Dave Moulton

Dave's Bike Blog

Award Winning Site

More pictures of my past work can be viewed in the Photo Gallery on the Owner's Registry. A link is in the navigation bar at the top

Bicycle Accident Lawyer






Powered by Squarespace
Search Dave's Bike Blog


 Watch Dave's hilarious Ass Song Video.

Or click here to go direct to YouTube.


A small donation or a purchase from the online store, (See above.) will help towards the upkeep of my blog and registry. No donation is too small.

Thank you.

Join the Registry

If you own a frame or bike built by Dave Moulton, email details to list it on the registry website at www.davemoultonregistry.com

Email (Contact Dave.)

 If you ask me a question in the comments section of old outdated article, you may not get an answer. Unless the article is current I may not even see it. Email me instead. Thanks Dave

« Hand Magic | Main | Remembering Tommy Godwin »


Often when out riding on the road or on the local bike path, someone will fly past me, tucked down on their aero bars. Invariably they will not speak.

It seems aero bars are only purchased by anti-social people, either that or after fitting these bars to a bike it gives one an extreme sense of superiority over lesser cyclists who don’t use them. But this is a whole different story.

The other thing I will often notice as this rider gradually pulls away to eventually disappear in the distance; his knees are splaying outwards as he pedals.

I find this kind of ridiculous, as any aerodynamic advantage gained from having the upper body in a low tucked position, is lost by the rider’s legs splayed outwards, therefore increasing the frontal area and catching large amounts of air.

The other point is to pedal efficiently legs need to pump straight up and down like two pistons. If the knees are splaying outwards the rider is not achieving maximum power.

The problem may be that the rider’s knees are striking his elbows at the top of the pedal stoke, and he is bringing his knees outward to clear. But more often it is a sign the rider is sitting too low, or too far back, or a combination of the two.

Often a cyclist will ride knees out even with regular bars; the problem is the same. There is a limit to how far your hip joint will rotate in its cup in the pelvis. To demonstrate this stand with your back to a wall and raise your knee forward and upward as far as it will go.

This is the limit of the hip joint’s rotation and the only way you can make the knee go higher is to move the thigh outward. You can also demonstrate this if you try to squat with your knees together, you will not get your butt very close to the ground; but squat with your knees apart and you can get much lower.

This is exactly what is happening when pedaling a bike. If your saddle height and/or the fore and aft saddle position is set where your hip joint reaches its limit at the top of the pedal stroke, your knees cannot help but splay outwards.

Also if you lower your back to achieve a more aero position, often you will rotate the pelvis forward, thus limiting the movement of your hip joint further. This is especially true if your spine is not flexible enough to get the back low without rotating the pelvis.

Don’t place too much importance on “Knee Over Pedal” (KOP) position, it is not that important. Look on it as a guide (A place to start.) rather than an absolute rule. The same with saddle height, this should be as high as is practical.

When the pedal is at the top of its stroke, (With your back in its lowest position.) you should be able to lift your foot a centimeter or so above the pedal, before your hip joint reaches its limit. In other words your hip joint should operate at slightly less than its limit at the top of each pedal stroke.

If your saddle is too high, your hips will be rocking as you pedal, and you will feel that you are stretching at the bottom of each pedal stroke. It is easy to feel if your saddle is too high, less easy to feel if it is too low.

This is why the person with the aero bars who passes me on the bike path does not know his saddle is too low; it feels fine to him. Whereas I can see immediately that his position is wrong.

Saddle height is not a constant thing; as you lose weight and gain fitness your saddle height can be reassessed and probably can go a tad higher. Always move in small increments and see if it is an improvement. (Feels better.)

Rather than follow exact formulas when setting up your position; think of what you are trying to achieve and adjust the bike to reach that end. If your back needs to be low for aerodynamic reasons, then realize that this will affect your pedaling action.

To achieve one at the expense of the other is defeating the object.



Reader Comments (9)

A helpful tutorial. Nice heuristic on saddle height ("...be able to lift your foot a centimeter...") I'll have to check this. No areo bars for me, but I imagine there is a similar issue when in the drops. Thanks!

I have seen that "knees out" position for riders on a variety of bikes; road and "fitness." I guess I hadn't attributed it to the seat being too low, although your explanation makes perfect sense as to why that would be the case.

May I say a word about aero bars on a multi-use trail? Why?!?!? Lots of blind corners on the trail I ride. Lots of dog walkers (using flexi-leads). Little kids with somewhat older sibs on foot and on small bikes. Elderly walkers who step to the left as you call "on your left." I know little about areo-bars but I have to imagine that steering, braking, reaction times are compromised. And, at speed?

November 26, 2012 | Unregistered Commenterbystander

Great article! For some reason, the most reasonable, logical approach is the one least found. I really enjoy your articles on bike fit and geometry subjects! Thanks for another good one!

