Motorcycle Geometry | EXPLAINED

|0 Comments |11:53 pm

If you are into racing, I’m pretty sure
you have heard riders talk about the so called ‘setup’ of their bike, and how this influenced
their pace. A layman probably assumes that this so called
‘setup’ has all to do with the suspension settings, and this wouldn’t be entirely
wrong but there is more to setting up a bike than you might think. Let have a talk about motorcycle geometry. Entire books have been written about motorcycles
geometry, these books are not easy to grasp, even for an engineer.

And this is for a good reason, because motorcycle
geometry and dynamics are not easy to understand. So in other words; one YouTube video on this
subject is not going to make you the next Jeremy Burgess. This video is meant to give you a basic understanding
of motorcycle geometry, and why it is so important. Now if there is one takeaway we can learn
from books on motorcycle geometry it is this; every choice made in setting up a motorcycles
geometry will be a trade-off. Wait, so what does that mean? Well, say you move the fork tubes up in the
triple clamps, which is a very common change racers do to their motorcycles, because it
makes their motorcycle steer in quicker.

I mean, that’s a good thing right? Moving the fork tubes up in the triple clamps,
will decrease the trail, and decrease the wheelbase, which will indeed make the motorcycle
steer in quicker. It will also make the motorcycle less stable,
and potentially harder to control. Moving the fork tubes up also lowers the ride
height, which means you might not have enough ground clearance, and it decreases the swingarm
angle, which decreases the the anti-squat tendency of the bike. Squat refers to the tendency for the rear
suspension to compress on acceleration. Countering that tendency is called anti-squat. A characteristic of the chassis that tries
to extend the rear suspension on acceleration. So moving the fork tubes up in the triple
clamps will make the bike squat more, which can cause you to lose traction on the front
wheel, causing you to run wide on corner exits. So sure, the bike will steer in quicker going
into the turn, but you also might not get out of the turn. You also need to consider that all of these
characteristics change when a rider is on the bike and the motorcycle is moving. For instance, the front forks will compress
when braking going into a turn, which will cause the same changes in characteristics
as moving the fork tubes up in the triple clamps.

I’m pretty sure this is you right now. But don’t worry, by the end of this video
these terms will start making a lot more sense to you. Just like in math, you have to start by learning
the basics, such as addition and subtraction, long before you get into multivariable calculus. To understand a motorcycles geometry, lets
start from the front and work our way backwards. Up front we have:
– Front forks – Triple clamps
– Wheel So the front forks are mounted into the triple
clamp and the wheel attaches to the lower fork legs. The triple clamp is mounted to the frame at
the steering stem.

The angle between the steering axis with respect
to a vertical line is called rake. The distance between the fork centers and
the steering stem is called fork triple clamp offset. The wheel + tire has a certain diameter and
lastly, we come to the most important measure, called trail. Trail is a measurement of how far the contact
patch of the front tire is behind the point where the steering axis hits the ground. You get the number by extending an imaginary
line through the steering axis to the ground, then you draw a perpendicular line through
the front axle to the ground. So why is trail so important? Well trail is what makes it possible to stay
balanced on two wheels, because it forces the front end to stray straight when the motorcycle
is driven forward. So the further the wheel is behind the steering
axis, the more stable the bike is and vice versa. So less trail equals quicker steering bike,
but also a more unstable bike.

The effect is obvious if you have seen a motorcycle
going down the track in a straight line when the rider fell off. The bike wants to go in a straight line because
of trail. As you can see, trail can be adjusted by changing
the rake angle, the fork triple clamp offset and front tire diameter. If you fit a smaller front tire, decrease
the rake or decrease the triple clamp offset, you will lessen the trail. To understand trail a bit better, lets compare
four very different motorcycles. First up, we have a KTM 450 SMR, with a trail
of 90mm Then, we have a Aprilia RSV4, with a trail
of 105 mm Ducati multistrada 1200, with has a trail
of 109 mm. Harley Davidson sportster with a trail of
117 mm Which of these motorcycles do you think turn
in the quickest? Cleary the KTM is going to turn in really
quick, with the Aprilia close by, since the Aprilia has a lower center of gravity as compared
to the KTM, but more on that later.

The Multi and the Sportster clearly turn in
slower than both of them. So why would you ever want more trail than
say the RSV4, I mean, it’s not like people who ride RSVs on the street are flying of
the tarmac because it is so unstable. Well, its not like its very comfortable riding
an RSV4 on a bumpy backroad.You see, the environment where the bike will be ridden is very important
when you determine what trail it should have. For instance, the off road version of this
KTM, which has a 21” front wheel, has a trail of 110 mm.

Because the front wheel has a larger diameter,
the trail is larger. So why would you want a bigger trail on a
dirt bike then? Well on a dirt bike you want to balance straight
line stability with agility. Since these bikes will be ridden over rocks,
in sand, essentially in environments where the front wheel will be pushed around, you
want the bike to help the rider keep it going straight. Remember what I said in the beginning of the
video, everything is a trade off. Of course, smart engineers have worked on
this for a while, and they have figured out how to get a stable motorcycle with a very
aggressive trail; the steering damper. Motorcycles that come stock with a steering
damper usually have a trail so aggressive that they would experience head shake going
in a straight line, but the damper keeps this in check. Now lets talk about the rear of a motorcycle. Remember I mentioned that thing called Anti-squat. So what exactly is that? Well, when you accelerate on a motorcycle,
intuition would tell us that the rear suspension should compress. I mean it makes sense right? Load is transferred to the rear, and thus
the rear suspension should compress.

