What is Motorcycle Rake and Trail?
By Alan Dowds
What do Rake and Trail Mean on a Motorcycle?
WHEN you’re talking about a new motorbike, the engine is, by far, the easiest thing to rate and discuss. How many cylinders, how much power, peak rpm, torque – it’s all very tangible and (relatively) simple to think about. More power and torque is always better, then you can choose the ‘character’ you want in terms of single-cylinder thump, twin-cylinder grunt, screaming inline four, or a variety thereof with a triple or a V-four.
Chassis tech is much more subtle though – especially nowadays, where things like tyres, brakes, frames and suspension generally just ‘work’ even in their most basic form. The days of old, where frames flexed, brakes and tyres didn’t really work and suspension struggled to keep things in check are long gone. But while design engineers have largely worked out how to make a bike chassis work well, it’s still not quite as easy to understand as an engine – for most of us anyway.
So – here’s the answer: Our quick and easy chassis tech jargon buster.
An idiot’s guide, if you will, to the main factors in frame, steering and chassis design. Read this, and you’ll be just as happy in the pub chatting about rake, trail, wheelbase and fork offset, as you are peak power and midrange grunt…
The way a bike steers is actually one of the more complex parts of motorcycle dynamics to think about. Compared with a car, say, it’s far more tricky to get your head around. The process is made complicated by the fact that above walking pace, a bike steers by leaning rather than as a car does, by simply turning the wheels.
To start with, there’s the idea of ‘counter steering’, which is to say that if you want to turn right, you first have to push the handlebars, er, to the left. Mad stuff. When you counter-steer, you’re moving the contact patch of the tyre to the opposite side of the turn, which makes the front wheel ‘fall over’ down into the turn basically, leaning the whole bike over. Then, the profile of the tyres is distorted, to effectively form a cone shape, and the bike rolls round, in the same way as an unopened Cornetto would roll across the kitchen worktop (if you put it down for long enough before eating it…)
Now, we hate to do this – but a diagram is really helpful here to talk about steering geometry. There are three factors here: Rake, trail and fork offset.
- Rake is, simply, the angle of the steering head/stem in relation to the ground. Some manufacturers measure it from the horizontal – calling this head angle – and some measure from a line perpendicular to the ground - which is rake. To swap from one to the other, take it away from 90°
- Trail is a little more complex – it’s the distance between the contact point of the tyre, and an imaginary line drawn from the fork to the ground, at the same angle as the fork
- Finally, Fork Offset is the distance between the centre line of the steering head/stem and the centreline of the fork legs, with the steering pointed straight ahead. This is determined by the yokes, and the offset plays a big part in determining the trail – more offset will mean more trail, less offset reduces trail
These three factors then, are the ‘secret sauce’ which determines how a bike steers and handles. They all have an effect, and they also affect each other. Broadly, less rake – a steeper head angle – will make a bike less stable and faster to turn. More rake does the reverse. You can do a little kitchen experiment here – next time you’re cutting pizza with a wheel cutter, try changing the angle you hold it at. If you have your hand directly over the blade – rake of zero degrees, effectively – you have much less control, and the blade can wobble all over the place. Sit your hand back, so it’s behind the wheel, and you can steer your way through the pizza with much smoother, gradual lines. Look at the pizza cutter as you twist your hand too – if the wheel is vertical (zero rake), then every degree you twist your hand (the steering if you like), the wheel rotates the same amount on the pizza. But if you have your hand at right angles behind the wheel (90° rake) and twist it, then the wheel doesn’t rotate on the pizza at all – it just leans from side to side. Imagine for a second that some mad custom hipster designed a bike with 90° rake – i.e. with the steering head parallel to the ground. It wouldn’t steer at all – the front tyre would point forward the whole time, just leaning to either side as you turned the bars. It would look insane (especially with a brown seat and no mudguard), but would be completely useless.
The amount of rake also varies the trail. Now, more trail again means more stability, because the contact point of the tyre is ‘behind’ the fork angle. The further behind, the stronger the ‘self-centring’ effect you get as you move along. Think of it as a lever – the further away the tyre contact point is, the stronger the lever effect it has on the steering position, pushing it straight again after any changes in direction. Less trail means smaller steering inputs have a bigger effect, since the self-centring effect is less.
Fork offset is generally used to fine-tune the relationship between rake and trail. So with more offset, you can have extra trail for a given rake, and the reverse also applies. In theory, you could then have the rake you want for a faster turn-in, but with extra trail to improve stability.
This is an easy one – sit the bike upright, and measure the distance between the points where each tyre touches the ground. That’s the wheelbase, and it’s a big part of a bike’s handling. In general, the longer the wheelbase, the more stable the bike’s steering is. That’s because the self-centring effect of the rear tyre contact patch increases as it moves further away, like the trail on the front tyre. Short wheelbase bikes will turn more easily and are less stable, because the self-centring or castoring effect of the rear tyre is less than on a long wheelbase machine.
A short wheelbase also affects stability in the pitch dimension though. So wheelies and stoppies are more extreme and harder to control on a shorter wheelbase bike. That’s why drag race bikes have massively extended swingarms, so they can apply more force to the rear tyre before it lifts the front end in a wheelie. For most bikes, the wheelbase is what limits the maximum acceleration in the lower gears, not the engine. You could have a 500bhp engine in your Fireblade, and it won’t get off the line any faster than a stocker, because the limit is how hard you can turn the rear tyre before the bike flips over backwards. Modern electronic wheelie control mitigates this by reducing the engine power/torque as the front wheel lifts – but then you’re just applying the same power as the standard machine, so will accelerate at the same rate.
