Bikes & Tech
The red plate under the cleat measures force via...

Technical FAQ: Cleat position vs. seat height, plus SRAM/Shimano shifting compatibility

Lennard offers advice on cleat adjustment and saddle height, and explains the mystery behind those annoying "snake bite" flat tires

Have a question for Lennard? Please email us to be included in Technical FAQ.

Dear Lennard,

I’m 6’1”, and 225-pound road rider, using 175 mm Campagnolo cranks, Time pedals, and Sidi wire shoes. I’ve been intrigued by the articles regarding moving your cleats back as far as you can. My setup is fairly standard, right over the ball of my foot. My question is: The amount you move your cleats back, should you lower your saddle by that same measurement?

— Brent

Dear Brent,
I usually end up with a bit more than one half of the adjustment. So, if you move the cleat back 14mm, I lower the saddle 7mm. Then I check knee angles with the goniometer, and sometimes I have to go down a bit more, but often that works out fine.
 — Lennard

Dear Lennard,
Recently I had a puncture on an almost new tubular tire in a road race. I opened up the tubular to find the cause of the puncture, I found two snake bite holes in the tube. In hindsight, the tyre was probably under inflated before the puncture and happened after hitting a large pothole.

I understand the cause of pinch flat punctures in clincher and tubular tires, what I can’t understand is the circumstances that cause the two holes, and why are there normally two holes? I assumed the tube squeezes out of the edge of the tyre bead and rim, it forms a bubble and is burst by the compression between the tyre bead and rim. If this is the case how can you pinch flat a tubular, as the tube cannot get out of the casing, and there are no sharp edges to form two puncture holes?
— Emmet

Dear Emmet,
You’re mistaken about how “snakebite” (pinch flat) twin punctures happen on clincher inner tubes, which also leads to your misunderstanding of how they can happen on tubulars. The tube does not squirt out under the tire bead to cause the pinch flat. If that were to occur, the tube would explode dramatically; it would not just result in two tiny holes.

A pinch flat happens because, as the tire compresses on impact with a sharp object, the tire casing folds, pinching the fold in the tube from top and bottom between the rim, the tire casing above and below it, and the sharp-edged object.

A pinch flat can happen on a tubular in exactly the same way it happens on a clincher. For tires of the same size and at the same inflation, it is less likely to happen on the tubular for two reasons:

1. A tubular rim is more rounded on the edge than a clincher rim, and,

2. Expensive tubular tires have latex inner tubes, which are much harder to pinch flat than butyl inner tubes. To test this, as well as to see the mechanics of a snakebite puncture, try smacking a butyl inner tube and a latex inner tube with a hammer on a hard, flat surface. One hit, and you will usually see the twin punctures of a pinch flat on the butyl tube. The more elastic latex tube, by contrast, will generally take multiple hammer hits before developing a pinch flat. But since a latex tube also bleeds substantial percentage of its air pressure overnight (and bleeds way down over a couple of days between pumping), it is more likely to be ridden with low pressure and hence be susceptible to pinch flats.

I’m hoping this is clear. The folded nature of a pinch flat is not easy to describe in words and ensure that everyone gets the picture; here’s how the late, great Jobst Brandt describes it on the late, great Sheldon Brown’s website: “Snakebites, otherwise known as pinch flats, are so called because they usually cause adjacent punctures about 10 mm apart (for tires with about a 25 mm diameter cross section). They occur when the tire casing bottoms on the rim, causing a compression failure in the tube for both clinchers and tubulars, much like pinching the cheek with thumb and forefinger. The fingertips simulate the tire casing and the cheek the tube.”

Here’s a photo and video that should further clarify it.

If you hit a sharp edge like a curb, railroad track, rock or pothole at a high enough speed, even tires at reasonable pressure can be pinch flatted. The variables making pinch flats more likely on a given square-edged object are, in rough order of importance: high speed, high weight on the saddle, low inflation, small tire diameter, clincher rim, and thin, butyl inner tube.

— Lennard

Dear Lennard,
In order to maintain the spring tensions when I store my bikes for long periods of time, I keep the front derailleur on the small ring and the rear on the small cog. If I think of it, I use the quick adjusters to open up the brakes so these springs under less tension as well. I realize this is a little obsessive. I recently got my first electronic shifting system. Since these are servo actuated, is there a still a need to store them in certain gear combinations like cable actuated systems? Maintaining the batteries is a whole other can of worms.
— Andy

Dear Andy,
No; you can store a bike with electronic shifting in any gear; just make sure the derailleur is not between gears and trying to shift to the next gear while stored (i.e., don’t hit a shift button when hanging your bike up).
— Lennard

Dear Lennard,
I am currently running a Campagnolo 11-speed Super Record group. I am considering switching to a 12-speed Campy drive train. Will the 12-speed chain work with my current 11-speed crankset and chainrings?
— Bart

Dear Bart,
I haven’t tried it. Officially, there is no cross-compatibility. Campagnolo says that cobbled-together components like this would not have reliable front shifting.

— Lennard

Dear Lennard,
Can you tell me if a Road SRAM Red 10-speed shifter and 10-speed cassette will work with a SRAM Red 22 rear derailleur?
Would a Dura Ace 10- or 11-speed rear mech work with SRAM Red 10 shifters and SRAM 10-speed cassette?
I ask because I have several bikes and wheelsets which are all SRAM Red 10, and trying to find a new SRAM Red 10 rear mech with a short cage is proving very difficult indeed.
— Simon

Dear Simon,
Yes, a SRAM Red 10-speed shifter and 10-speed cassette will work with a SRAM Red 22 rear derailleur. Here is an article I wrote that relates to that. The main point is that the shift activation ratio is the same on 11-speed and 10-speed SRAM road derailleurs (approximately 1.3).

As for your second question, no, you cannot use a Shimano rear derailleur with a SRAM shifter. Sometimes you can get away with a Campagnolo 10-speed rear derailleur with a 10-speed SRAM shifter. Here is an article I wrote about that 11 years ago.
You can often find Campagnolo 10-speed rear derailleurs on eBay.
— Lennard
Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder and purveyor of non-custom bikes for big and tall riders. A former U.S. national team rider, co-author of The Haywire Heartand author of numerous cycling books including Zinn and the Art of Mountain Bike Maintenance available also on DVD as well as Zinn and the Art of Road Bike Maintenance, Zinn and the Art of Triathlon Bikes and Zinn’s Cycling Primer: Maintenance Tips and Skill Building for CyclistsZinn holds a bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to veloqna@comcast.net