In the first two parts of this in-depth geometry feature we dealt with steering geometry and its effects on control of your bike and how your sit position on the bike influences the way the bike responds. This time we’re taking a look at what most people see as the minor detail.
But, as we said last time, nothing on a bike happens in isolation. Traction, balance, comfort and efficiency can be influenced by stuff that’s all too easy to overlook.
Bottom bracket height
Bottom bracket height is one of the major limiting factors in designing a frame. Too low and your pedals might clout the ground when you pedal through corners and/or over rocks and roots. Too high and the bike lacks stability – most noticeably at lower speeds – because the centre of gravity is too high, and you’ll find it harder to dab your foot on the ground if you start to lose control on difficult terrain.
Deciding on the right bottom bracket height for a hardtail with a rigid fork is pretty easy but as soon as you introduce suspension (front or rear) it’s a whole different ball game. Like the rest of the frame geometry, when suspension gets involved the bottom bracket height will be changing constantly as you hit bumps and dips. The more suspension you have, the more it will change.
When you see bottom bracket height listed in manufacturer’s spec details, it’s usually the measurement of the distance from the ground to the centre of the bottom bracket axle. On a completely rigid bike, 11.5in to 11.75in is normal. With a short travel fork (100mm or less), expect 12in to 12.25in. With a longer travel fork (120mm plus), expect 12.5in to 13in. Longer travel full suspension bikes vary enormously, depending on what sort of riding they’re designed for. For general cross country use, expect medium travel bikes to have a bottom bracket height of 13in to 13.5in, and for longer travel ‘freeride’ bikes, 13.5in plus.
Whether the bottom bracket height feels exactly right for you or not will depend initially on how you like to ride. Some riders prefer the extra stability of a low bottom bracket and are quite happy taking more care when pedalling through corners. Others like more ground clearance for technical terrain.
Several other factors have a bearing on how your bottom bracket height affects the way you ride. How soft or how hard you like to run your front and/or rear suspension will affect how easily your frame geometry changes and how much your bottom bracket moves. How big and how soft your tyres are and how long your cranks are will affect ground clearance too. If you feel that your bottom bracket is too low, try fitting higher profile tyres on your bike, as some tyres are way higher than others.
The Bermuda Triangle
A frame’s rear triangle can hide – or bring to light – all sorts of evils. For a start, what it’s made of and its shape can change the way the back end of the bike feels. That’s why you’ll often see carbon seat and chainstay tubes grafted onto an alu mainframe, as carbon tubes will often deflect a little more trail vibration than alu tubes. Steel tubed frames are still popular for the same reason, although when all is said and done, making a frame more comfy is far more effectively done through tyre choice.
So what about all those curvy shapes appearing in rear triangles these days? Well, there’s no doubt that a tiny amount of trail judder can be better deflected through a load of curves than it can through a dead straight tube, but once again that’s of little consequence compared to tyres and the amount of air in them. Those curves are there more because of heel and ankle clearance, and to make more room on the frame for big tyres.
The length of a frame’s chainstays (measured from the centre of the bottom bracket to the rear wheel axle) can have some effect on the way a bike rides. The average is about 17in. Shorter stays help you to sit more over the rear tyre’s contact point with the ground and tend to slightly improve traction, but longer stays can improve your bike’s overall stability and help to keep the front wheel grounded on climbs.
Everyone wants to know which tyres are best. The answer is that none of them are. Of course, some are better than others for certain conditions and for certain types of rider, but the perfect situation is only ever very temporary.
But, and as always it’s a big but, this isn’t about tyres per se. It’s about how they can be a crucial element of frame design. So what’s all that about then? Well, we’re all guilty of obsessing over frame angles, the material our frames are made from or how heavy they are, but we’ll often manage to completely overlook the fact that we ride our tyres too hard, or too soft, or run a tread pattern or size that’s unsuitable for the riding we’re doing. Or perhaps our tyres are way heavier than they need to be… lightweight tyres can transform the handling feel of a bike.
Bear in mind that tread ‘squidge’ on tyres – via the knobs or via the tyre air pressure – can vary enormously from tyre to tyre.
For example, a round-edged tyre makes the bike feel different to a square-edged one. A square-edged tyre makes the steering feel more like it’s ‘on rails’, in that it always appears to be trying to pull the steering straight. And while grip is often initially superb when you lean into a corner, it has a habit of breaking away when you get to the point where the knobs bend over. On the other hand, a round-edged tyre feels slightly more free and easy on the steering and tends to corner more predictably, although it may not bite as hard initially.
So, the message here is clear: Think about upgrading your tyres before you obsess about shaving a few grams off on componentry.
Does minor componentry really make a difference to the way the bike feels? Well, yes… but more in some areas than others. We’ve already had a look at the way stem, handlebar, saddle and seat post positioning and comfort are big factors in the way you feel about the bike, but there are other bits of the bike that matter, too.
We’ve already mentioned tyres, and how their weight can have an influence on the way a bike feels, and obviously rims and inner tubes do, too. You’ll really feel the advantages of less spinning weight in how fast the bike accelerates. It’s a big reason for the increasing popularity of DIY tube-free systems using liquid Latex sealant and light tyres. You save spinning weight and the tyres roll slightly better over the bumps as they can mould themselves better around bump edges without a tight tube pushing back. Oh, and you’ll stop getting punctures.
Other factors are less significant. There’s plenty of hype about the supposed vibration absorption of carbon handlebars and seat posts, and a little weight saving here helps slightly, but your choice of grips and saddle will have far more effect than the carbon construction in terms of ride comfort. Little things like this really matter.
Finding the right handlebar width – remember that you can trim your bars if you find them too wide for your riding style – and finding just the right position for your brake levers and gear shifters can make a difference to the way you ride and the way you feel about a bike. Some riders like bar end extensions for cruising/ climbing, as they put you into a very different position on the bike.
Last, but my no means least, the way your feet can use the pedals is absolutely crucial to the way you feel about a bike. Some riders hate the idea of having their feet attached to clip-in pedals – often referred to as ‘clipless’ because they don’t have toe clips and straps – but there’s no doubt that being properly attached to the pedals can give you a boost in pedal power, mainly because it places your feet in the most efficient position and allows you to pedal more complete circles. It takes a few rides to get used to the idea of being attached to the bike, but after that clipping in and out becomes second nature.
THE MAGIC NUMBERS
Different bikes aimed at different rider types and different terrain will typically have different head angles. We say typically because there are always lots of exceptions to the rule. While early beach cruiser-style MTBs had very slack head angles, a status quo for rigid forked hardtails intended for regular cross country use soon settled at about 71 degrees. It remained that way for ages, but these days you can expect the following (these are static angles so think of active angles as 1-2 degrees steeper):
– Hardtail (HT) Full Suss (FS)
– Rigid forked XC HT = Still about 71° (fixed)
– Medium travel (100-120mm) HT and FS = 70°
– Long travel (120mm+) HT and FS = 69°
– Long travel (130mm+) freeride = 68°
– Downhill specific = 67°
© BikeRadar 2007