Size Is Important – Bike Fitting Basics

Can smartphone technology help close the gap between your low cost second hand internet set-up & a custom fitting new machine from a professional?

The rise of internet shopping over the last decade has left virtually no retail sector untouched and cycling is no different. For those that know what they want, trawling the online bike shops to compare prices of bikes, components, accessories and clothing is a hobby in itself.  The hidden wonders (and nightmares) of eBay offer even more savings if you are willing to take a bit of a risk on people’s descriptions of second hand goods.

Buying a new or even used bike online is a bit of a gamble though, unless you have engaged in some significant showrooming.  Determining whether or not a bike is going to fit you really requires a lot of measurements, studying of specifications and probably the advice of a professional but not many people actually do this – especially if buying a second bike.  Most people should manage to get a bike they can physically ride but if you buy the wrong frame size in the first place you could be stuck with something that takes the fun out of cycling at best, or at worst causes pain and even injury.

Fast Bike Fit Photos-2

Assuming you have at least managed to get the right frame size that’s not the end of the story as we found out when trying out a new smartphone app Fast Bike Fit which uses standard measurements to ensure you are setup for optimum comfort and speed.  Currently available on iOS devices only for the princely sum of £1.99 this seems to offer a shortcut to spending £150 or more on a professional bike fitting service (serious enthusiast only).

So, what do you need to get started?  Well the bad news is that in order to get an accurate reading you need to setup your bike on a turbo trainer (rollers may do the trick too but you will have to be very careful) and you also need to find a way of putting your iPad or iPhone in the correct position to take the video.  We were quite impressed with our setup in the end and just needed a willing test subject and a bike – enter Dan.

Dan’s bike has already featured on Urban Limits and was an eBay special.  Dan has covered over 2,000 kilometres on the bike over the last couple of years and didn’t really consider fine-tuning the setup in the excitement to get out on the road.

“After tinkering with the set-up during the first few outings I have largely left the ride position untouched since. All in all the ride has been reasonable but on days when I have been in the saddle for two hours or more I had started to get a bit of pain in the lower back.”

Once the test rig was setup we needed to place coloured stickers on five points of the body, as detailed in the app.  These blue dots proved invaluable in the later phases of the process and allowed consistent positioning when the adjustments were later made.  Positing the iPad was crucial too.  It had to be exactly 3m away from the bike with the camera at the same height as the top tube.  If this can’t be achieved you are going to struggle to get good results.

Fast Bike Fit Photos-7Shoulder
Fast Bike Fit Photos-6Elbow
Fast Bike Fit Photos-8Hip
Fast Bike Fit Photos-5Knee
Fast Bike Fit Photos-4Ankle

Recording the video was very straightforward with a countdown timer letting you know when the recording was about to take place and when it had finished.  Once that was done we took a single measurement on the bike (in this case axle-axle) to feed into the calibration screen.  This allows the app to calculate other distances later on.

The next step is to head over to the analysis section and start calculating key measurements.  Each measure involves calculating an angle (three points) or a distance (two points) and you do this by moving the red dots to the correct position.  This is the reason we chose blue markers, to make them stand out a bit better in the analysis tool.  Someone thought quite carefully about the system and you move an associated green blob with your finger rather than the dot itself to get a more precise measurement.  You can also pinch-zoom to get more detail but this was limited by the rather rubbish camera on our iPad 2 – the pictures on an iPhone 4 or 5 are much better.

There are a lot of measurements to choose from but the most critical measurements are as follows:

This is a measurement of the maximum extension of your leg at the bottom of the pedal stroke. A common mistake is to have low measurements here (<140 degrees) which makes it easier to get on and off the bike but reduces your power output.  For road bikes this needs to be somewhere around the 140-145 mark and in Dan’s case it was 149 – a little bit too wide.  The way to adjust this angle is by raising or lowering the seat post.

