Trail is measured from the point on the ground intersected by a vertical line through the dropout to the point intersected by the head tube centerline. This is shown on the drawing in the photo.
Less trail causes the steering to feel quicker. In other words, it takes less turning on the bars to cause the bike to turn than a bike with more trail.
More trail causes the steering to feel slower. I other words, you have to turn the bars further to cause the bike to turn than a bike with less trail. Think of a chopper bike that has a really long front end. Its turning radius is very large.
Modifying the trail in a bike design will affect how the bike feels when riding slowly or at high speeds. A bike with a low trail can cause the steering to feel twitchy. A bike with a high trail will feel sluggish. It’s a balance that will favor one over the other or a compromise between the two.
A typical range of trail for a bike is 50mm to 70mm.
There are three variables the will affect the final trail calculation. They are wheel radius, head tube angle, and fork rake. Since each of those variables have impact on other aspects of feel of the bike, changing them has to be made with these in mind. I won’t go into all of those aspects. I’m just focusing on trail for now.
Here is the calculation:
wr = wheel radius
r = fork rake
hta = head tube angle
Trail = (wr * COS( hta ) – r)/SIN( hta )
If you want to know how to derive the formula, send me an email and I’ll send it to you. Those details aren’t needed to calculate trail. Just plug in the numbers to the formula and you’ll have the trail.
Increase the head tube angle or the fork rake and the trail decreases.
Decrease the head tube angle or the fork rake and the trail increases.
Put a taller tire on and the trail increases.
Put a shorter tire on and the trail decreases.
All of those changes will impact other areas of the design so keep that in mind. It’s a balancing act with tradeoffs.
Ok, so now you know how to calculate trail. Lets look at an example using my current electric bike project ShortHopper.
I was aiming for a trail of 50mm. I wanted to have quick steering because the bike will be used at slower speeds with lots of tight maneuvering. The parameters are:
Head tube angle = 73 degrees
Fork rake = 24.5mm
Wheel radus = 245mm
This gives a trail of 49.28. Ooops, I missed my target of 50mm. It turns out I made a mistake with my original calculation. To make matters worse, because of the anomalies that happen in fabrication, the head tube angle came out to 73.5 degrees. Also, the wheel radius measured is 242mm.
This gives a trail of 46.13mm. Oh wow. No wonder the steering feels twitchy. It could have been worse. I could have made the fork after fabricating the frame and introduced yet another error in the wrong direction.
These things happen, miscalculations and small errors that add up to big ones. As long as everything isn’t too far off or can be modified, it can work out.
So, how am I going to fix ShortHoppers twitchy steering? I can add a taller tire. That will help a little but not enough. I don’t want to change the head tube angle because that will require chopping the frame. The only thing left to do is change the fork rake.
I’m going to make a new fork with a rake of 11mm. This will give me a new trail of 60.21mm. I think that should fix that. I’ll make a new post to tell you how it turned out.
Tell me how your bike steers. Do you like it? Calculate the trail and compare it to how you sense the bike feels. Leave a comment.