out on the turns, leading to a complete lack of confidence, I was starting to believe the doubters again, was I wasting my time? This is where I once again spoke to Richy, he said all was not lost, there’s one more thing to try, the Angleset headset, he informed me how another local company, Works Components over at Cannock, made an array of them for loads of different bikes. So I was straight on Google, contacting them, they told me they did a 1.5 degree Angleset to fit the GT’s tapered head stock and would very kindly send one over, worth a try” I thought!

This was it, the last chance to make this work, the Angleset was fitted and I went off and did a massive trail ride around Llanollen, taking in some huge twisty climbs, fast open hilly downhills, rocky off-piste stuff and even one run of the terrifyingly steep and gnarly NPS DH track. Things were finally feeling right, and with a mid way lunch stop I slid the seat right forewords through the Reverb rails and dropped the stem through to sit on top of just one 5mm spacer and finally she was bang on, even got a Strava KOM to my name that day (not on the DH track I might add!)

So there you go, it did seem to take a lot of pissing about to get there but a point was proved in the end, with the bike ending up riding fast, stable and precise, with it’s 65.6 degree head angle and 353mm BB height and a wheelbase about 10mm longer than standard. So it’s definitely an option for those on a tighter budget, at this point I hand over to Richy, to tell us how he figured it out, how people can contact him, prices and where he thinks he can go with it.

Richy: “Cheers Jim, I first realised the advantages of the bigger wheels after a short ride on a mates’ bike, and I started thinking how I could squeeze these wheels into my frame. Then whilst out on a training ride out at Eastridge with my mate Tim Beedles (another UKGE racer) he mentioned that a new wheel size was emerging, 650b. A mid size that gives some of the advantages of the 29er, without the cumbersome size of the 29er. This is when I started looking into it seriously. My Intense Tracer 2 has replaceable drop-outs to accommodate the various hub options, eg quick release or through axle, so a drop-out switch was thought up.

Firstly I had to get the actual size of a 26″ wheel, which isn’t 26″! this is just the average diameter of a 26″ tyre and varies dependant on tyre size, type and manufacturer. Once the actual size was established, and after understanding the size increase to 650, I started the design. I didn’t want to alter the BB height or lengthen the swing-arm too much. Also being aware of the extra loading the drop-outs would need to take, due to their added length, I opted not to scallop them out similar to the originals, which only saves a gramme or 2. I had them machined out of aircraft grade aluminium for extra strength. I was keen to test my creation, had some wheels built up and ordered a pair of Schwalbe tyres, both courtesy of my local bike shop, The Trailhead in Shrewsbury. I had bragging rights to the first pair of 650b Hans Damp tyres in the country!

After bolting on the new 650b drop-outs and break calliper adaptor, there were a few final things that I needed to consider. The extra wheel diameter would alter my gearing ratio, so I opted for a smaller front chainring. Also the extra length in the swingarm changes the shock ratio, so the sag needed to be re-set with a bit more pressure. Plus I need to check the chain length throughout the full travel of the sweingarm, to prevent unwanted pressure on the freewheel body.The bike rode fantastic, and within a week or 2 I had loads of enquiries, this led me to the GT solution, and I am currently looking at other manufacturers frames that can be modified. As Jim says, there are lots of factors that need to be taken into account when changing wheel sizes, all of which, with the exception of fork brace clearance,can be resolved one way or another! For prices and advice on set-ups please contact me on my email below.”

Thanks a lot for those who helped in this project

Richy Lewis richylewis@virgin.net
Hope 650b Arch Evo wheels £125 front £215 rear www.hopetech.com
Burgtec Off-set bushes £46.99 www.burgtec.co.uk
Maxxis High Roller 2 £53.99 www.maxxis-bicycle.co.uk
Works Components Angleset Headset £74.99 www.workscomponents.co.uk

Words: Jim Buchanan Photos: Doc Ward (http://www.doc-photography.com/)