Yes Mark, you are right- the same POSITION can be acheived, but not the same FIT. The dimensions of the body contact points will be the same but the weight distribution, turning radius, wheelbase (location of contact patches), center of gravity, steering lever, level of responsiveness (and attendant stability)will all be different. FIT and POSITION are tow different things. I can get you a good POSITION on a bike that doesn't FIT, but it will, at best, handle poorly. When you get a good fit and position there is this marvelous synergy that takes place. You need both the right frame FIT and the right body POSITION to acheive that.Interestingly, that is why we have broken our fit and position bike buying process into two segments, first frame fit and selection- then rider positioning. Works for us......

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Tom Demerly
The Tri Shop.com

So, what you're saying is that the frame fit is established using the basic anatomical measurements, i.e femur length, inseam, etc., then the process of creating the proper position begins? Are there cases where finding a comfortable and powerful position require changing the frame size? It seems to me, that finding the proper fit (frame and position) is a series of compromises. As I envision it, a frame is designed with a neutral setup in mind, in other words, this bike is designed to accept a stem of xx length and seat of xx height to achieve designed efficiency. How far in left field am I?

Mark

Mark, I wish I had a an illustration handy to break this down to the lowest common denominator. Any given bike performs best with a certain distribution of weight and rider leverage. No matter where and how well a rider is positioned he will either be in or out of this "sweet spot." A bike that FITS allows the rider to get in his/her best position AND be in the "sweet spot." The multitide of variables that determine a rider's best position don't allow for a simple "make me fit on XXXXX bike" purchase decision. You might be dead set on a P3, but fit better on, say, a Litespeed (Me for instance). That's why we need guys like Tom.

Alright, I understand where both of you are coming from. Now the next question (there's always another question), which of the anatomical measurements equate to frame size? Is it primarily height and inseam length? It seems to me that the relationship of torso to lower body would be the primary factors in determining a frame size, is this too simplistic?

Mark

I use all the anatomical measurements along with the type of riding the customer is going to do along with my knowledge and expereince with a given bike to help determine the optimal frame. Every measurement is relevant, even shoe size.

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Tom Demerly
The Tri Shop.com

To my mind, static measurements are a good start but don't tell everything. Two people could have exact measurements but if one is considerably more flexible than another the bike fit will be considerably different. The more flexible person can ride a slightly longer stem or could ride with the handlebars lowered for example. The more flexible person could ride in a more aggressive position if on a tri bike. I find as the season progresses my ability to ride in more flexible positions means that I am constantly making minor fit adjustments.

An example is Giant TCR/OCR. Good road bike IMO but only offered in three frame sizes. Giant claims that by utilizing different stems and seat posts these bikes can fit anyone but then some of their Team Once riders get custom TCR frames because they don't fit the regular three sizes. My TCR seems to fit me fine but there must be a certain % of riders who don't fit one of the three sizes quite perfectly. When I adjust my own bikes if there is pain while riding then there is something wrong in the fit. Whether or not my own fitting is 100% optimum, I don't know. The only way to find out is to go to the wind tunnel and then also test with power meter to measure watts in different positions. Unfortunately this equipment is not readily available to the average age grouper.

By trying to lock in on frame size you've compounded your problem exponentially. No two bike manufacturers build bikes exactly the same way. One company's 53 is another's 55 is another's 51. In other words, a 54 is a 54 is a 54 IS NEVER THE CASE! If it sounds like I want you to consult an expert, I do. Either that or take 10 years and about 20 bikes and you will probably eventually stumble on the right bike. I don't have that kind of patience, let alone $$$$. Be wary of anyone who tells you that they can measure one or two things and fit you on a bike.

These are all good points, I hope I'm not coming off as abrupt or argumentative, I respect the abilities and knowledge of most of the folks on this forum. I'm just trying to get a better handle on "optimizing" bike fit. It seems to me that when most folks buy a bike, they do so from static measurements and what feels good at the time or the 15 minutes they spent riding around the parking lot. That being said, it makes sense to me that one could develop a chart that given certain anatomical measurements, equates to a certain makes and sizes of bikes. Essentially giving the salesperson/bike fitter a good starting point and the buyer a reasonable idea of what types of frame geometry they fit.

Mark

I agree almost 100%, except.....

Lets assume that the customer has not predetermined what they need, and require an expert's help. Ideally, shouldn't position come first, then frame selection? ie, interrogate and measure customer, figure out the necessary cockpit dimensions (in rough), set up on a size cycle or similar apparatus, fine tune the cockpit dimensions based on customer feedback, then look through the various geometries available to determine a shortlist of compatible bikes for that rider position. Or am I missing something here??

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Swimming Workout of the Day:

Favourite Swim Sets:

2020 National Masters Champion - M50-54 - 50m Butterfly

This almost exactly describes our process. Match rider to bike and establish position.

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Tom Demerly
The Tri Shop.com

One last question: Other than the fit article on Slowtwitch and Tom's site, what other resources are available on the net regarding fit. I think if I can raise my knowledge of fit, then when I eventually go for a professional fitting I will be able to communicate with them more effectively.

Mark

Now you're entering even deeper problem area. Static measurements alone can't determine the best bike for you, even if we had someone produce such a gargantuan table of bike sizes/fit. That multitude of variables that goes into your BEST position includes such things as your flexibility in several areas, as well as your tendon insertion points. Tendon insertion points? Yes. Slight genetic differences in these determine where you get your power from and ultimately how powerful you can be. In other words they determine your best point of leverage for any given joint. You might better leverage the cranks in the forward position. Then again you might be best in a slack roadie position. Most of us fall somewhere between those two extremes. I've left out a few other of these physiological variables, but you get the idea about how difficult it becomes to base fit recommendations on plugging variables into a spreadsheet.

"One last question: Other than the fit article on Slowtwitch and Tom's site, what other resources are available on the net regarding fit."



There is actually quite a lot of good info about bike fit on the net. I don't have the sites bookmarked anymore but do a goggle search and you'll come up with lots.

Unfortunately, there doesn't seem to be a lot of sites concerning the fit of tri bikes. The articles by Tom and Dan are probably the best that I've found so far.