single speed flat bar gravel bike build – how to turn a 26″ mtb into a 700c gravel crusher!

Ok now theres frankenbikes……. and then theres the wrong size wheels for the frame frankenbikes…… the wrong size fork for the frame frankenbikes…… the no gears on a geared frame frankenbikes…….. and this one ticks them all. My Saracen Dirttrax 2000.

With my Dawes gravel bike off the road and at Armourtex in London for powder coating I’d left myself with no bike for ripping up the New Forest trails on my doorstep. Big problem, I couldn’t be stuck for two months without a bike for that….. ooooofff. I needed a solution.

I had a scout in the garage…… I had parts left over from an old single speed road bike, and my Saracen frame from my teens (20 years old now!), plus a few spares kicking about….. The build was on………… Although there were some some part “compatibility” issues…….

Here’s a rundown of what I had in stock:

  • frame – year 2000 Saracen Dirttrax 2000 7005 alloy
  • headset – FSA Orbit XL
  • wheels – 700c Rodi airline evo road wheels
  • forks – Ritchey pro carbon 700c CX canti forks (from a Scott addict CX I can’t believe they are worth £370 lol)
  • brakes – STX RC brakes and levers
  • cranks – origin8 165mm square taper track cranks and bottom bracket
  • Chain – new singlspeed KMC Z1 chain
  • Gearing – singlespeed conversion with 42t oval chainring and 16t cog
  • tyres – Hutchinson overrides 700x38c with tubes from my Dawes
  • Saddle – Trek WSD from my wifes MTB
  • Seatpost – Planet X 27.2 alloy with shim to fit
  • pedals – Shimano Spd from my Dawes
  • Grips – some old charge one with lockrings

A real Heinz 57! Although it wasn’t quite enough to finish the bike. I needed handlebar and stem plus sundries like cables.

Handlebars and stem were easy to come by but, I would have a little wait. My current bicycle part supple love affair is with AliExpress, replacing eBay as my number one source. Spending a few hours trawling the listings I came up with an “EC90” full carbon stem and “Ritchey WCS Carbon Pro” handlebars. Both of which I am dubious of authenticity but would at least do the trick, oh and cost me £35.

Over a week of evenings, slowly cobbling it together…… it started to look petty cool. Its not an outright looker but it has something, plenty of people have actually complimented it! (although I cant workout if thats a sympathy vote….. “OMG he’s riding something that looks like it’s from a skip”). I actually love the oversized aluminium tubing and the mono stay seat stays, super 90s/2000s MTB.

starting to come together
the drivetrain setup

One thing I had to address though was making sure the 700c wheels would fit in a 26” frame. This is mostly to do with frame clearance, not so much the brake position. Yes it’s easier with discs but converting v brakes to fit 700c wheels in a 26” frame is pretty easy and shouldn’t be something to put you off.

First off let’s talk about wheel diameter and tyre sizes, for a gravel bike you really need tyres as a minimum to be 32c upwards to give the protection, grip, and comfort (use the word suppleness here if you are a trendy hipster) required for light off road. Don’t forget that the tire size is not just the carcass size, it’s the with of the tyre including any tread as well. Here’s comes the science bit, concentrate! Ok let’s work out the maximum wheel diameter that can be fitted in the frame (based on a 700c wheel tyre combo).

You can do this one of two ways. If you don’t have any wheels to try out you can fit an axle to the frame (or tape in a pencil?) and measuring a piece of string exactly 311mm (700c rims have a bead seat diameter of 622 mm) from the centre of that axle towards any point where the wheel may contact, ie the bottom bracket and seat stays. Then at the end of that 311mm point cut out a circle of card to the same diameter of the tyre you want to fit (like a lollipop on the end of the string, not through the centre of the circle) to gauge the clearance. This should give you a fairly accurate idea of maximum tyre clearance.

If you have wheels or you know the recommended maximum tyre clearance for the 26″ wheels fitted you can also work from that, but its a little more hit and miss. Firstly use an online tyre size calculator like this one HERE from BikeCalc.com to find the diameter of the max tire size for the frame. The Saracen could take 26 x 2.3 tyres easily from experience (it was the early 2000s and Tioga factory downhills were everywhere!). These equate to a total wheel diameter of 678mm, the direct swap for 700c wheel/tyre combo would be a 700x28c at 678mm. Now that may not sound like the ideal tyre size for gravel but don’t fret, thats the maximum diameter tyre that fits in the frame with a width of 2.3″, the narrower the tyre gets the larger the possible wheel diameter as the wheel fits further into the frame (this is where the first method is a little more accurate). So the tyre I wanted to fit into the frame was a 38mm and had a diameter of 698mm, an increase of 20mm over the old wheel set, that boils down to a 10mm difference in how close the tyre would run in the frame at its closest point. Luckily I had wheels and tyres that I could try but as the match suggested they fitted straight in with decent mud clearance.

