Bicycle on Tacx trainer

The drawbacks of bike fitting on a trainer

With the number of bike fitting methods around, you may be wondering which is the best one for optimising your position. Will the bike shop sizing serve me well? Should I have an angle based fit? A bike brand specific fit. What about having a biomechanist or sports scientist fit me? Is 3D video capture or saddle pressure mapping important?

It can be a bit of minefield to wade through the fitters in your local area to decide who you will trust to solve your discomfort or contribute to you achieving your goals.

In this article, I’m going to focus on just one of the differences between bike fitters: whether they solely use your own bike during the fitting process or make use of a dynamic fit bike.

At Winning Position, I use a Purely Custom dynamic fitting bike. Whilst I observe and assess the rider on their own bike early in the session, I perform the actual fit on the fit bike. I believe doing so greatly increases the prospect of finding the optimal position over a fit that uses a bike and trainer alone.  The reasons are as follows…

You need to already have a bike

I have laboured this point before, but you should get a fit before you buy a bike.

With a fit bike, there is no requirement to bring a bike to a fitting session at Winning Position. This means that those clients who don’t have access to a bike (for example those new to cycling or those whose last bike met an unfortunate demise) can still be fit. The session will involve exploration of positions on the dynamic fitting bike as well as valuable cues and instruction on cycling technique. You will walk out armed with direction and insights about which bikes on the market that will suit your fit coordinates and where you should begin your search for your new bike!

Limited saddle adjustment

For every bicycle, the range of saddle positions is restricted.  The range is limited by the geometry of the frame, the length of the seat post and the length of the saddle rails combined with the offset of the seat post. Even if the frame is the right size for you, you might not be able to find the optimal position if, for example, the seat post has the wrong offset.

Would you like to explore a more rearwards position? Would you like to trial a more forewards saddle position around the bottom bracket and experience the different muscle leg groups firing? Do you want to actually feel the difference? This can be done on the dynamic bike, with a much greater range of saddle positions possible with the turn of a knob.

Limited handlebar positions

On a ‘real’ bike, if you are lucky, the fitter will swap your stem out for a special sizing stem to explore options beyond the length and angle of the current stem. However this is still limited by the cable lengths and routing on the bike. (Swapping to a sizing stem might not even be possible with all the integration and internal routing the exists today.) Without an adjusting the stem, it’s guess work by the fitter and imagination on your part as to whether a shorter or longer, a steeper or shallower angled stem will improve your position.

Limited handlebar options

Whilst it is possible to put a different handlebar on a bike on a trainer, you have to do something with the existing bar that’s still attached by the cables. It’s a hassle and you can damage the bars and cables if you ride too vigorously with the bars dangling. In the end most don’t bother and you get fit with the bar that’s already on there. Even if it is too wide or long or deep.

On a fit bike, there is no cabling so it is far easier to change the bars (or the pad and extension position on tri bars).  I even have adjustable road bars so it is a matter of a minute to change bar widths and so on.

Fixed crank length

You may want to try a longer or shorter crank length.  Your morphology or problems on the bike might need a change in crank length from what came on the bike. When being fit using a bike on a trainer you are stuck with the crank length that’s on there.  It’s just not practical to be swapping cranks mid fit session. Even if it were, no fitter is going to be able to keep every combination of crank length and axle standard. An good bike fitter would be able to suggest what length might work for you but if you can’t try it out, how do you know that it will?  Cranks are too expensive to be buying on the off chance that it’s the right length.

Dynamic fit bikes typically have adjustable length cranks fitted to them. (The cranks on mine can be adjusted from 155cm to 185cm.) It’s quick, easy and convenient to explore multiple crank length options during a fit session. You can feel what it’s like and how the length interacts with the rest of your fit before you outlay a wad of cash on new cranks.

Let’s make those changes… just hop off your bike for a minute (or 10)

If you are being fit on a bike on a trainer, any changes required will mean you have to dismount whilst the fitter or mechanic make the adjustments. This can be a lengthy process, not only wasting time but also making it difficult to compare the new position with the previous one. Your proprioception has been reset during the time spent off the bike.

This issue is non-existent when on the dynamic fitting bike as rapid, efficient changes can be made in real time as the cyclist continues to pedal. You are able to compare small and large adjustments within seconds. We can explore and ‘play’ with various positions and monitor the resulting sensations for the rider.

Ending up with something less than optimal?

The upshot of the inefficient changes and positional limitations on your current bike is that you could well end up not moving too far from where you started. Additionally, your fitter will have spent more time wrenching on your bike rather than assessing your fit. You’ll never know how much optimisation you left on the table.

Dynamic fit bikes aren’t perfect

Lest you think that I am blind to the short comings of dynamic fit bikes, I thought I would finish this article with a brief discussion of some of them.

First it is very possible to discover that your optimal position is one that your current bicycle cannot support. This is disheartening to say the least! When this happens I work with the client to find the best compromise the bike can reach.

Second, the position found on the fit bike has to be transferred to the real bike. Measuring bikes is hard! I get around this problem by using an application that has been developed by Winning Position that takes the inputs from the fit bike, takes into account things like differences in handlebar dimensions and outputs the measurements for a given bicycle frame. Using my Bike Size jig I can then set up the bike quickly and accurately.

Third, a dynamic fit bike is fixed in the vertical plane. (This is true also bike attached to trainers, though there is sometimes a little side-to-side movement with those.) This just isn’t how bikes ride in the real world. For a minority of cyclists, the artificial nature of a fit bike (or trainer) can mask issues and problems that they experience in the real world. A few fitters get around this by getting their clients to ride rollers during the session. I don’t use this approach as not many cyclists have experience with rollers. I think that the tension caused by the fear of falling off the rollers changes the pedal stroke more than the rigidity of the fit bike.

To make up for this short coming, I include a tweak session with every bike fit so we can attend to any issues that arise as the cyclist adjusts to their new position. In fact, I believe the only way to really know that a position is right is to spend time with it in the conditions that it is going to be used in.