Seat height -vs- crank length

[Mikefule]

I got into unicycling many years ago but from a road-bicycling background. I have always set my saddle height according to the road cyclist’s rule of thumb: sit on the saddle with your leg comfortably straight. If you can rest your heel on the pedal at the bottom of its travel, the seat height is about right.

However, this rule was developed for bikes, where the standard crank length is around 170mm, and where more torque is needed because of the gearing.

I imagine that 170 mm is approximately the most efficient crank length for an average-sized adult male rider on a bicycle. I have no idea how much research has been done, but if there were a significant gain to be made with shorter or longer cranks, the market standard would have developed accordingly.

I’ve always preferred riding my 36 on 150s. I have found in the past that putting the pedals in the “short holes” makes the uni rideable, but hard work.

The other day, I swapped the pedals to the short holes but did not bother to raise the saddle. This meant that ”” according to the road bicycle rule ”” the seat was too low because the pedal at the bottom of its travel was now about 25mm nearer to the seat. To my surprise, it was easier to ride, and seemed disproportionately faster.

I’ve not ridden with many other unicyclists, and I’ve always ploughed my own furrow (US = “plowed”) but I wonder if I’ve missed a trick here.

For those of you who mainly ride distance, rather than technical trails, what “rule” do you apply to adjusting seat height when you put different cranks on? If you reduce your cranks by 25 mm, do you leave the seat the same, or raise it by 25 mm, or somewhere in between? What if the change in crank length is greater than that?

Thanks

[toutestbon]

@Mikefule

For those of you who mainly ride distance, rather than technical trails, what “rule” do you apply to adjusting seat height when you put different cranks on? If you reduce your cranks by 25 mm, do you leave the seat the same, or raise it by 25 mm, or somewhere in between? What if the change in crank length is greater than that?

For years, I applied the same rule you did. But in the last few months, I started to get the saddle a little lower. It helps me a lot for off-roading or to pedal faster on a gearless unicycle.
A higher position on the saddle gives you more strength, which is interesting on a geared unicycle or to climb a road pass.

My new rule :
High position = more strength (schlumpf and road climb)
Low position = more velocity and less chance of UPD in off-road

Between the high and low position, for me there is less than 2 cm (I still find it difficult to go very low)

[Mikefule]

@toutestbon

For years, I applied the same rule you did. But in the last few months, I started to get the saddle a little lower. It helps me a lot for off-roading or to pedal faster on a gearless unicycle.
A higher position on the saddle gives you more strength, which is interesting on a geared unicycle or to climb a road pass.

This is where I have my doubts, depending on the crank length.

When you pedal, your feet move in a circle (obvs) and there are about 4 key points: [LIST=1]

The highest point. With long cranks, having your seat too low means bringing your knees up further, or bending them more, which may be less efficient.

The lowest point. If your seat is too high, then your leg is at full stretch at the bottom of the pedal stroke.

The distance from the seat to the hub. If, for the sake of argument, you never changed this, then half way through the down stroke, your leg would be in pretty much the same position, regardless of crank length. How important is this part of the pedal stroke when considering power output? I suspect it is very important.

The distance forward and back that your feet go, depending solely on crank length. This is not affected by seat height. [/LIST] It is clear to me ”” I have known for many years ”” that with shorter cranks, you are using a smaller range of movement of your leg muscles and knee joints. Take to absurd extremes, think wht your legs would be doing with 300 mm cranks or 10 mm cranks. My point, therefore, is which part of the pedal stroke is the key to efficient and powerful pedalling? Does this change with crak length?

Over more than 30 years of riding, on sizes from 20 inch to 36 inch, I have used cranks from 75mm up to 170mm and have generally set the saddle height according to the “road bike rule” less about 10mm for very short cranks. I now wonder whether I should make any adjustment to the saddle at all when I change cranks. Is the key distance that middel part of the range which is dictated by seat to hub distance?

What got me onto this was reading a fixed/single speed forum. I have 2 bikes (2 x 10 gravel bike and a fixed gear single speed) and I was reading that many fixed riders lower their seat a bit to give them more control when applying back pressure to the pedals. This is what inspired me to try the 26 on the shorter holes (around 125mm) without raising the seat to compensate, and I was astounded by the difference it made.

[MrImpossible]

I doubt that there’s a “right” answer. When I watched pro bike races on TV I was always astounded at the range of seat heights (and general riding positions) that the peloton used. And those were professional riders, all doing the same ride, on basically similar bikes.

I personally find a slightly lower seat better for spinning along at high cadence and low pedal force, and a higher seat better for slow cadence and a lot of leg force. I set my fixed gear bike up for spinning with shorter cranks (155m) and a slightly lower seat, and just ride standing up for the slow, high force climbing stretches.

My geared bikes have the saddles a little higher, because I can keep my cadence and forces more in the middle range.

My bigger wheel unicycles are mostly set up like my fixed gear bike, with a little bit lower seat for spinning along, with the expectation that I’ll be standing for the slower stuff.

YMMV. Try different positions and see how it goes. Watch out for knee pain if you try something extreme. Bear in mind that the bike people have been at this for a long time, and know what they are doing…

[Mikefule]

@MrImpossible

I Bear in mind that the bike people have been at this for a long time, and know what they are doing…

I’ve been at this for a long time and I thought I knew what I was doing.

