Translating fit specs to bike specs
Written by: Dan Empfield
Date: Thu Sep 13 2007
1. F.I.S.T. axioms
2. F.I.S.T. protocol
3. Measuring conventions
4. Seat height
5. Cockpit length
6. Hip angle
7. Armrest drop
8. Tools of the trade
9. Your bike's "waistline"
10. Translating fit specs to bike specs <-- You are here
You should've read our article on Your bike's waistline prior to this, if you're following in order. Our series begins here.
Further, if he places the stem on the fit bike that conforms to that which a person ought to ride considering his stature, and he uses a stem pitch that he intends to use on the actual bike to be custom made, then he's removed this variable as well.
If this protocol is used, once the subject is positioned on the simulator, you've removed all guesswork above the frame's head tube. But still, how do you determine this frame's geometry?
The horizontal plate's gradations will tell you how far in front of the bottom bracket the stem is. In the case of the photo adjacent, you'll note that the stem is sitting 5mm above the plane of the horizontal plate. You'd need to normalize this for the measure above the top of the head tube you intend your bike to have.
In other words, the intended custom bike is going to have an integrated headset or not, and if not, then you need to subtract a bit of head tube to make room for the external headset cups. If you want to have 15mm of spacers, you need to adjust for those. You then adjust your stack appropriately. In any case, the reading on this horizontal plate, right underneath your stem's connection to the virtual steer column, is the frame's "reach," and tells you how many millimeters the top of the head tube is in front of the BB. What we've done is reduced the frame's length and height to a set of X/Y coordinates, that is, vertical and horizontal coordinates.
You need a third parameter, however, and this is the bike's seat angle. A few years ago this was an almost unnecessary parameter to consider, because you could always figure out a way to get the saddle where you wanted it. But these days bikes tend to have seat posts you can't change out, they're part of the frameset. So, you do have to consider this. So, you run a SmartTool through the BB and the center of the seat post clamp and you've got the seat angle of the bike you'll be riding. Now the astute frame builder has all he needs -- in terms of fit specs -- to custom build your bike.
Keep on thing in mind. Depending on the custom bike maker, the seat post clamp may be set right on top of the seat tube and post, or a cm behind, or a cm in front. The actual seat angle of the bike itself may be a degree steeper or shallower than the virtual seat angle of the bike. Taking QR as an example, if you want a custom TiPhoon, and your perfect seat angle measures 79 degrees from the BB through the center of the seat clamp, your TiPhoon should probably be built with a 79.5 or 80 degree seat angle, because the TiPhoon's seat clamp is offset about 1cm behind the centerline of the seat tube.
But why don't you use an X/Y measuring system in the rear of the bike, since you're using it in the front? I can ask a frame builder what the "X/Y" is from the BB to the top of the head tube, and for every size bike he'll give me a specific answer. But if I ask him the X/Y from the BB to the seat post clamp, he can't tell me that, because it depends on the height of the saddle. So, in the rear I find it easiest to use an angular dimension.
But how will the frame builder know how tall to make the seat post? Easy. He already knows where the top of the head tube is, because you've precisely positioned this for him by giving him the stack and reach. He just needs to project back in a straight line (that's the bike's top tube), and there you have the seat tube terminus (since tri bikes always have shorter head tubes than road bikes, you won't need to worry about standover height, and can just bring the top tube back in a horizontal line).
How does this work in real life? Any frame builder with a parametric program for framebuilding (framebuilders all have these) can extrapolate a geometry out of stack, reach and seat angle. If you take my idea of the best production geometry as an example, in a size I would ride (historically the 58cm version of my idea of optimized production tri geometry), that geometry generates a stack of 560mm and a reach of 460mm. What is the top tube length generated by this stack and reach? It depends on what seat angle you use. Personally, I would use a steeper angle than the 78.5 degrees I recommend bike companies use in production, because I ride steeper than the average bear. My preference in a top tube is about 56cm as opposed to the 56.5cm production top tube length I recommend on that 58cm size. So, my own perfect custom geometry would call for an 80 degree seat angle, and my custom frame's reach would increase to about 47cm. I like that 145mm head tube length called for in my optimized production geometry, and the 7cm of bottom bracket drop, so I'll stick with the 56cm stack.
