Bike Size measuring jig

Measuring bikes is hard!

Measuring bikes is hard.  There always seems to be some bit in the way of what you want to measure.  A lot of the desired measurements are from or to the centre of something.  It can be difficult, if not impossible, to determine the exact centre of a complex shape.  Parallax errors can cause problems too.  It’s just plain hard to get exact, or even repeatable inexact bike measures.

So why does this matter?  I have always been somewhat bemused by the all too common process where someone goes to the time and expense of getting a bike fit and then they (or their fitter/mechanic) takes a near-enough-is-good-enough approach to re-creating that position on their bike.  Think that it surely can’t be too far out?  Being 1 degree off when measuring a saddle height of 70.0 centimetres could mean a vertical error of about 4 millimetres and a horizontal error of about 1 centimetre!

An example: measuring saddle height

Let’s consider measuring saddle height in a bit more depth to really drive the point home.

The first conundrum is deciding exactly which point on the saddle you measure to.  There are a number of choices but I’ll discuss the most common.

  1. At the point where the top of the saddle intersects with a line extended from and in-line with the centre of the seat tube
    This is quick and convenient.  Most people are reasonably confident that they can eyeball the centre of the seat tube.  (Though as we have seen, small errors in alignment can lead to quite large differences in the result.)  Perhaps the biggest drawback of this approach is that it is dependent on seat tube angle and saddle fort-aft positioning. A different seat tube angle will cause this method to result in a different seat height.  Change the fore-aft position of the saddle and you will be measuring to a different place on the saddle.
  2. At a given distance back from the nose of the saddle
    I quite like this method and used it for a long time.  This approach isolates the saddle measurement from the seat tube angle and saddle fore-aft positioning.  The repeatability is pretty good too.  One downside is working out the distance.  Whilst it doesn’t really matter, it makes sense to try to measure to the point a person actually sits on the saddle.  In the past 13 centimetres seemed to be pretty close for the vast majority of saddles.  That doesn’t hold true anymore with the increasing popularity of short nosed saddles.  It’s got to the point where you would have to have at least a couple of offsets.  Or one for each saddle?  Not so simple to document how the measure was created.
  3. At a given width of the saddle
    This method is similar to number 2 above but has the additional advantage that it is largely unaffected by the length of the saddle’s nose.  It is affected by the shape/width of the saddle but this tends to be less variable in relation to where a person sits than distance from the nose.  It does require particular tools to be quick and repeatable – some sort of gauge or caliper which is set to the chosen width.
  4. At the point where the top of the saddle is directly above the centre of the saddle rails
    This is the method I currently use.  It involves finding the centre of the saddle rail, either measuring from the shoulders of the rail or from the clamping limit where indicated, and then measuring to the top of the saddle directly above that point.  There are disadvantages to this method.  The point being measured to changes as the angle of the saddle changes.  It is difficult to be precise without the proper tools – I find a laser level works really well.  But I think the advantages out-weigh the disadvantages.  One method works for all saddles.  It is easy to describe.  The centre of the rails tends to be close to where a person sits on the saddle (there are a few notable exceptions).  And the really big one for me: it means that a position determined using a dynamic fit bike can be accurately captured and replicated on the client’s bike.

So far all we have done is decided where on the saddle to measure to!?!

Now we need to locate where to measure from on the bottom bracket (BB).  The centre of the BB is the norm.  But how do you locate and measure said centre?  Do you eyeball it and just hold the tape measure in place?  Or use one of the nifty tools that fit into the non-drive side crank arm bolt head/opening, assuming there is one for your particular crank?  Again doing this accurately and in a repeatable manner is hard.

Now that you have the two points you want to measure between there is one final decision to make.  Are you going to take the measurement parallel to the plane that passes through the fore-aft centre line of the bike or the line that passes directly from the point on the BB and the edge of the saddle at an angle to that plane?  It makes a small difference.  Again looking at our 70.0cm saddle height and using a fairly typical BB and saddle width the difference in the resulting height is a few millimetres.

So now we have saddle height, just one of the measures required to specify a position on a road bike.

The Bike Size jig by BiciSupport

In the pursuit of accurate and repeatable measurements, here at Winning Position we have a Bike Size jig, produced by BiciSupport.  This is a precision made piece of equipment purpose built to measure and reproduce fit positions on bicycles.  The jig holds the bicycle in place and, relative to the bottom bracket, measures the frame geometry, saddle and handlebar position.

Bike Size measuring jig

I love this jig!  It might look like a medieval instrument of torture but it is invaluable for ensuring that the position we worked so hard to optimise is actually replicated on the bike.  As useful as the Bike Size jig is, they are vanishingly rare in Australia.  I had to import mine from Italy by way of the US (a long story!) at significant expense.  But it was money well spent!

My jig is even more unique as I have modified it to expand its utility.  In its original form, the jig only worked on bikes that accommodate standard road width quick release hubs.  With the recent proliferation of road bicycles that use wider through axle hubs, not to mention mountain bikes, changes to the jig were required.  With lots of thinking and a bit of tinkering, I now have a jig that can measure almost any bicycle on the market, irrespective of hub and axle specifications.

Measuring bikes is hard, but it can be a lot easier to produce accurate, repeatable results with the right equipment.

At Winning Position we have the equipment and we would love to help you find your very own winning position.  Why not book an appointment now?

By |2018-11-07T20:10:53+00:00November 7th, 2018|Fitting tools|0 Comments