This angle is a direct result of setting the seat post height for the Max figure and should always be more than 70 degrees (Dan measured 74.8).  If the Seatpost is adjusted correctly at the Max figure but the Min figure is less than 70 degrees it may indicate that the crank lengths are too long.

This indicates how sporty your setup is. On time trial and triathlon bike this angle is quite steep, often as low as 95 degrees for better aerodynamic effect.  On a road bike when you are in the drops the angle may get quite low (you will feel your legs getting close to your chest when pedalling) but for touring and climbing it needs to be higher.  In this case when Dan’s hands were on top of the shifters the angle was slightly low putting pressure on the back and restricting lung capacity.

Torso Angle is the upper part of the Hip Angle measurement and can be adjusted while riding by putting your hands on a different part of the bars.  Stem height and length is the other significant factor in setting this angle.

On a road bike the tip of the knee should really sit over the pedal axle.  If not you need to move the saddle on the rails to adjust.  Dan’s measurement was a little over the front of the axle.

This one is down to a bit of personal preference and can be adjusted using the stem. A 5cm drop is normal for most road bike with 10cm or more seen on pro bikes and triathlon machines.  Dan’s setup is quite aggressive with an 11.7cm drop.

We looked at a whole bunch of other measurements including brake levers (perfect), shoulder angle (a bit steep), handlebar angle (a bit upright), and foot angle (about right).  There’s also an option to look at shoulder width which required shooting a second video head-on but we didn’t do this (you can carry out the task quite easily with a tape measure – just make sure your bars are the same width as your shoulders for a road bike).

The advice provided is reasonably brief and if you want to know more about bike fit and biometrics you are best off looking at a more comprehensive guide.  We found it was enough to help us achieve the best position but it had made us wonder if there’s more to learn…

So, armed with the information we set about making some minor adjustments.  Knowing the Knee Angle Max was too high we dropped the seat post by 15mm and made some minor adjustments to the handlebar angle. Sorting out the Knee Over Pedal measurement to bring it back over the axle was impossible though as the seat was already as far back as it would go on the rails.  This means that Dan is stuck with a bit of a racy setup unless he gets a laid-back seatpost.  Adjusting the hip and torso angle, with its consequent effect on shoulder angle and arm angle, would require a new stem and possibly even a new fork steerer.  Achieving perfection on a factory-fit bike is often quite hard – especially if you didn’t go in for a fitting in the first place!

Once the adjustments were made we re-shot the video and re-measured the angles.  As the image above shows the Knee Angle Max was reduced to 142.9 degrees (perfect) and the Min value was still well within tolerance levels (74 degrees).  The arm angle is still quite steep, something that can perhaps be fixed be just relaxing a little!  As you can see on the screen grab there doesn’t need to be a lot of movement to achieve a more optimum position.

So, after an hour or so of fiddling, was it worth it?

“After seeing how I was overstretching at the bottom of the pedal stroke we dropped the saddle a fraction and then made a slight adjustment to the angle of the handlebars; making the overall position slightly less aggressive and bringing my shoulders and arms into a more relaxed position. While these were only small adjustments the ride position does already feel less taut.

Of course, securing a cash saving through buying second hand was desirable, though I wouldn’t want to risk any long term damage to my back! As a cost conscious internet savvy shopper I’d be pretty happy with using the app for future fittings; let’s see if it makes the ride more comfortable over time.”

Using the app has certainly made us think a little more about what’s important on a bike – whether it fits, rather than how shiny it is – and we will all take turns to get our various road, triathlon and mountain bike setup properly over the next few weeks.  It’s also shown us that getting the right sized bike in the first place is essential.  There’s only so much messing around with seatposts and stems that you can do so if the frame is the wrong size you have had it.

It must also be said that this app is only a taster of bike fitting and the experience of a professional fitter is worth seeking out if you are a serious athlete – for the rest of us though it’s £1.99 well spent!

To view all of the screenshots as well as the in-app advice click on the gallery images below

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App Screenshot Gallery
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App Advice Sections