Can a lightweight BMC Twostroke build keep up with the 9.3kg version?
I’ve always had a soft spot for hardtails and the art of being a weight weenie.
Hardtails may not be as popular as they once were, and cross-country race bikes are heavier than they used to be. But I wanted to see what I could do to flip that trend.
Could I make a lightweight hardtail, without sacrificing the heavier, trail-capable spec features we’re seeing at races.
Last year, BMC supported our ‘One hardtail to rule them all’ video. We beefed up an aluminium version of its Twostroke hardtail, with a burlier fork, wheels and tyres, and fitted a dropper post.
At the same time, we shaved the weight of a carbon version of the same bike, with lighter wheels. The result was a 9.3kg rocket ship.
Afterwards, I wanted to see how far I could push the limits of the carbon bike. How light could it really go? And would a combination of the two bikes prove to be fastest?
To find out, I put two builds for the bike together. A super-light, money no object weight-weenie rocket and a burlier version that still leaned towards the very light end of the spectrum.
“Simplify and add lightness”
The great F1 innovator Colin Chapman’s philosophy was, as he termed it, “Simplify and add lightness”.
Because the carbon Twostroke was already pretty simple, the process started with an even more dramatic weight-loss experiment.
The target? 8kg.
I wanted to first see how light I could go with this bike, and if it would still be good to ride. Spoiler, it turns out very light and yes, it was good to ride – in certain conditions.
For last year’s video, we replaced the stock DT Swiss XR1700 wheels with Hunt’s Proven Race XC UD Carbon Spoke wheelset. This dropped around 500g from the original build. Combined with the only other upgrade, a Garbaruk 12-speed cassette, the bike weighed 9.3kg without pedals.
The Hunts are an impressively lightweight set of hoops. I also love the obnoxiously loud freehub.
But I knew from my online bike nerding that I could chop more weight out here. Enter Pi Rope’s astonishing Duke Lucky Jack SLS4 29 wheelset. Using Pi Rope’s lightweight textile spokes, NonPlus hubs and Duke’s Lucky Jack rims, the result is a scarcely believable total of 952g – for a pair of mountain bike wheels!
After slapping these gob-smackingly light and bling wheels on, I dropped some more weight with Specialized’s S-Works Fast Trak tyres in a throwback 2.2in width. These slashed another 200g or so from the 9.3kg total.
My next areas of focus were the fork, cockpit, brakes and drivetrain.
I had wanted to swap out the surprisingly heavy (relatively speaking) Rockshox SID SL Select fork for Fox’s new reverse-arch 32 Step-Cast. Sadly, we couldn’t get one in time, so the SID SL had to stay in place.
Less or more carbon?
Undeterred, I set about changing up the cockpit. Again, this might seem strange at first. As stock, the bike comes with a carbon handlebar and seatpost.
So why swap them out? Carbon they may have been, but there’s always something lighter.
Enter German carbon meisters Schmolke. I swapped out the own-brand BMC carbon bar, seatpost, alloy stem and Fizik saddle for Schmolke’s gorgeous TLO items and an Extralite stem.
The 740mm bar and 27.2mm seatpost weighed only 99g and 114g respectively.
The saddle lacks any kind of padding and weighs a staggering 88g. The petite 70mm-long Extralite stem hits the scales at a mere 65g.
Overall, that represented a whopping 347g saving. My 8kg target was getting closer, with almost a kilo now saved.
The brakes were next and, again, lack of availability bit my efforts. Trickstuff agreed to send over a pair of ultra-light Picola Carbon brakes, but they didn’t arrive in time. Thanks, customs.
So I turned to BikeRadar’s brakes maestro Tom Marvin, who dug out a pair of Hope’s XCRs. Swapping these in for the stock SRAM Level TLM brakes saved a further 170g. I also dig the CNC finish a lot more than the Level’s cast aesthetic.
Marginal drivetrain gains
That left only the drivetrain for tinkering with. This is the area I knew would be most difficult. The BMC came with carbon cranks and SRAM’s lightweight X0 mechanical drivetrain – and I’d already swapped out the cassette to trim some fat.
This would be harder.
Like the cockpit, the first step was swapping carbon for yet more carbon. The stock crankset was a SRAM X1 affair, but there are lighter options, such as Race Face’s Next SL G5. I popped in a new Enduro Bearings bottom bracket to go with the Next SL’s bigger 30mm axle too.
Topping off the crank is a 34T oval chainring. It’s been a year or two since I rode an oval ring, but it didn’t take long to acclimatise.
Lastly, I swapped out the SRAM X0 rear derailleur and GX Eagle shifter for a matching Shimano XTR setup.
This saved a handful of grams, but overall, I’d cut 120g from the drivetrain. Not a bad return.
That rounded out a pretty comprehensive list of changes. The only stock parts left now were the frame, fork, grips and headset.
