While out testing with Josh Loncett we decided to look at the impact of adjusting the stack
height of his bike on his fork data. I've noticed since that there seems to be very little available online about this, maybe it's too basic? Either way, I did a bit of a test with my old DH bike to give some data that may help you understand some changes you can make to your DH bike.
One of the advantages of having triple clamp forks on you DH bike is that you can fine tune your head angle. But how much does changing different stack height and push through change the head angle of the bike?
By 'push through' I am referring to how much of the stanchion sticks out above the top clamp. If the stanchion is flush with the top of the top clamp I would describe there as being 0mm of push through. As shown here (excuse the dirt on my bike!)
If you loosen all clamps and slide the stanchions through like this then you raise the front end of the bike. because you've slid the stanchions out from the lower clamp you have raised the front from below the headset. This will force the wheel further out in front of the bike and you have therefore slackened the head angle and increased the wheelbase of your bike.
A slacker head angle and longer wheelbase will give the bike more stability at speed and it may help on steep sections of track. On the other hand the bike will be less responsive in tight turns.
By contrast you can loosen the clamps and slide the stanchions through. Like this:
This arrangement lowers the front end and brings the wheel in, shortening the wheelbase. This steepens the head angle. The steeper head angle will be slightly less stable at speed but will be more responsive in corners. You may find it a bit easier to lift the front end when required too.
But how much is the angle actually changing?
This will depend on the wheelbase of the bike, so these values are based on my old Trek Session, size medium. They will be pretty close though and if you want to be precise, read on for how to do it.
I worked from 0mm push through and took the head angle. Then pushed 5mm through and took the angle again. I repeated this to 20mm. Results gave this relationship:
So, basically, every 5mm you push the stanchion through will equate to a tightening of the head angle by 0.2% or vice versa. This assumes that you do not alter stack height (number and position of headset spacers).
Decreasing push through could be a good option if you are riding/racing a fast track with few tight turns, think Forth William. Increasing push through could be a good option on a tighter, twistier track like Innerleithen.
This alters the overall height of the cockpit which you may not want to do.
This time we're working with the arrangement of headset spacers. In this image all but one spacers are below the crown:
This time we are only working with the top crown, we are not altering the lower crown. That
means that we won't change the head angle but we will change the height of the cockpit. Lowering it will shift your weight forward which can help with front wheel grip. On the other hand you will be more 'over the front' on steep sections. Raising the top crown will have the opposite effect. Your bike is going to start looking a bit odd if you slam it and have the stanchion pushed right through, but, as long as you've left enough stanchion to get full travel, you can ride that. So, max push through and slammed will give the steepest possible head angle.
You may find that you change both though, particularly as you get to grips with a new bike. With Josh we moved 10mm of headset spacers from below the crown to above it. Lowering the cockpit. we stuck to the same amount of push through though. That meant that we effective increased push through (of the lower crown) by 10mm and slackened the head angle by 0.4degrees. This way we were able to slacken the head angle for that speed without changing the overall height of the cockpit.
So, assuming you keep the same amount of push through at the top crown, we can estimate the effect of taking out headset spacers and it's basically the same.
So, assuming you keep the same amount of stanchion perdruding the top crown the graph above will be roughly correct. Every 5mm headset spacer removed will correspond to roughly 0.2degrees slacker head angle.
For maximum effect you can run 0mm push through and slam the headset. Assuming you take 10mm from below the top crown and push 10mm through, you could change your head angle by 0.8 degrees. That's more than enough to slacken my 5 year old session off to match the head angle of most modern DH bikes.
This stuff is certainly worth experimenting with!!