The fore and aft measurement on a saddle is a very crucial measurement and was first measured in the early days by the famous plumb bob - fortunately these days are now behind us or at least should be, many bike shops still use the plumb bob to measure your knee over the pedal axle.
The knee over pedal axle is a standard way of setting up a bike however it is not a very accurate one, i will explain why
- A plumb bob can only measure a static knee and not a moving one - this can severely alter your measurement. in simple terms , this measurement is crucial for the pedaling power and efficiency so why would you measure it when you are not pedaling.
- A plumb bob only takes a single measurement - a vertical line from your knee down to the pedal, seems straightforward but bike fitting is a game of mm's and not inches and to be honest this reading is pointless if you are not 95%+ balanced on your bike.
- This method has no scientific value behind it and is used in the same way for every single rider (a persons riding position is as diverse as a fingerprint)
A much more modern,accurate and efficient way is to measure this in a dynamic or moving fashion - (the results of this method speak for themselves)
The benefits or this dynamic method are as follows
- better overall balance on the bike
- less pressure on hamstrings and glutes (the old plumb bob way seems to have riders sat towards the rear of the bike once the person is actually riding)
- More comfortable riding position and less chance of moving over the saddle and not being able to find that comfortable spot - this more often than not is due the to the rider pushing themselves back as they pedal after being set to far back behind the pedal axle.
- less pressure on hands and core muscles
- Most importantly a more consistent riding position over the duration of a long ride - your position needs to be as good on the road os it is during the bike fit.