This is not always desirable, for obvious
reasons if you think about it for a second. If the rear suspension compresses when you
accelerate, you would run out of travel, meaning you won’t have any rear suspension at all. Also, if you transfer too much load to the
rear the front wheel would lose traction and you would run wide in turns. What’s needed here is a balance, which is
easiest to explain as a percentage that indicate how much weight transfer is offset by anti-squat. If you have for instance 60% anti squat, its
effect offsets 60 percent of the weight transfer on acceleration, and thus the suspension would
still compress. At 100% anti squat, the suspension would not
move due to weight transfer. The amount of anti squat one wants depends
on riding style and overall bike setup, but generally speaking some squat is needed to
load up the rear for traction. So how do you magically tune this so that
he bike only squats as much as you want it to? Well it aint magic, its just geometry.

Lets have a look at the rear end of a motorcycle,
look at where the swingarm is connected, this is called the swingarm pivot point, now look
at the rear axle. Draw a line though the pivot point and the
rear axle, then imagine a horizontal plane that goes through the pivot point. The angle between the plane and the line is
called swingarm angle. Imagine what happens if the rear wheel is
driving the bike forward, because of the upwards angle of the swingarm, the suspension is extended. The second force enabling anti-squat comes
from the chain pulling on the axle in a direction parallel to the top chain run. The anti-squat tendency of the chassis changes
as the suspension is compressed, because the swingarm angle is changed.

Most motorcycles are setup in a way where
anti squat gradually decreases as the suspension moves through its travel. The anti-squat characteristic on a bike can
be altered by changing the chain angle or swingarm angle. On a MotoGP bike, the swingarm pivot point
and rear axle position can be altered quite significantly. So thus they can change the swingarm angle
a lot For most of us though, we can’t change the location of the swing arm pivot point. What we can change quickly though is the front
and rear sprockets. For more anti-squat effect you can fit a smaller
front sprocket and a larger rear sprocket on the bike. Vice versa, fit a smaller rear sprocket and
a larger front sprocket for less anti-squat. For more anti-squat you can also raise the
rear ride height, which will increase the swingarm angle. Lets zoom out now, and have a look at the
wheelbase. Or simply, the distance between the front
and rear axles.

A long wheelbase has the effect of making
the bike more stable but harder to transition, and vice versa. Lets again compare the same motorcycles. Aprilia RSV4 wheelbase 1,420mm
KTM 450 SMR has a wheelbase of 1,481 mm Harley Davidson sportster 1,519 mm
A Ducati Multistrada has a wheelbase of 1,530 mm Clearly, the Harley Davidson and the Multistrada
provide a stable comfortable ride, whereas the KTM and the Aprilia are more agile and
turn in quick. An important factor to consider here, is that
the wheelbase changes when riding the motorcycle. Because you now know about anti squat, you
know that the rear end will rise when accelerating hard. This will cause the wheelbase to decrease. The dynamics of all of the components together,
and how their attributes change in different scenarios is crucial when discussing motorcycle
geometry. Lets get into that after we have learnt about
center of gravity.

When talking about center of gravity we need
to know its vertical position and its horizontal position in the wheelbase. The horizontal position of the center of gravity
is important for the acceleration and braking characteristics of the motorcycle. A motorcycle with an extended swingarm, will
have a center of gravity more forward, which will prevent wheelies under heavy acceleration. A more forward CG will also put more weight
on the front wheel and provide less grip in the rear. The vertical position of the center of gravity
will have a big impact on the handling of the motorcycle. A low center of gravity makes the bike easier
to steer in, and provides more in corner stability, but requires more lean angle for a given speed. A higher center of gravity forces the rider
to use more force to lean the bike over, provides less in corner stability, but requires less
lean angle for a given speed.

A bike with a higher center of gravity is
also easier to flick from side to side. As you could guess, Sportbikes generally have
a low center of gravity, providing great stability in corners and making them easy to steer in. A Supermoto on the other hand, has a high
center of gravity, which requires slightly more force to steer in but also requires less
lean angle at the same cornering speeds. Now you know why the local Supermoto guys
are passing you in the corners, hah.

Generally sport bikes have a Center of gravity,
with the rider in the seat, in the midway between the front and rear wheels, or a 50/50
weight bias. A Supermoto has a weight bias of roughly 50/50,
as well, but the rider can affect this to a great extent, since they can move forward
or backward a lot in the seat. Now lets circle back to the so called bike
‘setup’, but now lets use the knowledge we just gained, and understand what the goals
here are. When the rider brakes, the weight will transfer
forward, the trail decreases, the wheelbase decreases and the swingarm angle increases.

This makes the bike unstable, but also very
easy to turn in. What you want is to setup the bike so that
it will turn in as aggressively as possible, while not causing the rider to crash because
of instability. That is the goal of suspension and geometry
tuning for braking. When the rider accelerates, weight will transfer
rearwards. So the trail will increase, the swingarm angle
will increase and the wheelbase decreases. The motorcycle will become stable and wants
to go in a straight line. What you want here is maximum traction and
stability. That is the goal of suspension and geometry
tuning for acceleration. As you know by now though, the increase in
trail and changes in the anti squat behavior of the swingarm can cause the bike to run
wide at corner exit. If you combine the two, what you want in a
race bike is a bike that turns in quick but provides adequate stability. Whereas in a touring bike, you want a bike
that provides great stability but still turns in at an adequate pace. You dig? I hope you learned something new today and
that this video inspired you to look more into this fascinating field.

That’s it for today, over and out..

As found on YouTube

Extremely light racing helmet >


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