There are two things to note here, any difference you make to the total wheel diameter will change the bottom bracket height (fork length dependant) and the toe clearance on the pedals (again fork dependant due to length/rake/offset). Luckily the 700c forks I was fitting had the perfect length and rake, there were no issues on toe contact and bottom bracket height raised a minimal amount. This is often a point of contention and one of the phrases flagged about on forums and facebook “changing wheels will fuck up your geometry”. It won’t, it really won’t. Oh unless you do something stupid like go to a 700c x 25mm on the back and 24 x 2.1 on the front. (As a quick note, a swap to a 650b wheel with a 50c tyre is another great option but usually 650b wheels are easier to come by in a disc hub/rim).

Next thing to manage was the brakes. The frame has v brakes and the calliper mounts are fixed in position to match the height of a 26″ wheel. Fitting a 700c wheel moves the rim further away from the brake pivots, meaning the brake would no longer contact the rim when pulled. It would pull below the rim. There is a chance you could get away with this depending on brake and frame manufacturers. The frame could have the calliper mounts mounted higher and you could have scope in your brakes to move the pad high enough but its a relatively rare occurrence. This is due to the distance the callipers need to move. A 26″ rim has a diameter of 559mm and a 700c a diameter of 622mm, a difference of 63mm. The brake calliper needs to move half of that to stay in the same position in relation to the rim, so 31.5mm. I solved the issue using some BMX brake risers, these are used on race BMX to change between wheel sizes for junior classes. They mount to the existing mounts and shift the position of the caliper up by roughly 23mm (406mm wheel to 451mm wheel). The remaining 8.5mm was easily taken out by pad adjustment. These can be used front or rear on the bike but, as I was fitting a 700c fork I only needed to use them on the rear.

brake mount risers from AliExpress
well used, heres those brake risers on the bike

As I was assembling I started to realise that it would actually be a pretty lightweight build. The absence of a full groupset and the choice of v brakes, alloy frame, carbon finishing kit, and not a lot else left it quite pure. All together the bike weighed in at 10KG. Impressive!

How did it ride then? this was the biggest surprise. I was expecting some either whippy or barge like steering, a weird riding position, or some odd peddling dynamics with the track cranks. But no, it went really well. The handling felt very natural. On its first couple of rides it even bagged a couple of strava leaderboards. The only thing that was a little off was the gear ratio to crank length combo. Whilst a 42×16 gear ratio is fine on a road bike in the new forest, its a little high on gravel bike. This is exacerbated by the increase in tyre size, a larger wheel increases the metres of development (how far the bike moves on a pedal stroke) of the gear thus making it harder to turn (you should always take into account the effect on the bikes gearing when changing tyre sizes, it can make a big difference!). Although, when riding through mud it actually seems easier to keep the bike rolling in a higher gear and avoiding wheel spin, rather than shifting down and increasing the torque to the rear wheel and causing it to break traction.

The bars and stem combo I nailed at 70mm long and 760mm wide, the ride position was perfect. I’ve never had bars so wide before so it was a bit of a gamble but it feels very natural and very comfortable. Carbon is naturally shock absorbing plus with the additional length they have a nice amount of deflection leading to an even smoother ride (this is all down to how the carbon is used though, ie the forks are constructed in a way to make and keep them stiff but the bars allow some flex). The handling feels super confident and the bike tracks really well. I always remember it being easy to ride without any hands, it hasn’t changed in its current form. I’ve completed a couple of circa 40 mile rides on it and returned home without feeling too knackered, it just keeps rolling!

the “ratchet” bar and “ec90” stem make for a great ride

The single speed Saracen gravel bike has been a steed for about a month and a half now and, I have to admit I’m a little in love with it, I’ve totally rekindled my feelings for it from my childhood (although finding out that at 37 you still fit on your bike from when you were 17 is a bit odd. I’m going with it was far too big for me as a kid as a cold comfort). To the point I quite want to push it a little further…… and make it look a little prettier. Yes I just can help myself……….

Such a fun thing to ride!

This will happen when I get the Dawes back from paint and its on the road……… im thinking some more carbon….. a lighter drivetrain setup…… tubeless wheels…. a new paint job….. can I get it below 8kg? now that would be crazy…. maybe lets just look to hit the 8s first…….

BC

see you on the trails!

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