I wrote back in May about the changes I'd made to my old crit bike to turn it into a sub-16 lb climbing monster for Wintergreen. My goal then had been to spend as little as possible to get the bike back on the road and just see what I could do with it. The result was a 15.7 lb 1x setup with a 40T ring driving an 11-36 SRAM 11-speed cassette. At the time it seemed perfect, even if I had failed to adequately tighten the cable in the derailleur. Oops.
But then I got a little greedy. The bike had been my main steed for 8 years, and getting it so cheaply back on the road made me want to ride it more. I had an idea to use it as a backup crit bike, just in case. That would be easy enough to accomplish with a cassette and ring swap. I raided the parts bin and found an old SRAM RED 11-28 and ordered a cheap ($25) 48T ring. 48 x 11 is kinda small for a top gear, but it would preserve the chain length perfectly. A few (dozen) turns of the B-screw, and I had a bike that was almost as fast on the flats as it was uphill, but now at an incredible 15.15 lbs.
This, of course, made me want more.
For months I've dreamed of finding a way to shave off that magical 0.15 lbs that would put the bike on UCI minimum weight. 2 principle options emerged:
1. tubeless -> TPU tubes
2. 32T chainring...somehow
The first option would be exponentially cheaper, and would seem like the obvious easy answer: TPU tubes typically weigh somewhere around 35 - 45g each, generally 1/3 that of butyl tubes, and also a fair amount less than sealant. I have about 2 oz in each 700x25 GP5000 S TR. Making the swap to TPU could net as much as 150g, or 0.3 lbs, in savings. That would put me under 15 lbs, and it wouldn't vary with sealant top-ups. But it would be a big job to clean out ALL of the sealant, and with rim-brakes, the lightest TPU tubes aren't really an option, as they're prone to heat failure from braking. Maybe fine for just pure climbing, but not for coming back down the mountain.
It also only achieves that weight goal with deeply sub-optimal gearing. The 11-28 cassette might rocket up the local hills with either a 40 or a 48T ring, but it would make Wintergreen harder than ever.
To hit that climb successfully, the math says the right chainring is a 32T. At 90rpm, 32 x 11 produces 20.5 mph, which is typically the highest speed I see at the base of the climb. 32 x 28, however, is a gear ratio of 1.14:1, very close to the 1.11:1 of 40 x 36 that was so good on the steepest slopes of the mountain.
32 x 11-28 also has 16 fewer chain links than 40 x 11-36.
It's physically less metal in the chainring, so reasonable to assume it would be lighter, as well.
But. My crankset is a 5-bolt 130BCD, meaning the 40T is really already the smallest it can support. To go lower would be expensive. It's also a BB30 crank in a BB30 frame, so the bottom bracket is an outdated, but insanely light, standard.
So here's where things get complicated quickly. Mountain bikes and road bikes typically cannot share components around the cranks and bottom brackets. The exact same standard, whether it's BB30 or Shimano Hollowtech or whatever is different based on road vs mtb. They may say the same thing, but they're not the same. This is usually in regards to the width of the bike frame itself, but also bolt-circles and chainring tooth profiles and all sorts of DEEPLY UNNECESSARY COMPLEXITY.
TL;DR: there is no such thing as a 1x 32T road chainring. There are plenty of 1x 32T options for MTB. If I want my unicorn, I must create it.
As luck would have it, though, the MOUNTING interfaces between road and mtb cranks and their chainrings--the spider--do appear to be identical. 8-bolt SRAM is 8-bolt SRAM. 3-bolt is 3-bolt. So if I get the proper crank arms and mount the correct spider, in theory I could run any size chainring I want. In theory.
In theory.
In reality, while there are a million bottom bracket specs and a million various crank manufacturers and standards, the options diminish significantly when factoring for matching road & mtb interfaces, future supportability, and cost. That pool basically comprises SRAM and Rotor.
While SRAM spiders are available in 3-bolt or 8-bolt interfaces, SRAM no longer makes the necessary crank arms & spindles for 3-bolt. So if you don't want to rummage through parts bins or hope for unicorn finds at the local bike swap, it's 8-bolt on a DUB spindle. Not a deal-breaker for most frames, but they used to make GXP and 30mm spindles, too. Shame they're out of that game or this conversation would already be over: my bike is a native BB30.
But because it's a true actual skinny BB30 frame, and DUB spindles are not designed for 68mm inboard-bearing frames, the only purported option that I could find was a $300+ adapter from BBInfinite. That ain't gonna happen.
But what about Rotor? They offer a 30mm spindle (98g, $35) with an extremely modular design that makes swapping cranks MUCH easier, so no bottom bracket work, and I could choose pricey carbon cranks in pretty much any length I want (all the way down to 155mm/255g for $351!).
Rotor's spline interface appears identical across their road & MTB ranges, and they've been around as a brand with reliable staying power for many years, so while there's 0 chance they'd stand behind this unholy union, at least the parts would be available?
Looking at the available spider options on PowerMeterCity, it seems there are a couple in the $400 range, but they're all out of stock for now, including the new Sigeyi AXO that allegedly only weighs 83g.
I really don't think I want to drop $825+ on a silly idea to tackle one single race, but dropping the crankset mass from 690g to ~485g would be absolutely insane, more than accomplish my rush to the UCI minimum--in theory the bike would weigh just a tick over 14.6 lbs, and I could still throw in a pair of TPU tubes for Wintergreen (the race explicitly prohibits riding back down), potentially rolling up the mountain on 14.25 lbs of carbon.
When the Aeroad arrived in June 2023, I never expected to ride my old Blue Axino again. She's harsh AF by today's standards, but her evolutionary cycle is not yet complete. For now, she's a dedicated trainer bike for the winter, but it's exciting to know there's a potential path to even greater glory.
