Ask the Experts: John Cobb - Part I

John Cobb, bike fit and aerodynamics expert, recently took the time to answer Endurance Corner team member questions. We'll share his responses over the coming weeks as part of a series. Up first: questions and answers about bike fit.

EC: Which method do you use to measure seat angle?
John Cobb: I measure seat angle from the crank center to the middle of the saddle.

EC: Can you briefly walk us through a bike fit in order of importance?
John Cobb: When fitting a rider to the bike you have to consider several things:

The rider's mileage
The rider's racing goals
The general fitness level of the rider

Most people run their seat heights too high, there is no power loss from a slightly lower seat but a big comfort gain. Hand and shoulder weight is very important to consider. Many riders come in and complain about numb hands or sore shoulders, raising the bars is almost always the wrong direction for this, lower the bars for more comfort. The goal for a good fit is to be sure that your hamstring group is not trying to contract at the same time as the quadriceps group. You can spot this by watching the activity in the calf muscles and the hip flexors, very small changes of 2-3mm in seat fore aft and seat height can really effect these muscles. You have to consider your feet, are your toes numb or do you have hot spots, you have to fix these things. You can’t get aero if you are not comfortable. Three to six hours on the bike can be a very, very, long time if you are not comfortable. Work on that area first then worry about aero changes.

EC: What are the main points of adjustment on the bike that lead to a good aero position?
John Cobb: During a wind tunnel session we usually already had the rider work out their best power position with a power meter and several miles or hours on the trainer. From that point, we look at elbow width and height, hand width and height and knee positions. It is very easy to go too low with the bars and pick up a lot of extra frontal area from the shoulder blades. Hand width is not as critical but clothing is a huge area to spend some time and energy on. Using the upper arms to mask the riders thighs is good. On women, selecting the right race top can completely “make or break” a position choice.

EC: Once you have estabilshed a decent fit, how does the average Joe fine tune adjustments going forward?
John Cobb: If you have a bike fit that you are comfortable with and it feels reasonably powerful, leave it alone. After several hundred miles, if an area hurts or goes numb, discuss that with the fitter but otherwise just ride it. Some of these things take some time to get used to. A good resource is a video camera and YouTube -- get someone to film you and then look at it. Do you look fluid or fast? Remember the mental picture you have stored and shoot for that.

EC: When adjusting fit, should one expect an adaptation period of discomfort?
John Cobb: I think that most fit changes should be adapted to within 30-60 miles. For really extreme changes it might take three or four rides. You should have that “new muscle” feel but no pains.

EC: Are there perfomance indications -- mainly w/kg versus speed -- that can tell you your position is more comfort/power based than aero?

EC: When is it appropriate to consider changing your position?
John Cobb: I find that many faster riders seem to plateau or level out in their performance after a couple of seasons. Changing your position will awaken new muscles by changing their contraction points and angles. This usually leads to a gain in speed that will need to be revisited again in a couple of years.

EC: What areas of position offer the most likely chance of improving aerodynamics?
John Cobb: Elbow width is a good safe area to begin with along with knee width. Generally it’s hard to go too narrow and for triathletes it’s easy to tell when you are getting too narrow. Too narrow will bother your breathing and kill your shoulders after a swim but you should still try to go as narrow as possible. Keep looking at your clothing, any wrinkle or loose fitting areas can cause problems.

John Cobb is a world leader in cycling aerodynamics and bike fit. He's based in Tyler, Texas and can be reached via his website (www.CobbCycling.com).