Bike fitting – expert advice from Bike Science’s Andy Sexton

Bike Science head fitter Andy Sexton answers BikeRadar readers’ fitting questions

A good bike fit is essential for all riders

Founder and head bike fitter of Bike Science, Andy Sexton was one of the very first Retul-certified fitters in the UK and has helped scores of athletes of all abilities get on better with their bikes, whether they be roadies, MTBers or triathletes.


Andy hosted a Q+A session on BikeRadar’s facebook page last week – and here are the highlights.

What size bike should I get?

Andy: We get asked this a lot. My advice is always to ask around riders and clubs in your local area to see which shops or bike fitters have a good reputation when it comes to sizing advice before you buy.

Ideally you should look for someone who can offer you the service of sitting you on a fitting jig (an adjustable fitting bike) and using that to advise on which makes and models would work for you best. You’ll find each brand will often offer a number of bikes at your price point with differing geometries that will fit different shapes of rider.

I’m not a strong believer in suggesting a bike size just based on height and leg length as there’s so much more involved in selecting the right bike. Your posture, flexibility and how you sit on the bike will make a huge difference in which bike will work for you.

How do you know if your bike is set up properly?

There’s never really a definitive answer and even if you put 10 bike fitters in the same room, chances are they wouldn’t all agree, but the general picture we’re looking for is the same. Our key pointers are that, above all else, you are balanced and relaxed on the bike. I look to distribute your weight in such a way that you don’t have to push with your hands to maintain your riding position. Weight too far forward will often result in rigid arms and shoulders, painful neck and hands.

Think about it in this way – if you pose (off the bike) in a position that looks like you’re riding your bike, then gradually move your weight forward, eventually you will fall on your face unless you do something about it. What most people will do on a road bike if their weight is too far forward on the bike is lock out their arms and shoulders and use them as ‘props’. It’s really hard to relax in this position.

I’d move your weight back until you feel you’re at the point that, when riding under load, you are on the verge of tipping forwards (with your hands on the hoods position). When you’re pedalling (especially when going hard) you should almost reach the point where you want to pull on the bars rather than pushing on them.

How often should you be re-fitted on a bike?

A key thing to keep in mind is that your fitter will have done the best job he or she can in the time available during the fitting session you booked. It will often be that a re-visit a few weeks after your initial fitting will help. Once you have some feedback for the fitter on how the positional changes have worked (or not) that’ll often help them improve your riding experience further.

If you’re making significant changes to riding equipment or you feel your flexibility, experience or strength has changed significantly (better or worse), then I would also recommend re-visiting the fit.

Is there a common source to getting more pain on one side of the saddle or sit bones than the other?

During a fit session, we would check your strength and flexibility off the bike, as well as looking for possible signs of leg length difference. There are a number of factors that would possibly make you get more pressure on one side of the saddle. The first place to look is to see how balanced you are when it comes to flexibility. If you do a single leg hamstring and glute stretch, are you equally as flexible on both sides of your body?

We often see riders who show large asymmetries on the physio bench exhibit large asymmetries on the bike. When stretching keep in mind how symmetrical you are. If you find you’re consistently tighter in one area than other, stretch that area more.

Do shorter cranks make you faster?

This depends on a lot of factors [including] your ability to produce torque. Some riders are better at producing torque than others. These riders will manage well on a longer crank relative to their height. Crank length becomes more critical on time trial bikes, where we’re trying to get the rider into a flatter back, more aero position. This in turn closes the angle between the thigh and torso at the top of the pedal stroke.

Ideally we want to keep this angle open. Picture yourself doing a weighted squat from a low squat position – this is harder than squatting the same weight from a higher starting position because your hips are more open to start with. Current research does seem to be showing that given a long enough adaptation period, short cranks are more mechanically efficient than long when riding in the aero position.

How do you transfer cleat position when changing cleats?

Ergon makes a really useful tool that helps match cleat position. If you’re happy with your current cleat position, the tool can be used to match the position on the same, or another pair of shoes. Alternatively, draw round the cleat, ideally in a metallic marker as it’s easier to see.

How should you adapt a road bike fit to a cyclocross bike?

From a pedalling point of view, keep in mind that when changing from road shoes to MTB shoes, there might be a small difference in the pedal/shoe stack height. If your SPD shoes are slightly thicker soled than the road shoes (which can be the case) you may need to put your saddle up a little to maintain the same pedalling sensation.

Above all else, if it feels the same (and feels good), it’s probably not far off. With regards to bar position, the speeds are much lower with cross than with road riding and handling priorities are different. Often a slightly wider bar will help slower speed control. You may need to combine with a slightly shorter stem. I wouldn’t radically change the reach of the bike. Assuming your posture is good in the road position, then no need to change too much for cross riding.

Does lower back pain usually mean my bike is too big or too small?

Lower back pain can be caused by a number of different things. Firstly it’s worth eliminating other ‘lifestyle’ factors – desk / chair set up at work, driving position etc. If you feel that it’s only really riding that’s causing the pain, then during a fitting session, we’re really looking at your posture on the bike to see what might be causing the issue. The assumption most riders make is that if their back hurts, they should put a shorter higher stem on the bike. This can work, but certainly not always.

I’ve seen cases of lower back pain on bikes both too big and too small. One of the main things I’m looking for when fitting a rider is that they are relaxed whilst in each riding position. I’ll often find that pain (be it back pain, neck pain, pain in the hands etc) is caused by the rider being off balance on the bike. By this I don’t mean they keep falling off, but that that their weight is positioned on the bike in such a way that they need a significant amount of muscular effort to maintain their riding position. Often getting the saddle position (height and fore/aft) is the key to unlocking a relaxed position and curing all kinds of pain.


If you’re interested in finding out more about Bike Science or booking a fitting, visit the Bike Science website, call 0117 9273444 or email: