Thursday, November 08, 2007

How To - Fitting a Bike in This Day and Age

It used to be that a serious road rider would have to go to a local shop to be fitted by the master ex-pro/racer. The guy would put you on a bike, put on different stems, wrench your foot here and there, push down on your forearms and back, and lift your chin. He'd grumble and measure and write, squeeze your calf, look at you suspiciously, and then grumble some more. Then he'd declare that he's fit you and you should come back in 8 weeks for your custom bike.

Okay so that's sort of mythical.

25 years ago it was a little less mythical, a little more awkward. One guy holds the bike up. Another guy does some stuff like ask you if you can see the hub or is the handlebar blocking the way. They eyeball your seat height, moving the seat up and down in somewhat random amounts, trying to hit "that spot". They have you pedal backwards (because you certainly can't pedal forwards while the bike is being held). After a few revolutions they drop a line from your knee which should bisect the pedal axle. They fiddle with the stem, putting different stems on the same bars (Cinelli 66 for big guys, 64 for smaller guys, 65 for guys who look like they should be playing football) and figure out which of the stems fit you best. The wrote all the numbers down and then you were done.

That's when Cyril Guimard, Greg LeMond, Bernard Hinault, and some other people (Bill Farrell, I think, who invented the FitKit) came into the picture. All of a sudden a fit included actually measuring someone (!). Inseam, torso, arm, shoulder, quad length, things like that. The FitKit was great because it included excellent tools for measuring, even if you didn't subscribe to their numbers. Fit became getting a good measurement and multiplying it by some ratio to get seat height or perhaps looking at a chart to get top tube and stem length.

A bonus was that by this time stationary trainers were common and the rider being fitted could pedal forward to "settle into his position". This made for more accurate fits as the rider wasn't perched abnormally forward or back on the seat.

The FitKit got upstaged by more sophisticated measuring devices - like the Serotta FitCycle, a totally adjustable stationary bike on which a "fitter" could adjust the different key dimensions (seat tube, top tube, stem, crank length) with virtual impunity. Typically the fitter would use the position derived from the various ratios as a base, adjusting for individual preferences and limitations.

This is all good for brick and mortar stores. Nowadays though many people buy their bikes online. How should they be fit? The question came up and I came up with some recommendations.

Ultimately, you are looking to get position the contact points correctly, with the pedals (really the bottom bracket) as the base. The contact points are the two pedals, the seat, and the bars (two hands usually). The contact points are determined in that order - that is critical. So critical that I'll put them in a list.

1. First figure out where the seat will go in relation to the cranks.
2. Second figure out where the bars will go in relation to the seat.

You should never, ever move the seat in order to adjust the distance to the bars. You always adjust the bars. If you can't, you have the wrong frame and need to start over again.

Since an improper fit might lead you to buying the wrong frame, I would definitely get your first "serious" bike fit at a shop that knows what it's doing. I don't have any recommendations on how to tell if they're good or not but personally I'd use the "gut instinct" method. Are they fit? Nice? Do they ride? Do they have a team? A local shop here is owned by a former pro and his (ex-)World Champion wife. They'll have experienced a lot and will know how to fit you, even if you have a bum knee or some other issue. My gut instinct told me that shop was good.

On the other hand, I've been to shops where my alarm bells started ringing as soon as someone opened their mouth. I rode to (and into) a huge, and I mean huge shop in California. Bikes were dripping down from the rafters, there were two open floors, and the floor was just a sea of road bikes. Impressive, even just at an inventory level. I figured I just walked into bike heaven.

Then one guy asked me if I race track. Track? He pointed at my criterium bend bars.

"Well aren't those track bars?"

I would NOT get fitted in a place like that. Luckily I was there to buy a frame pump.

As someone who's fit hundreds of cyclists, am very particular about my fit, and whose frames tend to be ordered sight unseen, I look for the following when checking frame fit on a sight-unseen frame:

1. Seat tube height - leg length related, simple. This can vary by a lot due to different types of frames. Ultimately you should choose your size on other factors - the only thing I'd worry about with seat tube height is "Can you straddle the frame without getting (as my mom used to say about the cats) neutralized?" I have frames that measure 44 cm center to center and 52 center to top. They all fit me correctly due to the next two measurements.

2. Top tube length, horizontal/virtual - this relates to your torso and arm length. It's affected by your cycling fluency - a more experienced or performance oriented rider will want a longer setup. A newer or less experienced rider will want shorter overall length. This is due to lower back strength, glute strength, and comfort sitting on a saddle. You should strive to keep the stem length away from the extremes (i.e. length) unless you are extremely experienced. In other words, if you're not really sure about your fit, you haven't been riding too long, and the only way a bike will fit you is if you put on a very long 14 cm stem, you probably need to go to the next size up. That size up will have a longer top tube.

3. Head tube length (height) - this is CRITICAL for proper bar height. I cannot say how important this is. It used to be a given with level top tube frames because the head tube always ended at same level as the top of the seat tube. With such a set up, for experienced riders you slam the stem down, for less experienced you raise it a bit. This has changed with the non-level top tube bike designs - the seat tube no longer determines the headtube length. The head tube can vary by 3 or 4 cm for a given seat tube size.

You can get a bike that has the right seat tube and top tube and the bars might be 2 or 3 cm too high or low, and without a radical stem it won't fit you. A shorter headtube is good for those more experienced/performance-oriented riders. A longer headtube is geared for less experienced or more comfort oriented riders. A good example is the difference between Giant's TCR (performance) line and their OCR (recreational) line. The OCR frames have longer headtubes for a given size but they are otherwise virtually identical.

