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Tech FAQ: Crank Outcry and Tire Selection

  • By Lennard Zinn
  • Published Oct. 19, 2010
  • Updated Oct. 11, 2012 at 5:05 PM EDT

Dear Lennard,
With all the knowledge on biomechanics and trying to get the best out of the human engine on a bike, I am still floored that mountain bike cranks come only at intervals of 5mm, i.e., 170, 175 and 180mm.

I ride 172.5 cranks on road, TT and cross bike and actually have a set of (now older) 172.5 XTR cranks on my mountain bike. I now see that even XTR has dropped this size and no other company specs a 172.5 mtb crank that I know of. I noticed a huge difference when I switched from the spec’ed 175mm crank and put on the 172.5mm, everything felt “normal” switching from bike to bike to bike now.

Can you explain why the mountain bike industry has this very narrow view that it is unimportant and you really-won’t-notice attitude? Is it really as simple as they have created no/little option to carry/produce fewer SKUs? As I am sure I will have little effect on the manufacturing schedule of Shimano, can you tell me if anyone still produces a 172.5mm mtb crank please?
-Richard

Dear Richard,
Far be it from me to know the answer, but I’ll venture a guess. The fundamental reason, in my opinion, is that you and others like you are solitary voices in the wilderness and without a large outcry, the big crank manufacturers see no advantage in spending all of the extra money required to offer lengths in 2.5mm increments.

You could not be in the road crank business and not offer a 172.5mm crank. It’s as simple as that. The world is full of devotees of that length, and if you can’t sell to them you’d probably be losing more than half of your potential crank sales.

The same is not true with mountain bike cranks. With very few exceptions, mountain bike riders make no fuss about accepting the 175mm crank length that is on the vast majority of bikes.

Contrary to your contention, the cost throughout the supply chain of offering that extra SKU is actually quite large. Especially an XTR crank, which is 3D hollow forged in a single go — the tooling to make a single crank length is enormously high. It takes a series of very high-strength precision forging molds with gradual changes in them as the crank moves from an amorphous aluminum blob to a beautiful refined piece. Those are costly.

Then you have extra inventory at the manufacturer, distributor and shop level. Everyone would just as soon pocket that money as profit if they’re not going to sell a significant number of more cranks by offering a 172.5mm. And I’m sure Shimano has data to show that the 172.5mm is not cost effective or it wouldn’t have dropped that size.

Your only hope is to somehow band together with enough other like-minded individuals that your opinion can hold sway.
-Lennard

Dear Lennard,
I am new to the mountain bike world and have a question about tire selection. In a recent race on dry, hard-packed ground I crashed twice when my front tire slid out from under me going across a side hill. I am thinking that I either have the wrong tire or air pressure and am looking for advice. The bike is a Specialized Stumpjumper FSR and I am using the stock tires that came with it running 45 psi. Specialized claims that the tire is “all-purpose” and it has fairly tall tread blocks that are relatively far apart.

I looked at the tires other people were riding and they mainly had lower, closer together tread blocks. Would a different tire selection help?  If so, what? How about pressure?
-Dave

Dear Dave,
I’d say it was both tire selection and too much air pressure. Big knobs are good for loose dirt (and for mud, provided they are far enough apart and the tire is narrow). Low, stiff, closely spaced knobs are better on hard surfaces.

On side hills, low pressure keeps a lot more tire on the ground. More surface area of contact equals more traction, and 45psi is a lot of pressure for all but the heaviest of riders. Thing is, depending on your weight, you could run the risk of pinch-flatting your inner tube as you drop pressure. That’s one of the biggest reasons for running tubeless tires. Even 175-pound riders can run pressures under 30psi with tubeless tires.
-Lennard

Readers can send brief technical questions directly to Lennard Zinn.

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

Follow Lennard on Twitter.

FILED UNDER: Bikes and Tech / MTB / News / Technical FAQ TAGS: / / /

Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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