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Bike fit math revisited
While reading an article on bike fit by Matt Russ (TriSports.com) I came up with what was to me a new set of calculations to point toward proper bicycle size.
1. The traditional inseam measurement is done standing against the wall with a book (large is better) pressed firmly up into the crotch and square against the wall. If you measure it in inches, convert to centimeters by multiplying by 2.54. Edit: Removed note to wear shoes......do it barefoot. 2 Your height, also measured in centimeters. 3. First result: the approximate frame size for you is the inseam measurement multiplied by .67 4. Second result: Divide your height by your inseam. If the ratio is greater than 2.2 a slightly longer frame is called for. If the ratio is less than 2.0 a slightly shorter frame is called for. My example. Inseam 79.5 cm........proper frame 53.3 Height 180.3............ratio...2.27 indicating a longer top tube or larger bike. Edit: My example edited to reflect change in inseam measurement. Observations: I have been buying 57cm frames and they were too long for me by about one size. |
Easier is sit on the Bu**er and see if it is comfortable. :):)
I am one of the NON standard sizes. Short legs for height and that loss of leg length is in the thighs. So I normally go for a shorter top tube than required and adjust out with a longer seat post. Then the stem length has changed over the years to accomodate an aching back. And does trisports make any allowance for bike colour. Gold bikes require that you have a longer stretched out position for the extra speed that comes from this colour wheras you can comfortably get an upright position on a white bike as it will never get up to the critical speed where aerodynamics will play a part. |
Originally Posted by maddmaxx
(Post 5885829)
While reading an article on bike fit by Matt Russ (TriSports.com) I came up with what was to me a new set of calculations to point toward proper bicycle size.
1. The traditional inseam measurement is done standing against the wall with a book (large is better) pressed firmly up into the crotch and square against the wall. (wear your bike shoes) If you measure it in inches, convert to centimeters by multiplying by 2.54. 2 Your height, also measured in centimeters. 3. First result: the approximate frame size for you is the inseam measurement multiplied by .67 4. Second result: Divide your height by your inseam. If the ratio is greater than 2.2 a slightly longer frame is called for. If the ratio is less than 2.0 a slightly shorter frame is called for. My example. Inseam 81.3 cm........proper frame 54.5 Height 180.3............ratio...2.22 indicating a longer top tube or larger bike. Observations: my inseam is longer than I thought while wearing bike shoes. M height is less than I thought because I am older. :( I have been buying 57cm frames and they were probably too long for me but not more than 1 size too large. Interesting stuff! Our inseams are about the same, mine perhaps a bit more since I've done barefoot measurements. You are coming up with a larger frame than I usually calc for myself by about 2 cm, but it's a sizing I've wanted to try. Arnie Baker wrote an E-book on bike fitting and sizing (available at RoadBikeForums I think?) where he also uses this 2.2 factor to assess body proportions and to recommend the need for a non-standard TT. Road Fan |
Do you have a link to the actual article?
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Age. Style. Number fits don't accommodate. Use.
I have an upright bike and a low long bar bike. Both fit. Wouldn't think from looking at them that the same person rides them. Both comfortable. One for loafing to work. Another for cranking. |
Originally Posted by Road Fan
(Post 5885860)
Do you have a link to the actual article?
Many articles here. This is an assosciated site to trisports.com. |
Originally Posted by stapfam
(Post 5885839)
Gold bikes require that you have a longer stretched out position for the extra speed that comes from this colour wheras you can comfortably get an upright position on a white bike as it will never get up to the critical speed where aerodynamics will play a part.
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I have always heard to measure in bare feet, not in cycling shoes. My way puts me on a 58cm frame, which matches my seat-of-the-pants observations. Measuring while wearing shoes would put me on a size larger frame which would stretch me too far imho.
<edit> I just followed the link to the article and it also says to measure in bare feet. |
Originally Posted by BluesDawg
(Post 5886459)
I have always heard to measure in bare feet, not in cycling shoes. My way puts me on a 58cm frame, which matches my seat-of-the-pants observations. Measuring while wearing shoes would put me on a size larger frame which would stretch me too far imho.
<edit> I just followed the link to the article and it also says to measure in bare feet. |
Originally Posted by BluesDawg
(Post 5886459)
I have always heard to measure in bare feet, not in cycling shoes. My way puts me on a 58cm frame, which matches my seat-of-the-pants observations. Measuring while wearing shoes would put me on a size larger frame which would stretch me too far imho.
<edit> I just followed the link to the article and it also says to measure in bare feet. |
Originally Posted by George
(Post 5886474)
I read the same thing many, many times. I guess we'll all have to buy larger bikes now;)
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Originally Posted by stapfam
(Post 5885839)
Easier is sit on the Bu**er and see if it is comfortable. :):)
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It all starts with the top tube. When you are bent over in the position you like, and your elbows are bent a bit... that's the distance you want. Knock off a bit for the stem and look at frames that size.
