Teach Me Suspension (Part 2): Simple Geometry Changes

As I said in the first part of this series, the words ‘geometry changes’ often strike fear into the hearts of many riders thinking about making changes to their setups.

In part 2 we are going to be talking about the most basic types of geometry changes that we can employ in order to change the natural characteristics of our bikes.

When looking to make geometry changes, there are two more typical ends of the handling characteristics scale we need to think about.

At one end you have steering ability, and at the other you have stability.

By steering ability I mean how your bike acts to steering inputs made by you. A more agile bike will have a tendency to turn quicker with less input needed to achieve your desired steering rate.

The trade off comes in the way of stability. The more agile you make your bike with geometry changes, the less stable your bike is going to feel.

Less stable simply refers to the bike feeling more twitchy and one that reacts more harshly to unwanted rider inputs or less than ideal surface conditions. This could mean a bike shaking in a straight line, or being more prone to the odd head shake at corner exits.

What you’re happy with for your setup in particular could differ depending on your experience and skill level.

Less experienced riders aren’t going to want a machine that’s bucking and weaving everywhere, because that will more than likely sap confidence.

Experienced riders on the other hand will have more refined techniques and skill to minimise these bike reactions – like being loose on the bike, for example – or they just won’t worry about them too much if they never threaten to get way out of control.

That’s something to bear in mind as you move forward – how you like the bike to feel.

What Creates These Characteristics?

The most typical changes that riders make in order to change their bike’s handling characteristics is to alter the ride height at either end of the bike.

Changing the ride height at either end will ultimately change the rake and trail at the front end, and as we learned last week it is changing those measurements that alters how the bike handles.

Decreasing rake angle and the amount of trail you have will make the bike turn quicker, but feel more twitchy.

Increasing them will make it turn slower, but feel more stable all round.

For every rider it becomes a trade off between getting the desired steering performance without sacrificing too much stability to the point where the bike becomes a bucking mess.

The best way to actually learn where on the scale you are most comfortable you need to get out there and test the differences. Before you do that you’ll never know.

Making Changes

When making changes to the front end, you may well have heard people talking about raising the forks or dropping the triple clamps.

What this is doing is lowering the front end by moving the triple clamps so they sit further down on the forks than they did originally.

As you can see in the (rather extreme) diagram below, dropping the triple clamps down the forks changes the angle of the forks relative to the road and in turn reduces the rake and trail.

What you’ll also notice is that the wheelbase shortens when you lower the front, which is one of the factors that causes the bike to feel more agile. This also happens when the rear is raised.

As a general rule, changing the ride height at the front by 4mm will equate to about a 1mm change in trail.

Making changes to the ride height at the rear can be done in a couple of different ways.

If you’re lucky enough to have a ride height adjuster, this is going to be the easiest way to not only make the change, but also the easiest way to actually change the trail.

The adjuster is typically found on aftermarket shocks and comes in the way of a threaded collar found at the bottom of the shock. There will be a locking nut that needs to be slackened off, then you can simply turn the adjuster.

Lengthening the assembly will raise the rear ride height, bringing the forks to a steeper angle relative to the road (like the above example) and will once again reduce the trail. Shortening the assembly will do the opposite.

Some bikes have a different setup and use a large rod in the rear suspensions linkage. This is most commonly found on Ducatis.

Measuring Changes

The ratio at which one turn on the adjuster, for example, changes the ride height will be different for different bikes and shocks.

For this reason it’s wise to take a measurement from a solid point on the swingarm to the rear subframe/seat in order to track the difference each change makes.

Typically one turn will equate to about a 2-4mm change in ride height, and as with front end changes, a 4mm ride height change at the rear equates to about a 1mm change in trail.

When using an adjuster, always make sure you are within the adjustment limits set out in your bike’s or shock’s user manual.

Don’t’ Have an Adjuster?

If you don’t have an adjuster, what you’ll find with a lot of bikes is that the manufacturers and some third parties will sell special shims that you can slide into the shocks top mount, effectively lengthening the whole arrangement.

The shims are designed so that you don’t have to completely drop the clevis so they can slide straight in. However if you can’t find the required shim for the bike, you can always get creative and place a couple of spacers in there yourself to do the same job.

The shims are typically 1 or 2 mm thick.

How Much to Start?

As I said, a 4mm change in ride height at either the front or rear will equate to a 1mm change in trail.

This might not sound a lot but to a seasoned racer or track day rider this change can be easily noticeable.

You may not notice the change, but it’s still well worth taking things slow to being with, that way you can feel the changes gradually coming into where you want them to be.

Always note down how each change felt to you. If it didn’t give you the desired result then keep going in the same direction until it feels right. If it got worse, go back the other way.

At this point it should be noted that while changing the ride height at either the front or rear will have the same effect on trail, they don’t make the exact same changes to your bike’s handling.

Keen racers and riders will favour using one method over the other, but less keen track riders or road riders may not need to concern themselves as much with which end the change in trail comes from, in which case I would simply say use a method of adjusting trail that is the easiest (front or rear).

The Limits

It goes without saying that there’s only so far you can take adjustments either way.

At the front, once the top of the fork is flush with the top clamp you’ve reached maximum height. When the tyre is in danger of touching the radiator or fairing, you’ve gone as low as you can.

At the rear, make sure you stay within the adjustment limits recommended for your shock.

If you reach the end of adjustment at either end, it’s possible to use one in conjunction with the other to get a little more out of it.

For example, lowering the front AND raising the rear is going to have more effect on trail than just lowering the front.

When you make changes at both ends though i.e. raising both or lowering both you will be having a bigger effect on the bike’s centre of gravity which will further change how the bike handles.

Other Things the Affect Geometry

Adjusting ride height will change rake and trail across the board and give you different characteristics during every part of riding.

However, you can also change how your rake and trail are affected during different areas of riding.

Changing damping settings is going to change how quickly your suspension extends and compresses, meaning that in the braking zone, for example, you can set your bike to have differing characteristics during that time.

This is something that we will also be covering in future parts, but for now concentrate on the simpler geometry changes we have covered here.

Preload also changes the front ride height and affects trail. Something that will be covered in the next part.

Until then, hopefully the above has given you some understanding as to how geometry changes affect the bike, which I hope makes the whole thing feel a little less scary for you.

And while there are other things that can be considered, the basics of geometry changes are really quite simple. Changes I hope you will now be more comfortable making.