Tech Mailbag #8: Can I turn an old mountain bike into a gravel bike?

Also: quelling crosswind instability with wider tires, and how to measure chain wear without a chain checker tool.

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Welcome to the CyclingTips Mailbag column, where you send us your tech questions, and our team of nerds gives you answers. Trying to track down some odd noises? Curious about some possible upgrades? Wondering how the heck you get that darn tire off the rim?! If you’d like to be featured in an upcoming CT Mailbag column, send your questions to tech@cyclingtips.com or post them on the CyclingTips Forum using the #askanerd hashtag.

In this week’s Mailbag, we talk a reader off the cliff before diving too deeply into a gravel bike conversion, discuss the merits of the old-school way of measuring chain wear, and try to dissect the cause of some curious crosswind instability with wider tires.

Hi CyclingTips,

I just bought an old hardtail in excellent condition for C$300. I am hoping to convert this into a gravel bike for my son. Everything is almost stock, and I’ll be getting a gravel drop bar and replacing the fork.

Is this a good candidate for a gravel conversion? Are 650B or 27.5” wheels possible? Should I convert to disc brakes or is the v-brake more than adequate? Would you suggest that I keep the current drivetrain? This bike will be mostly used on packed gravel roads, fire roads, and pavement with very little use in gnarly or extreme conditions.

Thanks!

Sincerely, 

Tom Lee

Hi Tom,

Older mountain bike hardtails can be great starting points for gravel bike conversions. The geometry is often not far off from modern gravel bikes, there’s usually very inexpensive, and they’re generally in plentiful supply. I think you’ve got the right idea here, but I also think you’re heading down the rabbit hole. While everything you’ve mentioned is possible, I wouldn’t say it’s cost effective. By the time you’ve done everything on your list, you may as well have just bought a proper gravel bike to begin with!

Judging by the pictures you sent, my suspicion is that 650b wheels and tires will be a tight fit. And it’s also likely that the chainstays bow out too much behind the bottom bracket shell to fit a more gravel-friendly crankset (be it 1x or 2x). In other words, you might be stuck with 26″, much of that existing drivetrain, and the linear-pull brakes. However, that’s not the end of the world.

Keep in mind that it doesn’t take much to get going on gravel and dirt roads, which is one of the beauties of that style of riding. If your son is planning to do some longer distances, then drop bars would certainly offer a lot more hand positions. I would skip over integrated brake/shift levers, and instead, go with Tektro RL520 levers (which are designed to work with linear-pull brakes) and Microshift bar-end shifters, which should index just fine and are much less expensive. Alternatively, you could also just go with something like Surly’s new Corner Bar, which would provide a lot of the benefits of a drop bar while keeping all of your original controls.

A rigid fork wouldn’t be a bad idea as even a cheaper one would still lop off about a kilogram. However, be sure to get one that’s suspension corrected to keep the bike from getting too steep up front. Older suspension forks have axle-to-crown lengths of about 450 mm, so use that as a starting point.

Otherwise, all I’d probably do is switch to some faster-rolling tires — and by faster-rolling, I don’t necessarily mean narrower. There are lots of good high-volume 26″ options from Continental, Schwalbe, Maxxis, and others, with low-profile tread designs and supple casings. You won’t be able to safely go tubeless on the stock rims, but that’s fine; just run some ultralight butyl inner tubes.

Overall, try to push aside the need to have your son’s new project bike look like a modern gravel machine. With a few key changes, there’s no reason why he couldn’t enjoy getting out there on what you just bought. Once you know for sure that he enjoys it, start looking around for a used gravel or ‘cross bike, as that’ll be a far better starting point for future upgrades.

James

Hi CyclingTips,

I have never been very good at checking chain wear. Instead, I just change them on a semi-regular basis. When I get a replacement chain, I figure out the correct length by laying the old chain next to the new chain and then cut it to length. 

After I started cleaning and waxing my chain, I noticed that the length of the new and old chains are almost identical after my usual replacement interval. So I know Dave Rome will hate this because it doesn’t involve a tool of any kind, but is it possible to do a good chain wear test by just looking at the difference in the lengths?  

Michael Williams

Hi Michael,

While measuring the chain in the manner you describe most certainly isn’t Dave’s preferred method (and he details why in this extensive article on measuring chain wear), it still provides decent information since you’re talking about comparing the difference in length of the entire chain, and not just a 12-inch segment of it.

What you’re looking for is a 0.5% difference in length for the whole chain, but there are some caveats. That length discrepancy is only somewhat valid if you’re measuring two chains that are similarly tensioned and also clean. And even if you did go through the trouble of satisfying those two conditions, there’s still the matter of getting accurate and precise measurements considering a 0.5% difference works out to about 0.7 cm. 

My advice would be to get yourself a good chain checker. Not only would this take the guesswork out of the process, but it could also tell you that you might be replacing your chain more often than necessary.

James

Hi CyclingTips,

I’m a 176 cm/90 kg rider on top of a 8.3 kg 2020 Specialized Roubaix Comp with Zipp 303-S wheels (bike fit properly done). I’ve tried those wheels with 28 mm tyres but I was too close to the 72.5 psi limit since we’re talking straight-side. But when I switch to 30 mm tires, now I feel like the wheels are way less stable in crosswinds.

I ride on the 30-32 km/h range solo as well as 36+ group ones, and my rides are in the 1-1.5 hour range during weekdays, mostly flat, and 2-3 hour normal weekend “long ones” with 500-700 m gain and five fondos/year that are around 5-6 hours. These are on road (medium/bad pavement is common) with the occasional dirty path.

The Montreal (Canada) area has winds in the 20-30 km/h range at all times with gusts sometimes reaching the 50s. At those speeds (both mine and the wind’s), I have a bit of wobble but it’s not too concerning. It is when I go up-down Camelien-Houde (GP Montreal climb) and I can reach 70 km/h that I feel the instability the most when using 30mm Pirellis. I do not have the same problem with Vittoria Corsa 28s. This kills the joy of the descents, which makes me avoid climbs even more.

What should I do for increased stability with 30 or 32 mm tyres? Should I go with 32-35 mm wide wheels? What are the most stable? I don’t necessarily care about them being the fastest mid-depth (45-60) wheels.

Thanks for the help, and cheers!

Leo Nascimento

Hi Leo,

First things first: you shouldn’t be running the Corsas on those wheels regardless as Vittoria doesn’t approve them for use with hookless rims! 

As to your main question, though, this is an interesting one. 

Zipp says those 303-S wheels are optimized for 28 mm-wide tires (actual width), but my guess is those Vittorias are already getting close to 30 mm in terms of how they measure when inflated. For those Pirellis, it’s probably closer to 31 or 32 mm. Considering the external width of those wheels is 27 mm, you’re starting to induce a “light bulb effect” where the tire is getting a bit too wide for ideal aerodynamics. It’s probably not a big deal at lower yaw angles, but as you’re apparently discovering, the airflow is getting messier when you start introducing crosswinds into the mix.

If you’re set on running 30-32 mm tires, I might consider switching to wheels that are even wider than what you have now, and I’d also think about going a bit shallower. A slightly lower-profile rim might be slower in a wind tunnel, but if it’s easier to handle and lends more confidence in a broader range of conditions, it’s faster overall since you’ll feel more comfortable letting your bike really run.

I certainly haven’t tried every aero wheelset on the market, but ones that come to mind for your situation are the Enve SES 3.4 AR (32 mm external width), Zipp 303 Firecrest (30 mm external depth), Bontrager Aeolus Pro 3V (32 mm external width), and Roval Terra CL/CLX (30 mm external width). 

Good luck in your search!

James

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