The above three dimensions are critical when selecting a frame. To a lessor extent, I check the following when checking if a frame fits me:

1. Seat tube angle - based solely on quad length, but unless the angle is extreme (less than 72, more than 75), it's probably going to be fine. This is due to various seatposts which have or don't have setback, a measurement of how far behind the centerline of the seat tube the post holds the saddle.

2. Stand over height - make sure you can get off the bike without squealing.

Once you get the bike, you'll install the following parts. Based on your previous fit knowledge, you'll probably have the proper lengths already:

1. Stem - this adjusts the overall length of the bike for you. A newer rider should aim towards frames which use shorter stems - this way they can use longer stems as their body acclimates to cycling. An experienced rider will probably be using a 12-14cm stem, perhaps a 10cm for a shorter torso rider. This is because the experienced rider will not be making position changes as their body adjusts to cycling - all that's been done already.

2. Crank length - this is based on riding style/preferences but a longer/shorter crank will move your seat forward/backward and down/up respectively. 5mm is 5mm. It'll be unusual to need to change frames for this but realize it's a factor. Probably stem or bar changes will suffice.

3. Bar width, drop, and reach - all affect the overall length of the bike. I rarely change bars (I have the same bar on my bikes) but if you're getting a snazzy carbon bar, make sure it doesn't throw off your length too much. More width increases length slightly. Drop can be corrected using a stem (unless the adjustment is out of range). Reach is critical since a 2 cm longer reach will mean you need a 2 cm shorter top-tube/stem combo if you want to maintain the same position in the drops, a significant difference when selecting a frame. On the other hand, if you end up with a frame that's not quite right, a radical bar change can make a big difference towards correcting fit.

The following have absolutely nothing to do with fit but everything to do with how the bike handles, a totally different topic. If a shop claims these factors are involved in fitting the bike, nod your head politely, look at your watch, and say, "Hey, I just realized I have to go to the hospital for emergency surgery! Bye!" These factors simply help determine the bike's riding style.

1. Head tube angle - a 73 or so will give you a responsive bike that is still stable. A shorter bike (under 52 cm or equivalent size) will be very shallow to give the rider's foot more room to clear the front wheel. The bike will handle correspondingly slower. No way around it.

2. Chainstay length - shorter keeps more weight on the rear when standing. Longer is softer/comfy. If you're looking to do very aggressive riding, look for a short stay. 40.5 cm seems to be the "short stay" measurement for now - it gives some tire clearance and allows you to make big efforts out of the saddle without having the rear wheel skip around too much.

3. Wheelbase - reflects the above two factors, but basically a shorter one is more agile. A long one is better for comfort. Pros typically use 1 cm longer wheelbase bikes in Paris Roubaix, the cobbled and very bumpy classic in France. Most if not all of the wheelbase difference is made in the chainstay. This way the front end of the bike (where it's all fit to the racer) doesn't change.

A good fit will recommend:

1. An overall seat height (measured from the sole of the shoe to the top of the saddle). This means the following affect it: pedals, shoes, crank length, frame size, and saddle. This is virtually unaffected by anything out there - only individual preferences and physical disabilities factor in here.

2. An overall length - seat to bar, depends on how detailed they get. The bike is adjusted by using the saddle, top tube of frame, stem, bar, and height/angle of stem. The rider's experience level and riding goals affect the recommended length.

3. Crank arm length - I've decided this really depends on riding style and has virtually nothing to do with leg length. However a fitter may recommend a crank arm length to you. I'd keep an open mind on this topic.

4. Bar width - easy enough - about the width of your shoulders - but affected by riding style.
Based on the numbers and questions about your riding, the fitter will probably make some recommendations on bike and equipment.

Remember you only contact the bike in five spots - bars (twice), pedals (twice), and saddle. As long as those spots are in the right place, you can have anything between them, within reason. Once you know where those five points are, your job (and whoever else - LBS etc) is to figure out a way to connect the points in a logical and efficient way.

There is one more factor when choosing a frame. It has to do with bar to seat height - the vertical drop from the saddle to the bars.
With virtual top tubes, it's hard to tell what's right.

Dave Moulton (frame builder) has an interesting chart on his blog relating to this (and associated commentary):
http://davesbikeblog.blogspot.com/20...ebar-drop.html

His numbers seem pretty good. I have a 53.5 + 13 top tube (66.5 cm total) and his chart says about 8.1 cm drop. My primary Giant has about a 5 or 6 cm drop to the top of the bars, and I'm setting up the Cannondale with perhaps another 1-2 cm of drop. His numbers seem to be aggressive but pretty close.

I have shallow drop bars so the delta between the two positions is less than with a deep drop bar. This means riding the tops is still pretty low but the drops don't put me that much lower. I could get deeper drop bars to get a lower low position but my lack of fitness (i.e. my stomach) gets in the way. For now I'll have to lose weight and have a relatively high position.

If you have a lot of doubts, buying an adjustable stem helps a lot. My stem is a Ritchey 12cm adjustable and it's a great tool for experimenting with bar height. I think they're $60 from Excel.

Add seat to bar height in the list of things a good fit provides (it would be #5).

1 comment:

suitcaseofcourage said...

Amazing how much goes into it. I loved geometry in HS, but I'll always go to a pro for fit. I did the FitKit at a local shop last spring before I got into racing and had a tune up from a coach this past spring. Only thing he changed was flipping the stem to drop the bars a little for a more agressive position. Fortunately, everything else was done "right" (or at least the same as the coach would have done it). And it bears out - I can ride pretty long distances in comfort despite the bike having "race" geometry. Any pain comes more from me being out of shape (lower back, shoulders, hips) rather than the fit. Once I ride more, there's no pain.

Another great post - actually, this is more like a full-length magazine article that should be published!