Motorcycle: Front Geometry

Due to motorbikes having 2 wheels instead of 4, they have significantly less geometry to think about. However, the geometry that they do have is adjustable and affects the handling of the bike in a big way.  The geometry categories are split into front geometry and rear geometry. This section will cover front geometry and how to adjust it.

Front Geometry

There are 3 main areas of geometry for the front wheel. They are:

• Offset
• Rake
• Trail

Offset is the distance from the steering axis to the centre line of the front axle. Alternatively it is the distance from the steering stem to the fork centre line. The diagram below shows the offset distance of the front wheel geometry. It is often set and governed by the fork triple clamp that mounts the forks a set distance away from the steering axis.

Rake is the angle of the steering axis from a vertical centre line through the front wheel down to the contact patch. The rake angle is one of the areas used to set trail and is governed by the frame steering axis mount and ride heights.

Trail is the horizontal distance measured along the ground between the extrapolated line of the steering axis and the vertical front wheel centre line. The trail is the main factor that determines how a bike steers and how stable the bike is on the road or circuit. If you increase the amount of trail on the front wheels you will increase the stability of the bike but will require more effort to change direction.  The increased stability is especially useful at high speeds where the rider needs to have full confidence in the bike.

How to Adjust Trail

The main areas used on the road and on circuit to adjust the trail of the front wheel are by adjusting either rake angle, offset or front tyre/wheel diameter.

The most common way to adjust trail on a street bike is by adjusting ride height as road bikes have a limited amount of adjustable components with relation to geometry. However, adjusting the rake angle by increasing or lowering ride height can have negative effects on how the bike handles due to the other areas being altered when ride height is changed. Raising or lowering an end of the bike will also angle the steering axis and angles the entire chassis. This can raise or lower the centre of gravity position with increases or decreases in ride height which will alter the way the bike likes to change direction. It also changes the static loads on the front or rear wheel which means that the bike will respond to throttle or braking inputs differently with the ride heights are altered. The ride height can be altered at the front by sliding the forks up or down in the triple clamps and is adjusted at the rear by adjusting the rear shock length.

In order to increase trail at the front wheel the front of the bike can be raised or the rear of the bike can be lowered. As a rule of thumb used for street bikes a 4-5mm ride height alteration will produce a 1mm change in trail length. Therefore, significant ride height changes can be required in order to produce a noticeable difference in the handling due to trail.

On racing bikes the trail is usually adjusted with an adjustable triple clamp which changes the offset distance and can be adjusted in fine increments to fine tune the handling of the bike. Aftermarket triple clamps can be purchased for road bikes to allow this adjustment and remove the need to change ride heights instead.

Another method used on racing bikes is having inserts in the frame which adjust the steering head angle directly which will alter the rake angle and therefore the trail of the bike. The frames are custom designed to include this point of adjustment and is not a possible aftermarket fix on a road bike without carrying our extensive work to the frame.  Alternatively a larger or smaller diameter tyre can be installed on the front wheel which will also change the trail of the bike.

In order to increase trail the following adjustments can be made:

• Install A Larger Front Tyre
• Increase Rake Angle
• Decrease Offset Distance
• Raise Front Ride Height
• Lower Rear Ride Height

The opposite adjustments can be made in order to reduce trail.

The Effects of Adjusting Trail

When making adjustments to trail through offset or rake angle it must be done carefully as adjustments in these areas will directly affect the stability of the bike. Not only is the stability important when cornering but also when hitting a bump in the road that causes the wheel to deflect off centre to the right or left.

Why Adjust Trail?

Giving your bike a longer trail dimension or “more trail” will increase the self centering capability of the front wheel and will increase stability. This is beneficial on long fast circuits or roads where you want the bike to feel secure at speed in a straight line. The longer trail dimension will also make the bike respond to bumps in the road less as it naturally wants to stay on a straight line. However, the longer trail dimension also means that the bike will be more difficult to steer as it will require more rider effort to turn the bike. This means that on fast switchback roads, the bike will feel heavy and unwilling to change direction quickly. It will improve the corner exit behaviour of the bike because when you exit the corner and begin applying throttle the bike will naturally begin straightening up and self centering down the straight.

