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Sizing up

| Buyers Guides, What Mountain Bike | 23/07/2010 12:02pm

Ask our test team about bike fit and feel and they’ll tell you that every bike is different. Most require perseverance to get the best out of them.

Suspension set-up and parts choice will come into it, but it’s basic fit and feel that matter most. If a bike feels initially strange, it can often be improved by fine tuning aspects of the ride position, the parts, the suspension, or all three. These are all factors that dictate ride feel and handling more than many riders imagine. Our testers can usually manage to work out why some bikes feel right/wrong.

It’s rare for one thing to dictate, or even dominate, bike feel. Feel is dictated by a combination of several factors, none of them can really be considered in isolation. The recent tendency among frame designers has been to build most frames as compact as possible. Compact triangles make for a stiffer, stronger, lighter structure, all positive factors in these days of suspension doing so much of the donkey work.

Originally, the listed size of a bike was the seat tube length, measured from the centre of the bottom bracket. Some brands still measure their bikes like that, but others measure to the top or centre of the top tube, and some simply prefer to work around small, medium and large sizes.

Most MTBs are built with sloping top tubes for good standover clearance and lots of seat post length so that saddle height adjustments are easy. The aspect of frame sizing that makes the biggest difference to a rider is the reach. This is dictated by a combination of top tube length, stem length and saddle position. But it’s not as simple as it sounds. Because the slope of the top tubes and the frame angles vary between bikes, the top tube measurement that matters is an imaginary horizontal top tube.

Moving geometry
When MTBs all came with rigid forks for fairly steady cross-country (XC) use, the geometry was very similar between brands. The head angles were usually 71 degrees, seat angles were usually 72 or 73, the bottom bracket height was 11.5in and the wheel base (front to rear wheel axle) was usually 42in. There were a few minor fluctuations, most notably in top tube length, but it was often things like stem length and fork rake rather than basic geometry that dictated the ride feel of a bike.

With the advent of suspension, geometry became more confusing because of the differences between static geometry and active geometry: static geometry is the geometry of a frame when a bike is built, the tyres are pumped up but no one is sat on it; active geometry is when the bike is moving and the suspension starts working.

As soon as you sit on a bike, you have to take suspension ‘sag’ into account. Suspension sag is how much suspension sinks when you sit on a bike. On hardtails, fork sag will steepen frame angles by a degree or so, then another degree or two when you start hitting stuff on the trail. On a full suspension bike, front and rear sag may balance out the geometry, but bottom bracket height will be more affected than on a hardtail. As soon as you’re hitting stuff on the trail, the feel of the bike is influenced as the geometry changes as the suspension compresses.

So the way you set up your suspension affects lots more than just comfort and shock absorption. To make matters more complicated, lots of bikes have adjustable suspension travel now. Frame geometry will usually change as you alter the travel, especially if you alter one end but not the other. The skill is to make sure those travel adjustments offer you real advantages. Think before you adjust. Experiment. Usually, full travel at the back works at its best when the fork is set to full travel too. If you reduce fork travel for a climb, you may forget to wind it back up again, leaving you trying to tackle a stretch of gnarly singletrack with nervy steep steering and a bottom bracket height that means you slam your pedals on the ground.

One geometry formula won’t work well all the time. Static geometry for all-rounder bikes, full suss or hardtail, will vary between 68 and 71 degrees at the head and between 71 and 74 degrees at the seat. A steep seat angle sits you forward, so you end up getting better use of the fork than on bikes with slack seat angles that sit you back. Bikes with slack geometry rely on downhill terrain and/or an out of the saddle attacking ride style, to get everything feeling right and working properly.

Bottom bracket height varies enormously these days. Average mid-range hardtails (assuming 80mm travel fork) should have static bottom bracket heights of 12-12.25in. Average hardtails with long travel forks (100-120mm) and average XC full sussers (100mm both ends) should be around 12.75-13.5in, to allow for more suspension compression. For long travel full sussers, expect anything between 13.5 and 16in. A low bottom bracket’s better for stability, worse for ground clearance, and vice versa.

Contact points
As your front and rear suspension compresses, the position of everything on the bike, including yourself, shifts in relation to the contact patches of the tyres on the ground. Think of these constant changes as ‘Rolling Chassis Geometry’– this changes in relation to the ground and in relation to the way the bike responds to your input. A skilled rider reacts instinctively and pre-emptively to terrain variations with weight shifts on the bike, but adjustments of where you sit on the bike and where you hold it are important too. So you need to get the position of the saddle, bars and stem right. It’s the position of these that most dictates how a bike feels over different types of terrain…

Saddle position
Inline seat posts will sit you further forward than laid-back ones. This can be a good thing if your seat angle is slack, not so good if you have a short top tube. Saddles can also be moved a couple of inches backwards or forwards along the rails. However you do it, positioning a saddle further back or further forwards changes the working seat angle of the frame and changes your sat-down weight distribution on the bike.

The height of your saddle will also change the way your seat angle works for you on the bike. As you put your saddle down you shift your gravity centre further forwards over the bottom bracket. At the same time you’re moving your gravity centre lower. If your saddle is too low, you’ll have to stand and hover over the saddle more and shift your weight back and forth more to get the best traction from your tyres and feel from your bike. Of course, this won’t be a problem if you’re riding the sort of steep, technical terrain where you want your saddle to be out of the way.

While all this standing, hovering and weight shifting can go some way towards making up for a poorly positioned saddle, it still makes a lot of sense to set everything up for steady sitting-down trail pedalling, at least until you actually need to put the saddle down for more radical manoeuvres. You need to feel at ease with your ride posture and the bike handling all the time.