November 26, 2012 | Unregistered CommenterSJX426

The biggest laff that I have when been passed by riders in the 'TUCK' position, is watching them TRY to take corners. Often I am passed like you by these super men and mostly WOMEN I just wait for the next corner and there they are right in front of me.Another type of rider that I have problems with is WOMEN usually who have the saddle so high they have to pedal with there TOES! I get passed everyday by a young? gal who rides on the drops fighting her way around bends and pumping the pedals with her toes NO ankle drop at all. I let her go and then get into MY tuck with the hands in the center of the bars drop my elbows which lowers my back and pedal right past her. She of course gets out of the saddle and humps to try past me again. GOOD LUCK!

November 26, 2012 | Unregistered CommenterJohn Crump

Another chap that passes me on the path is a a chap as you discribe KNEES OUT MRS BROWN, Now I know him and know the reason he rides this way, He is in his late 40s VERY muscular is a weight lifter and runs a local health club.He use to have a mountain bike that of course I could leave him in the dust on BUT he got a road bike on Ebay he told me, Well he took the chain rings 52X42 off and got a I think about a 60tooth big ring, NOW he goes like the clappers, It is like watching someone riding in slow motion, He must only be doing 50rpm BUT with his untold power he avgs about 30mph! WITH HIS KNEES OUT!. I didnt see him for a few weeks and one day he was running on the path, I asked why no bike to day Dave? He said "My docs says I have to give my KNEES a rest!" Go figure that one out!

November 26, 2012 | Unregistered CommenterJohn Crump

I know I'm riding well - uphill or on flats - when I can look down and see my knees popping straight up toward my face in a nice rhythm.
On the mountain bike, it is a little different. At times, when I come upon a very steep (30% or better) and sudden climb I kick my heels out away from the bike a degree or two. This seems to deliver more torque to the pedals for the few strokes I need to get over the hump.

November 26, 2012 | Unregistered Commenterskylab

Excellent post! I was actually contemplating this same exact thing this weekend while riding.

Another factor to consider is that the range of motion in the hip joint is functional as well as anatomical (so to speak). Meaning: one's degree of flexibility / tightness in the hip flexors, glutes, and hamstrings will also affect their ability to keep knees relatively in-line with the feet when riding a high(er) seat-to-bar drop and/or stretched out aero position. For example, when you lie on your back, try pulling your knee to your chest with lower leg relaxed, you will feel a stretch. If you do the same, and extend you lower leg as you go, you will feel more of a stretch. If you are placing yourself in an on-bike position that exceeds your functional flexibility, you will have to compensate in other ways to make it work, e.g. rotating the hip & knees outward.

Aside from potentially losing power from riding in a 'knees splayed out' position, a bigger issue may be the lateral stress that it places on the knee, which unlike the hip or ankle, is a joint designed primarily for two-dimensional 'piston-like' movement, as you said.

November 26, 2012 | Unregistered CommenterT

About a year ago I had my right hip min evasive surgery done, For quite a while I had trouble getting down on the drops, I worked at it and after a month or so all was normal again. I have talked to other riders from the dark ages that ONLY rode on fixed wheel with clips and straps and they all have had some hip problems, Plus of course the knee problems, So in my mind the clipless pedals of to day are a god send to us old ems that still do ride a lot.

November 27, 2012 | Unregistered Commenterjohn Crump

Another thoughtful post dave,

I often chuckle to myself when I see the folks on aero bars as more often than not their position often produces diminishing returns by only using a few of the muscles available while not being able to effectively use hamstrings, glutes, lower back and heel and toeing the pedals round. More often than not they don't know or understand when to come out from the aero tuck to use a more conventional position, E.G. being able to slide back in the saddle when climbing when the speed drops to engage other muscles, open up the chest to increase oxygen intake, pull on the bars and ride with increased stability. Usually not much point in mentioning it to these folks as they just don't get it, but know every dotted I and crossed T of the latest go faster goodie.

My positioning is fairly conventional with the knee back from the pedal axis to help when climbing, and a tad higher than would be considered optimal. When a position is sorted and documented it's uncanny how close you can get if you adjust it on the road with no tape measure, just feel and allen keys.

November 27, 2012 | Unregistered CommenterKeith - British Columbia

I talked to my son Paul about the photo and the aero bars, He is Cat3 racer and when not at sea flying on and off a aircraft carrier rides a lot of time trials, He told me that in a 40k (25M) TT Aero bars CAN make a difference of TWO mins in the time time done! Think back DAVE to our days racing TWO MINS in a 25TT! Of course everyone else would have have the same adantage.

November 28, 2012 | Unregistered Commenterjohn Crump
Comments for this entry have been disabled. Additional comments may not be added to this entry at this time.