1. Home / Bikes and Tech / Technical FAQ: Crank length, saddle position, squeaky brakes

Technical FAQ: Crank length, saddle position, squeaky brakes

Feedback on seat position and crank length

Dear Lennard,
I saw your answer to Bill about changing his crank from a 170mm to a 180mm. Is there a way to be more exact on how to get your seat position correct when changing crank length?
— Tim

Dear Tim,
To maintain the same pedaling position when switching from 170mm to 180mm, ensure that the seat height from the bottom bracket becomes 1cm shorter, and the saddle setback relative to the bottom bracket also becomes 1cm shorter.

I did four drawings of the same bike to illustrate this (click each link for more):

The first one has 170mm cranks.

The second one has 180mm cranks and has the saddle lowered by 1cm relative to the bottom bracket (so the seat height from the pedal is the same as with the 170mm crank).

The third one also has 180mm cranks and has the saddle moved down and forward so that the seat height is 1cm lower relative to the bottom bracket and the saddle fore-aft relative to the bottom bracket is 1cm further forward than with the 170mm crank (so the seat height from the pedal is the same as with the 170mm crank, and the seat fore-aft position relative to the forward pedal at the 3 o’clock position is the same as with the 170mm crank).

The fourth one has a 10mm longer stem as well as the same changes as the third drawing: 180mm cranks with the saddle moved down and forward so that the seat height is 1cm lower relative to the bottom bracket and the saddle fore-aft relative to the bottom bracket is 1cm further forward than with the 170mm crank (so the seat height from the pedal is the same as with the 170mm crank, and the seat fore-aft position relative to the forward pedal at the 3 o’clock position is the same as with the 170mm crank).

You will notice that the horizontal distance from saddle to bar is approximately the same on the first and fourth drawings, along with the saddle being at the same distance from the bottom pedal and at the same horizontal distance behind the forward pedal in both drawings. The main difference besides the crank length is that the handlebar is about 12mm higher relative to the saddle on the fourth drawing than on the first one. Again, as I said in the previous column about this, my recommendation would be to leave it this way or perhaps take out a 2mm spacer so that the bar with the 180mm cranks is 10mm higher relative to the saddle than with the 170mm cranks, to accommodate the fact that the knee will come up 10mm higher above the bottom bracket with the 180mm crank than it will with the 170mm crank.

As for the exact answer to your question, you can see how much the seatpost has slid down (208.3 – 199.7 = 8.6mm); admittedly, 8.6mm is quite far off from my guess in the previous column’s answer of 4mm. I, unfortunately, can’t display on the BikeCAD figure how much the saddle has slid forward on its rails, but I can tell you that I had to move it forward 7.9mm, and that is right on with my guess in the previous answer of 8mm.

These numbers are dependent on seat tube angle since the amount the saddle moves forward or back with a given movement in or out of the seatpost depends on that angle. This bike in this drawing has a 76-degree seat angle; if it had a more standard 73-degree angle, the saddle would instead have needed to come down 8.3mm and go forward 7.6mm in order to achieve the same saddle height from the bottom pedal and the same knee position over the forward pedal. The shallower the seat angle, the less the seatpost needs to slide down and the less the saddle needs to slide forward to get the same saddle position relative to a longer crank.

Another thing you may have noticed from this exercise is that stack and reach of a frame cannot be compared properly without taking crank length into consideration. Stack and reach, which are the Y- and X-coordinates, respectively, of a point on the frame or a part of the bike relative to the center of the bottom bracket, have made comparing bike-frame dimensions in this day and age of sloping top tubes, extended head tubes, and oversized and non-tubular frame members much more accurate (since the length of the seat tube to the center of the top tube and the actual length of the top tube are meaningless as frame dimensions in terms of fit unless the top tube has a fixed diameter, is level, and meets the head tube only a few millimeters below its top).

You will notice that the stack and reach of the frame (measured to the top of the head tube) have stayed constant at 530.3mm and 422.6mm. However, the stack and reach of the center of the saddle (687.1mm/-170.8mm) and stack and reach of the center of the handlebar (605.4mm/502.8mm) have changed (to 679.2mm/-160.8mm and 609.5mm/511.9mm, respectively, and we would ideally want to drop the stack of the stem down to 607.5mm, as I mentioned above) when going from 170mm cranks to 180mm cranks and increasing the stem length by 10mm. So this means that you have to make a similar accounting if you are comparing the stack and reach of the frames of two bikes if the bikes have different crank lengths; otherwise, you will be comparing apples to oranges.

― Lennard

Dear Lennard,
You are correct that you can replace 7900 DA shifter with 5700 (105) and 6700 (Ultegra), but you cannot use 4700 (Tiagra) on the 10-speed setup because Tiagra 10 speed 4700 series shifter is indexed and the cassette is spaced the same as new Shimano 11-speed road series (5800, 6800, etc.), but Shimano only reduced the cassette to 10 gears at the back for the Tiagra 4700.
— Ammiel

Thanks, Ammiel.
― Lennard

Follow up on switching brake pads

Dear Lennard,
In your recent response to an inquiry about switching carbon pads based upon the manufacturer’s recommendation:

Dear Lennard,

I have several wheelsets I like to use and switch around. When switching between carbon and aluminum wheels, I change the brake pads out for the specific ones for each wheelset, but when switching from one carbon rim to another, do you really need to switch the pads? Pretty annoying to have to remember. I have a pair of Reynolds tubulars I use with Reynolds blue pads, then some Rolf carbons that I use the Rolf pads for and some Enves with the Enve pads. Can I just use the same pads for any carbon wheel or should I keep switching them out every time I change wheels? This is when discs sure have an advantage for sure.



I own a set of Enve 7.8 rims with the textured brake track. Enve supplies a special formula brake pad specific to this textured surface, different than previous wheels. In conversations with Enve, they won’t recommend another pad and will not warranty the rim if another pad is used.

But this rim/pad combination is driving me nuts with the brake squealing with almost every use! I’ve messed with the angle of the pads, changed to new Enve pads, etc., and the squeal always returns shortly after replacement or adjustment.

Any suggestions?
— Tim

Dear Tim,
If you want to maintain your Enve warranty and don’t want to have the squeal anymore, there is really only one thing to try: a different set of brakes. I would say that the chance of success at eliminating the squeal is low, but if you are sticking with those pads, that’s all you can do. The squeal is related to vibration, which those big, deep rims would amplify, so stiffer brakes might actually work.

If I were to attempt this, the only brakes I think might possibly make a difference would be Cane Creek eeBrakes, designed by cycling genius Craig Edwards. Trek followed this design with its Bontrager Speed Stop Pro brakes, but those only work on direct-mount frames and forks; if you already have those, there is probably little else you can do with the calipers themselves. The reason Edwards’s design might have a possibility of working is because it’s not a layered brake like every other road rim brake. All standard road brake arms have to counteract the twist created by having one arm laid over the other arm. But the eeBrake’s arms are both on the same plane and, therefore, have no twist to counteract. They also have large-diameter, hollow pivots to further reduce flex. Of course, since they have so much more stiffness, they can get away with making each caliper half the weight of other manufacturers’ brake calipers, so they may give back much of the stiffness in the interest of weight loss. Still, it might be worth a try.

Otherwise, your options are:
— Ignore the squeal.
— Get different wheels.
— Follow my January 30 recommendations and try different brake pads, wheel warranty be damned. Obviously, as I said in that column, first ensure that the pads will stop you and won’t get rapidly ground down under hard braking.
― Lennard

Related Articles