[JimT]

My saddle is lower that what a normal bike recommendation would be. It works for me and makes sense. The higher the saddle is in relation to the pedals the more leverage you have but a slower cadence can be maintained easily. A high cadence on a bike may be 90 rpm but a normal cadence on a uni may be closer to 120 rpm. The higher the saddle is in relation to the pedals the more the knees have to flex and that restricts higher speeds.

[Setonix]

I also learned to use your road bike rule when just riding. However some months ago, I decided to saw 1 cm off the saddle pin of the 36" and it became much easier to mount and handling became much easier. With unicycles it depends on what you are doing. For freestyle a higher seat is required to more easily keep the balance in the center and do things like pirouettes and possibly for wheel walking a higher seat is nicer, because your legs will bend much more. For municycling a lower seat height is recommended so you can apply more pressure and have some free space for bumps and pits. With the freestyle seat hight, a bump will prolly break your jewels ^_^. For road riding I would put the seat slightly higher, but keep my legs bendy enough to get that extra stability and a bigger balance area. As for crank lengths,

I’ve decided now after 5 years, that 150mm is the most comfy length for the 26" and up. Then sure I might not got as fast as other unicyclists, but 150mm is nice for going uphill and mounting and having all unis at the same crank length, then they all more or less mount the same.

Maybe for different people there is a different sweet spot, which mostly is just below the road bike rule.

[johnfoss]

Seat height for me:
Like you, I use the basic bike rule, which was also what Bill Jenack recommended for beginners, and can be found in many respected sources for learning to ride. That gets you a baseline for riding on a smooth, flat surface. Great for learning to ride, Freestyle and some other activities. Not necessarily the best for other types of riding. Another reason for using the “bike” or “Jenack” method is to educate beginner unicyclists to not have the seat miles too low. Higher seat gives you more power, which equals more time before your legs die, when learning. From this baseline setting you then adjust for other types of riding, and in almost all cases this would be by lowering it.

Street and Trials-type riding are the most obvious example; get the seat way low, to get it out of the way so you can compress your legs a lot. And you won’t necessarily be sitting on it that much compared to other types of riding. Many types of riding require a lowered seat so you don’t get your feet bounced off the pedals. A little bit for Road riding, more for rough road riding, and a lot for rough trails. I don’t measure, but I’d guess it’s a minimum of 1" for Muni, and 1/2" for Road. I’m 6’ tall.

When riding Muni, I also sometimes change the seat height during the ride. Long climbs are greatly aided by raising the seat. One of my favorite trails starts with a flat-ish section, followed by a couple of miles of uphill on dirt, followed by pavement, followed by all dirt, leading to the fast and technical ride back down. So I raise the seat at the beginning of the ride, and lower it back before starting the big downhill.

For Road, which is the type of riding I do the most, I would not want to have my seat all the way up either. So it’s a bit lower. Those measurements above are guesses; I’m not sure how much it really is. I generally do it by feel. My Road uni rarely gets its seat height changed (because I rarely change away from the 150 holes), so it’s usually right where I want it. But for Muni, I will often stop and adjust the height during a ride if it’s feeling a little high or low for the terrain.

@Mikefule

• The distance from the seat to the hub. If, for the sake of argument, you never changed this, then half way through the down stroke, your leg would be in pretty much the same position, regardless of crank length. How important is this part of the pedal stroke when considering power output? I suspect it is very important.

Sounds important to me as well. If anything, the “peak power angle” may change slightly with different crank sizes, but I imagine not very much. Small changes in seat height probably don’t have a large impact on that angle.

…I was reading that many fixed riders lower their seat a bit to give them more control when applying back pressure to the pedals. This is what inspired me to try the 26 on the shorter holes (around 125mm) without raising the seat to compensate, and I was astounded by the difference it made.

That’s fascinating. Having ridden some major descents on no-brake Munis, I’m very familiar with the need for back pressure, and how it can really wear you down. But at the seat heights I usually ride, I can’t see that making it any lower would give me more power; it feels like my power is compromised for that anyway. Maybe my seat is already low enough to do that, or I’ve got more to learn… Got to try it!

[pierrox]

What’s interesting about bikes and saddle height is how the “bike fitters” find the starting point: they measure your inseam and multiply it by a factor (which depends on which “school” of bikefit they come from, but is usually around 0.85) and that gives you the distance from the top of the saddle to the center of the crankset. Not from saddle to bottom pedal. Now ok, bikes cranks don’t vary in length as much as our uni cranks (170mm is the norm, 165 on track bike but now 175 for taller riders and 160 for people who prefer spinning are becoming popular). Having said that, it’s a similar idea to what is discussed here: the center of the circle drawn by the feet is what matters, not the bottom of the circle. And if cranks are shorter then the leg won’t extend as much on the downstroke.

[JimT]

The preferred method of adjusting the saddle height on bikes now seems to be the knee angle at the bottom of the stroke. The most common recommended angle for bikes is 25 to 35°. My unicycle measured knee angle on flat road with pedal near the ball of the foot is about 50°. On a 12% up grade with pedal near the foot arch, pulling up on the saddle and butt off the saddle is about 15°.

This makes sense. On bike the cadence and torque are close to the same because of the available gearing. On a uni on level road at speed the cadence is fast but the torque low so a large knee angle works. On hills with a uni the cadence is low and torque high so a small knee angle works. If uni’s had 10 or 15 gears maybe the recommended knee angle would be similar to a bike.