So my perfect custom bike would have a stack and reach of about 56cm and 47cm respectively. But so far we're talking about fit specs only. Now there's the handling-specific specs to consider, and this is where you either have to rely on the framebuilder's expertise in how tri bikes ought to handle, or you can use my article referenced above on the sorts of techniques used by production bike makers (as well as my own ideas of handling-specific bike specs). Accordingly, choose chainstay lengths, front/center and wheelbase specs, steering geometry, to suit.
You do not need Mandaric's fit bike to generate stack and reach. Serotta makes an X/Y tool that also does this. Making an X/Y tool would not be that hard, and somebody probably ought to make one for sale to those who have fit bikes that don't automatically give you X/Y dimensions.
What about production bikes that might work instead of getting a bike custom made? I anticipate, within a very few weeks, publishing another article with the stack and reach of every meaningful tri bike company, throughout their line and for each size. I already have some of these stats databased. Using my own 58cm size as an example, QR's new 2007 geometry says its 58cm Kilo and Tequilo have a stack and reach of 54.5cm and 45.5cm respectively. Alternatively, its 61cm size has specs of 57cm and 47cm. Since my own "perfect" specs would call for 56cm and 47cm respectively, it's apparent that QR's 61cm is actually very close to my perfect size -- the head tube is 1cm taller than perfect, and the top tube is perfect. That's certainly something easy to accommodate by moving a 1cm spacer out from underneath the stem.
Cervelo was the first company to start publishing stack and reach dimensions, fully 3 years in front of anybody else. Should one consider its geometry chart, one sees that the 61cm P3C Cervelo has a stack and reach of 56.4cm and 45.4cm. Again, this is very close to what I'd need. My stem on a Cervelo needs to be 1.5cm longer, and I'd need to take a 5mm spacer out from under the stem, and I'm there.
Then there's Felt's new geometries for '07. In my size, the seat angle is 78.5 degrees, so I can get my 80 or so degrees by moving the saddle forward on the rails. The stack and reach for its tri bikes in 60cm are 56cm and 46cm. My "perfect" spec, remember, is 56cm and 47cm. So, I add a cm to the stem I'd normally ride an on any '07 Felt I'm good to go.
Once all the stack and reach dimensions for all the popular tri bikes are posted here, it ought to be easy to see what bikes will work for you and which one's won't. Just keep in mind that you can't go to a FIST session, get your stack and reach numbers, find corresponding stack and reach specs of current production bikes, and jump on these bikes and ride off happy. Assuming your fit was executed with your own preference of aero bar and stem config, you'd need to take your Cervelo, Felt, QR, Giant, Litespeed, and stick your front end on that production bike in order for it to fit the same. And, of course, that bike's seat angle must be such to accommodate the angle you'll choose to ride.
What I hope to express here is that there is a path from "points in space" to finished bike geometry. But the path requires a fitter with state of the art tools, and a lot of expertise. When the article databasing the stack, reach and seat angles of each of our sport's production tri bikes is published, I'll also publish stack and reach of my optimized production tri geometry. I'm doing this for a variety of reasons, and one such reason is a sort of "buyer beware." If you're getting a custom bike built, I would recommend you look at how much its geometry differs from the production geometries of the bikes built close to your desired seat angle. If it differs a lot, then you might want to inquire why before you buy.
Read this first, if "tri bike fit" is new to you. It's a guide to the dozens of articles in the bike fit section. The articles are grouped, and we offer our advice about the order in which they should be read. 12.30.08
I ride a 59cm bike or, if it's a road race bike (as opposed to a tri bike) I'll ride a 60cm bike. If it's a Litespeed tri bike, I'll ride 57cm, and so it goes. How are these bikes measured, and why do I ride different sizes depending on the manufacturer? 8.06.02
This second in a series of articles describing our tri bike fit system lays out the axioms on which our F.I.S.T. system relies. 9.13.07