Author Topic: Correct Measurment?  (Read 4806 times)

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Offline popeyespal

Correct Measurment?
« on: October 19, 2010, 11:58:04 am »
While browsing the forum yesterday I came upon a response to a question about proper bike fitting. It referred to a measurment called VTT? Virtual top tube or something like that? (I may have just butchered the term). What is this and why is it more valuable than stand-over height?

Offline DaveB

Re: Correct Measurment?
« Reply #1 on: October 19, 2010, 12:07:35 pm »
Bikes with level top tubes can have their top tube length measured directly.  Bikes with sloping top tubes (aka "compact frames") have their top tube at an angle and the length along the actual top tube isn't the correct measurement needed to fit a frame for reach. 

The "Virtual Top Tube" is an imaginary line drawn where a level top tube would be and the length of that line is the actual top tube measurement needed to select a frame size. 

Standover on a level top tuber is easy to measure.  Standover on a sloping top tube is usually much less than on a level top tube of the same nominal size and, of course varies a bit depending on where you are standing. 

Offline rvklassen

Re: Correct Measurment?
« Reply #2 on: October 19, 2010, 12:42:37 pm »
What is this and why is it more valuable than stand-over height?
What it is was already addressed; its value is as follows:

If you are of average proportions, and the bike was built for someone of average proportions, the ratio of standover to reach is appropriate for you.   Most people are not of exactly average proportions, but within a reasonable distance of average.  The three most important measurements are 1) standover - but only so you don't hurt yourself when stopped; 2) top-of-saddle to top-of-pedal when crank arm is parallel to the seat tube; and 3) reach: the distance from the saddle to the bars.  #1 is a function only of the frame design and wheel size, and puts an upper bound on frame size - but you can use any smaller frame as far as #1 goes.  #2 is very adjustable with seat posts that have 10-20 or even 30 cm of travel.  The forward back position of the saddle is set according to getting your knee bend right, so that only leaves one thing to adjust to change reach, which is the stem length.  Most stem lengths give about a 10 cm range: the effective top tube length needs to be within 10 cm of ideal for the rider.  By going to an extreme stem you can go slightly further.

Where variation in proportions comes in is the ratio of your torso height and arm length (combined in some way that depends on how upright a position you find most comfortable) to your leg length.  This can vary widely.   I know a fellow who's about six inches shorter than I but it's all in his legs - meaning his torso and mine are the same.  If his arms match mine (I don't really know) we'd want the same effective top tube length, but he wants about 6" less standover.

Offline popeyespal

Re: Correct Measurment?
« Reply #3 on: October 19, 2010, 04:41:15 pm »
Alright! Two answers combine to give me a complete picture of bike measurments to look for. However, that leads me to another question.

Which measurments of my body are necessary to make sure I get the right bike? Stand over is easy... measure the inseam. What numbers do I need to determine proper reach, etc.?

Offline rvklassen

Re: Correct Measurment?
« Reply #4 on: October 19, 2010, 09:40:01 pm »
There are at least three answers to that question.

1) There are some vendors that claim to have the perfect formula, and begin with many measurements to derive the "perfect" geometry for you.  There may even be a web site or two that claims to have it all worked out. 

2) Get 'fit' at a bike shop that has a specialist in bike fitting.  If you buy from them, they will often not charge for the service.  If you just want to be fit, or have your existing bike adjusted, it runs around $100.

3) Measure a bike that fits you well.  Don't have one of those?  If you have one that's close, you can measure it and guess at the adjustment.  If not, see 1) and 2).

The difference between 1) and 2) is that 2) is based on an adjustable fit-bike, which has the various parts of a normal frame connected in ways that make it totally adjustable.  Whereas 1) is typically based on hundreds or thousands of measurements of people and the measurements that resulted of them being fitted by method 2), and then some sort of linear regression.   Even if the quality of fit is very good, a key source of variability is individual preference - things not necessarily measured easily.

Offline DaveB

Re: Correct Measurment?
« Reply #5 on: October 20, 2010, 09:09:10 am »
1) There are some vendors that claim to have the perfect formula, and begin with many measurements to derive the "perfect" geometry for you.  There may even be a web site or two that claims to have it all worked out. 

2) Get 'fit' at a bike shop that has a specialist in bike fitting.  If you buy from them, they will often not charge for the service.  If you just want to be fit, or have your existing bike adjusted, it runs around $100.
The fit process at a bike shop is still based on someone's "perfect formula".  The only benefit is that you will actually be there and can tell if their concepts match what you think is comfortable. 

Offline rvklassen

Re: Correct Measurment?
« Reply #6 on: October 20, 2010, 11:37:00 am »
The fit process at a bike shop is still based on someone's "perfect formula".  The only benefit is that you will actually be there and can tell if their concepts match what you think is comfortable. 
When we got fit for our tandem, the guy took all sorts of measurements (of us) but then apparently didn't use any of them, except perhaps to get a starting position for the fit bike.  On the bike, he adjusted seat tube angle, crank arm length, saddle height, saddle fore-aft position, height and fore-aft distance to the bars.   After any significant adjustment the rider being fit "rode" for several minutes.