The machine’s centre of gravity also plays a part here – the lower the centre of gravity the harder you can accelerate before the front wheel lifts (and the harder you can brake before the rear comes up). So, again, drag race bikes are very low, with the front forks compressed, a smaller 16-inch front wheel and as low-profile a tyre as they can get. There’s a unique Michelin A59 front tyre from the early 1990s which was in great demand amongst drag racers for years – it was off the Ducati Paso, which used a 16-inch wheel and a 130/60 tyre. Sadly, Michelin stopped making the A59 in the early 2000s, and drag racers are now looking at 15 and 14-inch maxiscooter wheels for their low-slung front ends instead…
Seems simple – but the devil is in the detail. The first job is to hold everything together – engine, swingarm, forks and yokes – and cope with all the forces going through it. But you also want it to be as light as possible, without costing a fortune. And, it turns out, you want it to be flexible in certain very narrow areas. As an example, when you’re leaned right over in a corner, especially on track, any bumps will be moving the wheels across the frame rather than up and down into the suspension movement. So you want a little bit of flex there to help absorb bumps while the suspension can’t do it so well. The same applies to swingarms: several race teams in the early 2000s ended up with enormous braced and triangulated swingarms which were too stiff.
There are a few main types of frame construction technologies used today – fabricated steel tube trellis and aluminium – either cast or cast/extruded fabrication. Some bikes use both: steel tube trellis beams attached to cast aluminium swingarm pivots and steering head. Cunning computer-aided designs allow engineers to incorporate the amounts of directional flex and stiffness needed, by using narrower steel tubes for certain parts, or by making the aluminium castings thinner in certain areas and adding extra strengthening ribs elsewhere. KTM’s Duke 690 swingarm design shows this off really nicely – the triangulated ribs are on view from the outer face of the arm, rather than hidden on the inner faces.
I tested a Kawasaki H2C 750 triple a few months back, and was prepared to be terrified by the legendarily-bad handling and crazed power. Now the power (maybe 70bhp on a good day) is frankly commuter-spec these days. And, as it turned out the bike handled fairly well for an old ‘un. “Ah!” said the Kawasaki expert I spoke to afterwards. “The ‘C’ model is a much better handling machine than the earlier models. The swingarm is a good bit longer and that made a massive difference.”
Sure enough, comparing the swingarm with the BMW S1000RR I had on test at the same time, and it was nearly as long. So – the length of the swingarm is a big part of how a bike feels. It improves stability – a little like having a longer wheelbase, without the downsides. Making the engine as short front-to-back as you can lets you move the swingarm pivot point further forwards, right up against the drive sprocket, while keeping a fairly short wheelbase.
Tyres and wheels
Arguably the most important part of a chassis setup, tyres are probably the area of bike technology that has improved the most in the past forty years – certainly in terms of grip (wet and dry) and mileage. At a more fundamental level, though, tyres do have a massive impact on the way a bike handles. Starting with the sizes – narrower tyres steer faster and give a more direct connection between the bars and how the bike moves. Wider tyres slow down the steering – but give more grip from a larger contact patch.
Wheel diameter also has a big impact on steering, and also affects the rake/trail measurements. A larger wheel has more trail, all things being equal, so is more stable. The big firms went from 19-inch front wheels on roadsters in the 1970s, down through 18” and 17” to 16-inch rims in search for better handling, before settling on 17 inches as the best all-round choice.
As with most aspects of chassis design, it’s a compromise, but modern bikes have converged on a narrow range of sizes: 17-inch wheels with a 120 width front and a 160-200 width rear tyre depending on power output. Dirt bikes, big adventure bikes, cruisers and mega-tourers often diverge from this, but even many of those are now moving towards common 17-inch sizes.
WHAT IT ALL MEANS IN PRACTICE
As ever, a story like this can only give you the basic mental foundations for what it means in reality. The best way – by far – is to try and get a spin on a few bikes with radically different chassis geometry, and have a think about the differences in how they feel compared with each other. Jump on an extreme custom cruiser like a Harley-Davidson Breakout, with very long forks and a raked-out front end, and you’ll feel how the steering ‘falls’ into slow corners, how turning the bars has much less effect on direction, and how slowly it changes direction. Similarly, riding a small sporty bike, like an old Aprilia RS250 will let you feel how steeper steering geometry plus a short wheelbase makes turning super-fast, with a much less stable overall ride.
Next, get on a proper dirt bike with a 21” front wheel and feel how it compares with a ‘normal’ 17-inch front tyre in terms of steering feel. Finish off with a ride on a Honda Melody with a 10” front wheel for the ultimate contrast.
One of the biggest lessons I got in chassis tech came about 30 years ago when I bought a 1976 Suzuki GT500 for a summer commuting while my Kawasaki GPz550 was getting an engine rebuild. For the first time, I understood what a flexible chassis feels like, with old-school tubeless tyres and weak steel tube frame and swingarm. Add in brakes which don’t work in the rain (not advised in Glasgow) and you have the perfect recipe to feel much better about modern machinery – while also understanding more about how chassis tech affects how your bike feels.