Stem position
The way your bike feels through the steering, getting your stem length and height right is almost as important as the manufacturer getting the head angle bang on.

Ten to 15 years ago, the fashion for long stems was trying to compensate for the fashion for short top tubes. Most of the stems were very low too, an attempt to mimic the race posture of roadies. But long and low doesn’t work for many riders. Many of the bikes were lumbered with steering that felt both sluggish and twitchy and many riders ended up so stretched that they couldn’t do those subtle shifts in body weight that help with handling over difficult terrain.

Steering a race-bred MTB was like steering a race-bred rowing boat, the stem being the tiller. Fortunately it’s now been generally accepted that length is better designed into top tubes. Gary Fisher’s ‘long top tube short stem’ Genesis geometry designs effectively brought the inherent stability of downhill bikes to the cross-country market, while at the same time keeping the lively steering of racey cross-country set-ups. Most manufacturers have been influenced. Over the last five years or so, stem length has reduced by 25-40mm and top tube length has increased by about the same.

An overlong stem tips your weight too far forward over the front wheel, particularly if it’s positioned very low too. A shorter and higher stem helps you to sit in what we refer to as ‘the centre’ of the bike. If a bike is the right size for you, you’ll feel well balanced.
Stem height is obviously a very personal thing relating to finding a position on the bike that feels comfortable for your muscle structure. But, as with stem length, stem height also has a lot of influence on how well balanced you feel when you’re controlling the bike on tough terrain.

The combination of saddle and bar position, stem length and height dictates how much body weight you’re supporting with your arms and how much with your arse. A high stem might initially shift you up and back and put more weight on your arse, but it also allows you to bend your elbows more when you want to shift your weight down and forwards. Put simply, a shorter higher stem will allow you to change position and move your weight around more easily.

Handlebar position
Bar width, shape and position is also something that has far more influence on the overall feel and handling of a bike than most riders imagine. The old-school cynics think that riser bars are a fashion, transient design. They may be fashionable, but they’re a lot more besides.

A ‘flat’ classic 22.5in handlebar may have a bit of back-sweep to accommodate your natural wrist and hand position, but it’s essentially just a lightweight adjustment-free rod acting as a steering lever. The riser bar, in its many height, width and sweep guises, offers you a lot more control and adjustment possibilities. Okay, the extra height could be more efficiently accomplished with a higher stem, but the width and sweep can be personally tailored to suit the feel and position you want. Most risers are easy to trim on the ends, and most will move through an adjustment arc that changes the grip position and angle enough for you to notice changes in your ride posture and control. Persevere with back and forth adjustments and get a feel for the way they change your posture before you settle on one position. Like stem height/length, bar position changes your whole weight distribution and body posture on the bike

A – Head angle
Average mountain bikes have working head angles of about 71 degrees. Steeper angles make a bike steer faster, to the point of feeling nervous. Slacker angles make a bike more stable, to a point of the steering feeling slow. Suspension forks change the static angle by enough to change steering characteristics as the fork compresses and extends. It’s crucial that a fork suits a frame. A smaller frame might be built with a slacker head angle, a larger frame a steeper angle. A big bike has a longer wheelbase and often carries a heavier rider, so tends to be inherently more stable.

B – Seat angle
A good custom frame builder will determine your seat angle by taking body dimensions such as inside leg, upper leg and foot length as well as considering what sort of rider you are. An off-the-peg bike manufacturer has to make assumptions about such things. They usually get it right. A typical seat angle on mountain bikes intended for all-round use will be near 72 degrees. As with stem length and head angle, a different seat angle will affect a rider’s weight distribution and change the overall balance and feel of the bike, as well as the efficiency of the suspension.

C – Stem length
As a rule, short stems create a more light and lively steering feel and long stems slow things down. You shouldn’t use stem length as the main way of adjusting bike fit though. Top tube length is more important. All frames have an ideal stem length relating to both top tube length and a combination of head angle and seat position. Rider weight distribution can vary considerably with different stem lengths and saddle positions.

D – Saddle position
If everything about frame size, geometry and stem length is spot on, you’ll probably be looking to have your seat post clamped dead centre on the saddle rails with the saddle top lying flat. Remember, moving your saddle back and forth changes the effective seat angle, worth bearing in mind if an off-the-peg bike isn’t quite right.

In terms of getting the right saddle height (disregarding low saddles on jump or downhill bikes) there are all sorts of formulas. The rule of thumb is to get almost full leg stretch when your foot is at the bottom of the pedal stroke.

E – Top tube
Sloping top tubes make a lot of sense. They result in more standover height, more seat post in use and better absorption of trail vibrations.

Length wise, trends over the last few years have seen a move towards longer top tubes and shorter stems, often with steeper seat angles to place the rider slightly further forward and get the best out of longer travel suspension forks.

F – Chain stays
Longer chain stays make the bike more stable. Shorter stays improve climbing traction, but that has to be considered against the disadvantages of less stability, particularly with longer forks, and less tyre room.

G – Wheelbase
A longer measurement between the front and rear wheel axles results in a more stable bike. A shorter measurement adds agility. However, the wheelbase is simply a result of all the other geometric configurations.

H – Bottom bracket height
Higher bottom brackets keep the pedals clear of the rocks, especially crucial when pedalling through corners, but raises the centre of gravity of the bike and rider. A low bottom bracket is inherently more stable, but you risk ground contact more often, particularly on a plush suspension bike where a sudden compression takes you by surprise.


This entry was posted on Friday, July 23rd, 2010 at 12:02 pm and is filed under Buyers Guides, What Mountain Bike. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

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