If you reduce the trail dimension of your bike or give it “less trail” it will reduce the effort required to make the bike change direction. This means that the bike will feel much more responsive to turn in and change of direction so is ideal on fast switchback circuits as the bike is now much more sensitive to steering inputs. However, bumps on the road will be fed back through the steering much more aggressively meaning that a lot more rider effort is required to keep the bike in a straight line which can be un-nerving and dangerous on high speed straight sections. Therefore, it can be unwise to set a road bike up with race bike geometry as potholes and large bumps in the road are not featured on track so you could be putting yourself in danger when riding on the road.

What Is The Best Way To Adjust Trail?

Adjusting trail by altering the rake angle can have a negative effect on the steering lock of the bike. By reducing the rake angle, in order to reduce trail, most bikes will experience a reduction in steering angle making the turning circle much larger which can be a nuisance on a road bike where tight turning conditions are frequent. Also, adjusting the rake angle does not tend to move the centre of gravity positon of the bike. Although this sounds like a benefit, it can be a negative as race bikes often use adjustments that also move the centre of gravity slightly forward when trail is being reduced.  This slight increase in mass on the front wheel will help bring back some stability in the front end even though the trail is reduced, helping the bike to corner and change direction faster without all of the losses of straight line stability.

Race bikes usually adjust trail with adjustable triple clamps which allow the offset to be adjusted in small increments. Adjusting offset also has the effect of moving the centre of gravity forwards or backwards. A reduction in trail will increase the mass on the front wheel slightly. Alternatively, an increase in trail will reduce the mass on the front wheel as well. Moving offset will also alter the bike’s wheel base which is another feature contributing to the stability of the bike.

On many road bikes, having adjustable triple clamps as standard is not an option. Therefore, rake angle is selected via ride height adjustments to change the trail. Adjusting ride heights to have an effect on trail often has large effects on other areas of the bike that would not otherwise be adjusted if the offset was the altered aspect. To show why offset is the preferred method of adjustment the below example figures have been taken from a Ducati Corsa.

For a 1mm decrease in fork offset:

• Trail Increases 1.1mm
• Wheelbase Decreases 0.9mm
• Centre of Gravity Height Increases 0.2mm
• Percent of Front Wheel Mass Increases 0.05%

For a 1mm increase in rear ride height:

• Trail Decreases 0.4mm
• Wheelbase Decreases 0.2mm
• Centre of Gravity Height Increases 0.8mm
• Percent of Front Wheel Mass Increases 0.03%

For a 1mm fork raise in the triple clamps:

• Trail Decreases 0.2mm
• Wheelbase Decreases 0.5mm
• Centre of Gravity Height Decreases 0.4mm
• Percent Of Front Wheel Mass Increases 0.06%

You can see from the figures above that in order to change the trail by 1.1mm the rear ride height would have to be moved 3 times more than the offset distance making the centre of gravity height move 12 times more than if the trail was adjusted using offset. This large increase in the height of the centre of gravity will make the bike respond much differently in corners so needs to be done with caution. Also, the figures show how raising the rear ride height and lowering the front are not equivalent adjustments. This is due to the rear ride height increase raising the centre of gravity and the front ride height decrease lowers the centre of gravity which have opposite effects on how the bike corners despite them both reducing trail. The effects of centre of gravity position will be covered in our “Centre of Gravity” article.

Therefore, it can be greatly beneficial if you are keen to adjust the trail on your bike to install adjustable triple clamps which are available for some bikes as aftermarket parts. Also, as mentioned previously be careful when setting up your bike particularly if it is used on the road as the demands of a road bike are much different to that of a racing bike.