Lifting the bike onto the scales, they stopped at 8.02kg, a mere 20g from my target. I could have got under that magic 8kg mark with some slide-on grips, but I wanted to see how the bike rode first.
On the trail, the difference between the original build and the new one was dramatic. It felt even faster under acceleration, but also, much twitchier. Maintaining momentum in fact became a little harder, with the bike struggling to stay planted on rougher trails.
It was more of the same on the descents. In its original guise, the bike felt reasonably planted, but screamed for a dropper post.
In its 8kg form, the need for a dropper felt even more acute. Such was the flightiness of the bike, I needed to shift my weight around even more to keep it online. With the post up high, this was a challenge.
Is bigger better?
So while I’d proved I could build an 8kg hardtail, I wasn’t sure I’d made the bike better. Or faster. Had I, in fact, made it worse?
Time to break out the trail mods. I say ‘trail’, but these mods are things we’re seeing more and more on the cross-country race circuit.
I largely left the bike alone, changing only three parts; the fork, tyres and seatpost.
All three take inspiration from the bikes used by the world’s top racers. The SID SL was swapped out for a 35mm-stanchion SID Ultimate, with 120mm travel. Surprisingly, this saved a few grams, because the bigger-stanchioned SID weighs a whopping 8g less than its slighter sibling. This is likely due to the SID SL being the cheaper Select+ model. This doesn’t get the lightweight Charger Race Day damper of the Ultimate models.
With the front end looking beefier, it was time for more fresh rubber. I slid on a set of my favourite XC race tyres, the Maxxis Rekon Race. In a 2.4-inch size, with the brand’s MaxxSpeed compound, these added close to 450g to the weight of the bike. Would the extra cushioning and grip overcome that weight gain?
Last but not least, the upgrade the Twostroke needed from the start – a dropper post. OneUp’s V3 comes in a skinny 27.2mm size to fit the BMC, so I slotted in the shortest-travel 90mm version. It also happens to be pretty light. Including the remote and cable, it weighs 403g.
That’s a 290g improvement over the Schmolke post. Along with the other changes, would the added capability and weight make the Twostroke faster?
I also swapped in an SDG Bel Air carbon-railed saddle because the Schmolke carbon saddle didn’t agree with my posterior. I’m happy to sacrifice a little weight here (109g to be precise) in order to maintain comfort.
Overall, the bike now weighed 8.84kg without my Favero power meter pedals. With all that done, it was time to revisit the trails.
Proof in the pudding
Spoiler alert, the heavier build is indeed faster. On a timed climb and descent at Cwmcarn, it was quicker to the tune of two seconds over a lap of just under six minutes.
I’ll caveat that with this note. The burlier build was ridden on a different day due to the weather and the trail conditions were marginally worse. Despite that, it still came out on top.
Had I been able to ride both builds in the same conditions, I fully believe the margin would be wider. So where did it feel different and make up the time?
On the climb, it was only two seconds slower than the hyper-light, 8kg build. The extra weight was slightly noticeable. It definitely seemed a little slower to get up to speed, and the difference was most noticeable in traction and comfort.
The bigger-volume Maxxis tyres enabled the bike to track the ground better and bounced off trail features less. Not only was it more comfortable, the lack of bouncing meant I could put the power down better and focus on my line choice more. I could be precise, rather than praying I could stay on my chosen line, as on the lighter build.
On the looser section, grip was better too. It felt much easier to maintain traction and I noticed the rear tyre slipping a lot less. While it was slower overall, it felt a lot more manageable and less tiring to ride.
On the descent, it went four seconds faster. That's not to be sniffed at over a two-minute section of trail.
Despite the wetter conditions, I felt an awful lot more confident on the burlier, heavier build. The dropper post was a game-changer; it enabled me to move around on the bike a lot easier. Not everyone has Nino Schurter’s high-post riding skills!
While I felt the dropper post made the biggest difference, the tyres and fork shouldn’t be overlooked. The tyres in particular felt much more secure than the skinny S-Works rubber of the 8kg build. The bike felt more planted, had more grip and inspired a lot more confidence.
The extra travel of the fork wasn’t really noticeable. The bigger stanchions, however, were. The improved steering precision, combined with the improved grip from the tyres, meant I could stay on-line far easier.
Heavier = faster
Overall, the heavier build felt exactly as intended – more capable. I had an inkling it would be faster overall and so it proved to be.
While it's impressive you can build a mountain bike to around 8kg, I don’t think ultra-lightweight is the holy grail for modern cross-country riding.
The filtering down of tech from burlier bikes means cross-country bikes are more capable than ever. So even if they’re heavier, they’ll still be faster.
I’m suggesting we start throwing DH tyres and triple-clamp forks on XC bikes. Weight weenying was once the ultimate goal of Lycra-clad off-roaders, but some smart weight-gaining isn’t such a bad thing after all.