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Analog methods are nice but when they tell you something is very very wrong then it is nice to have an idea about where to start over.
As built, the White Rabbit was very uncomfortable when in the aero position. At first I thought...naaa...your just old and fa* (slightly larger than optimum) you'll get over it with training. Looking back I can say that BluesDawg and Hermes spotted the clues and gave good advice. The bike was torn down this morning and rebuilt with the aero bars at a higher level and slightly farther away using a collection of parts from around the lab. Much better. The math helps when you are banging around and cant find the bread crumbs. Just to make my day worse, I violated rule 7.....Don't wrap your bars untill you know that all is right. |
This is consistent with my experiences.
According to your algorithm I fit a 50cm bike but need a longer top tube (ratio 2.29). And when I've ridden road bikes, I find that a compact geometry (i.e. lower standover but longer top tube) 52cm comes very close. On some bikes, even a 54cm was a reasonably decent fit. |
Originally Posted by maddmaxx
(Post 5886630)
Analog methods are nice but when they tell you something is very very wrong then it is nice to have an idea about where to start over.
Well, I agree that you have a point but there's still this: Imagine a room full of people doing toe touches. Some can flatten their palm against the floor, some can barely touch their knees. Now how does that stand-over height and total height formula adjust for those flexability differences? Getting the fit dialed in is still an art. |
Originally Posted by maddmaxx
(Post 5886630)
Just to make my day worse, I violated rule 7.....Don't wrap your bars untill you know that all is right.
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Works like a charm for me:
height = 173 cm inseam = 81 cm recommended frame size = 54.5 cm (Bianchi & Capo = 55, Peugeot = 54) ratio = 2.125, right in the middle of the range |
Wrapping your bars is regarded as making an appropriate sacrifice to the Bike God, who then blesses you with insight into what you forgot to do.
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Originally Posted by Retro Grouch
(Post 5887944)
Well, I agree that you have a point but there's still this:
Imagine a room full of people doing toe touches. Some can flatten their palm against the floor, some can barely touch their knees. Now how does that stand-over height and total height formula adjust for those flexability differences? Getting the fit dialed in is still an art. |
Would one's physical condition have a greater effect on the size of one's bike, or the configuration thereof? Or even the type of bike (i.e. drop bar road bike vs flat bar road bike vs hybrid)?
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Originally Posted by Tom Bombadil
(Post 5889114)
Would one's physical condition have a greater effect on the size of one's bike, or the configuration thereof? Or even the type of bike (i.e. drop bar road bike vs flat bar road bike vs hybrid)?
Something that has often amused me is that bicycles are traditionally sized by the length of the seat tube. Seat height is the easiest thing to readjust for. |
+1 on the irony of labeling bikes by seat tube height...especially in a time of compact frames and seat tubes that are extended above the top tube. (Especially with long saddle posts so easily available.) Standover clearance is significant, but what is most critical is top tube length and overall reach. To a degree, geometry also: surprizing how a slack 72 degree head angle can pull those bars back.
I'm happy on a 57cm Romulus as well as a 53cm Bridgestone...both drop bar bikes with somewhat similar geometries. Certainly I have a Nitto Technomic on the Bridgestone...otherwise both have fit set-up adjusted the same...and certainly they handle a little differently. Forumulas are a good place to start, but even then, personal dialing in is still required. **Incidentally, the formula puts me on a 54.5...which is the size of my Specialized and which also makes me happy because I've adjusted with stem and set back seatpost. The formula does not seem to account for my ungodly long, orangatang like arms. Or my short torso/longish legs. Hence, personal dialing. |
Are you stating the frame size as top tube measurement or seatpost? It seems every maker has some different way of sizing their frames.
Did the calculation and my size is a 52.26 but the ratio says I should have a smaller frame, which is what I normally ride. How much smaller or larger by the ratio is the magic question. |
A couple of notes on these formulae:
-- on the .67 x inseam measurement, (a) i.s. is with barefeet, and (b) the multiplier gives you what was (BCG [before compact geometry]) the rough centre-to-top (of top tube, along the seat tube) sizing for a traditional frame. E.G. my inseam of 84 cms. x .67 puts me on a c-t frame of about 56, or about 54 centre-to-centre (56 c-c would be too big for me). With compact frames, you really have to size more by tt length, and by headtube length. -- the '2.2' formula does seem to have some validity. Again, my own e.g.: 174 cm height divided by 84 inseam gives 2.07 -- well within the range, but suggesting a caution against too long a top tube. I think another worthwhile approach is Dave Moulton's chart (can find the whole thing with explanatory comment on his bike blog). |
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