1. Home » Bikes and Tech » Tech FAQ: Seat height vs. crank length; cassette swaps, 10-speed shifters

Tech FAQ: Seat height vs. crank length; cassette swaps, 10-speed shifters

First of all, I want to thank all of you who sent me words of encouragement, inspiration, and suggestions on my column from last week about the failed procedure on my heart. I am overwhelmed by the outpouring of support and the kind words many of you had about how my writing has made a difference for you over time. I really appreciate it and will never forget it. You are the best readers anyone could ever hope for.
― Lennard

Dear Lennard,
If I need to switch crank lengths from 170 to 180mm, do I adjust the fore and aft component or saddle height?
— Bill

Dear Bill,
Both. Primary is to ensure that the distance from the pedal to the top of the saddle stays the same. One way to do this while maintaining almost the same reach to the bars is to simply drop the saddle one centimeter. Your legs will reach the same extension at the bottom of the stroke, but your bar will now be a centimeter higher relative to your saddle than it was, and your pedal when at the 3 o’clock position will be a centimeter further forward relative to your knee. These things may or may not be something you care about.

If your setup is good and efficient currently, then in order to maintain that, you would probably want to do a combination of dropping your saddle and moving it forward. You might want to lengthen your stem as well. It might end up something like moving your saddle forward 8mm and dropping your saddle 4mm or something like that to end up with the same saddle-to-pedal distance as well as the same relationship of the knee over the forward pedal. The stem increasing by 1cm as well would keep your upper body at about the same reach to the bar. I think you likely will not want to drop your stem by the centimeter that your saddle dropped by because your knees will still be coming up just as high as they were, so your hamstring extension would stay the same.
― Lennard

Dear Lennard,
I’m not sure I agree with your point four for Murray’s question.

You say there would be too much slack if he switches between 32 and 36 cassettes.

Would there not be the same amount of slack if he downshifts from the 36 to the 32 cassette? As if he was at the top on the 32 teeth cassette? The derailleur can take up enough slack all the way down to the 11 cog, so this does not make sense to me.

I’ve been running an 11-36 on my race wheels and 11-42 on training wheels w/o issue. I do have a bashwich [a guide on either side of the chainring] to help keep the chain on the chainring, though. I leave the B screw in place, and it is fine though I am not super-picky as long as it runs well enough.
— Marc

Dear Marc,
I stand by that statement, although I could have clarified it better. What I wrote was:

“4. The best performance will be if you have the same cassette on both wheels. That’s because the chain will have to be considerably longer for the 11-36 than for the 11-32, and you will probably also have to crank down further on the b-screw with the 11-36 as well to avoid chain noise from the upper jockey wheel pinching the chain against the cog. If you then slap an 11-32 on there, there will be more chain slack than need be, and the upper jockey wheel will probably be constrained to stay further away from the cassette than is ideal, so shifting will be more sluggish. The potential for a jumped chain will increase with all of that extra chain slack having to be taken up by the jockey wheels.”

What I wrote is correct, although I could have specified “chain gap” more clearly. If you have a chain long enough to handle a 36T cog, then it is longer than what is required for a 32T largest cog. So when you’re in the 11, the derailleur has to take up more loose chain. While the rear derailleur may have enough range to take up all of that slack and not leave a loop hanging, it certainly will have less tension than on the same bike with a chain cut to the proper length for a maximum 32T cog. And less tension in the lower run of the chain means less chain retention.

As for the other point, if the b-screw is adjusted so that the chain runs quietly on the 36T cog, then the “chain gap” will be bigger on the smaller cogs than otherwise. In other words, the upper jockey wheel will be further away from the ten smallest cogs, or from all 11 cogs on an 11-32 cassette, than it would have been if your drivetrain had been optimized for an 11-32 cassette (i.e., if the chain were the proper length for the 32T maximum cog, and the b-screw were tuned to have minimal chain gap and no noise when riding on that cog).

In your case, you have a chain long enough to handle your 42T cog. That chain is longer than it needs to be for an 11-36 cassette. So when you put your 11-36 on your bike, your derailleur is taking up more chain slack than it would have to if your chain were cut to length for the 11-36. Thus, your chain retention is reduced.

And if your upper jockey wheel is not bumping along when on the 42T cog, going up and down from as each tooth rolls by, then your b-screw is tighter than it would need to be for a 36T largest cog, and your chain gap will be bigger throughout the range. Snappy shifts are produced by the chain being deflected laterally over the shortest length, so shifting is slower with a bigger chain gap.

This effect is obvious if you think about a derailleur being far below the cassette and you move it over one step so it lines up under the next cog. If the jockey wheel is too far below the cassette, this same lateral movement will result in too small a lateral chain angle for the chain to climb up or drop down to the next cog. A tight chain gap is what makes for snappy shifting.

I may be splitting hairs with your particular setup since you seem satisfied with its performance. But there is nonetheless some performance you’re leaving on the table when you put your 11-36 cassette on.
— Lennard

Dear Lennard,
I managed to crash my 10sp Shimano DA 7900 bike and in the process destroyed my right DA shifter. It seems hard to find these old 7900 shifters now, and if you do, they are very, very expensive. I was hoping I could replace this shifter with a lower end shifter instead if possible. Would Tiagra 4700, 105 5700, or Ultegra 6700, for example, work with DA7900 drivetrain? The bike was relegated to commuter after the crash so no longer need the lightest and best on this rig.
— Jonas

Dear Jonas,
Yes. I haven’t tried the Tiagras, but I’m sure the 105 or Ultegra lever will work.
― Lennard

Related Articles