Jump to content

Geometry


faffi
 Share

Recommended Posts

I'm currently reading The Suspension Bible and have found it to provide a lot more information than just suspension stuff. Like geometry.

Many of us have heard that if the rear axle sits above the swing arm pivot, we get pro-squat under acceleration, and that if it sits below we get anti-squat. Well, turns out it is far more involved than that.

 

First off, virtually every streetbike have a built-in anti-squat effect. The amount of the force, however, can be manipulated. You can get so much that it tops out and lock the suspension solid, or you can get so little that the rear end squats under power. But it squats from weight transfer being greater than the anti-squat effect, not because there is any pro-squat forces at work. This was news for me.

 

If you have a bike/rider combination with a 50/50 weight distribution you can theoretically accelerate at a rate of 1G before you'll start to wheelie. If the anti-squat forces run directly through the CoG, you will have a 200% anti-squat effect, locking the suspension from movement as it will support the whole weight of the machine. What a chassis maker will want, is about 100%, that is where acceleration does not affect the suspension in either direction. That is not easy to achieve on a regular motorcycle since the anti-squat effect will vary with suspension travel - hit a bump and it is reduced, hit a dip and it is increased - but they can get reasonably close. If the output shaft and swingarm pivot sits on the same axis, suspension travel will not effect anti-squat and explain why these two are so close together on a modern sportsbike; to minimize the effect. A longer swingarm will also influence the anti-squat less for any given amount of wheel travel than a shorter arm. Other things that will influence the anti-squat effect are sprocket sizes, wheel placement, wheel diameter, ride height and more.

 

Another thing that influence the behaviour of our bikes far more than I imagined is trail. Yes, more trail equal more stability and heavier steering. But according to the book, more trail, at least to a point, also increase traction. Improper trail is apparently also why so many motard riders are backing their bikes into corners.

 

However, what I've never thought of is that trail can become negative quite easily. If you hit a bump, the tyre's contact patch moves ahead. On a big bump, it can get far enough forward to create negative trail. Imagine what that will do to the stability of your bike, especially if you hit several big bumps in succession. The same happen if you ride slowly in deep sand (or snow); you push a load of it front of the tyre, in effect moving the contact patch ahead and quickly end up with negative trail. So the lack of control is not just lack of skill, it has a physical explanation.

 

The more I learn about motorcycling, the less I know. Motorcycles are incredibly complex, yet usually also highly forgiving. When I think about all the quite drastic changes I've made over the years to suspension, both in length and in springing/damping, it's amazing handling haven't been more messed up. Usually, the changes have been a success. As such, I'm glad I didn't read this book sooner as it probably would have prevented me from making changes in the first place in fear of making a good bike into a poor one :D

Link to comment
Share on other sites

It's funny how many people are afraid to touch their suspension, especially if they have already had it "set up" by someone. Turns of preload, clicks of dampers, spring lengths, ride height, oil height - they can all be measured and recorded, and returned to previous settings if necessary. So many riders look at their motorcycle suspension like great grandpa looks at a computer - afraid to press the wrong button in case he ruins it.

 

Thanks for the review of squat and anti-squat. It's something I don't think about much because I tend to leave rear preload and ride height alone for long periods (like, seasons) and concentrate on damping adjustments, but it's good to be reminded that a rear preload adjustment might best be compensated with a ride height change if I don't want to alter the geometry in a manner that will affect squat (amongst other things).

 

To be honest, I am not sure that all of this is quite as sensitive as some make it out to be. For example, I recently made a huge front preload change (like, 5 mm) without touching fork height. In theory this should change the geometry enough to make an important difference....but all I noticed was that I was not using as much of the front travel, as intended. My point is that, if you are riding well within the performance envelope of the chassis, moderate or even kind of large adjustments don't really have such profound effects. Maybe it's only when you are really pushing things to the limits that you start to see major benefits (or otherwise!) from small tweaks.

 

Of course you can make mistakes that will matter, especially on aftermarket components that tend to have a larger adjustment range than OEM. For example, I could set rebound so stiff at either end of my bike that the suspension would be practically locked down. That's very bad - but it is an extreme example. Minor ride height adjustments are not something that the average rider is really going to notice much.

Link to comment
Share on other sites

I also generally find that I can make quite big alterations without much impact on handling, at least at my pace. I fitted 20 mm custom made Ikons to the back of my old Z650 and the most noticeable change was increased stability around bumpy corners due to sufficient rebound damping. Some bikes have been more sensitive; my VT500 Ascot was fitted with 40 mm longer and stiffer rear shocks and suddenly couldn't be cornered unless I hit the throttle early. Once I fitted stiffer front springs that raised the front static height about 30 mm it once again acted neutrally.

 

However, when you start looking for tenths or even less, I'm sure these issues matter. The book mentioned some top rank motard rider who dropped his lap times by 1.5 seconds with no other changes than getting proper trail due to a new triple tree - which is substantial on a 45 second lap. The author even claimed riders with less skill could gain twice that!

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...