Rake, Trail, Offset and Your Bike...
I get a lot of questions on why people need different offset triple clamps than stock or why they should buy clamps from me and I guess the simple answer is you don't NEED them (Done! Shortest blog post ever, kidding) but you might want them. You don't need them if your not chasing specific numbers in set up on a bike. Same as you don't need sticky track tires but those touring tires just don't quite have the same grip on the track do they?
Lets seem if I can't help muddy the water further for you...
So what is Rake?
Rake is simply the angle neck angle vs a vertical line as illustrated above in my crude drawing above. Generally speaking the numerically lower the number the the lower your trail number will be on a bike with the same length forks.
Example would be say a touring bike might have a rake angle of something in the neighborhood of 30-33 degrees, while a sport bike generally speaking might be in the low to mid 20 degrees or closer to vertical.
So what is Trail?
Trail is the the distance from the center axis of the steering head or stem and a vertical line coming down from the bikes front axle or tire contact patch.
And finally what is offset?
Ever notice your triple clamps aren't in a straight line across with the steering stem and forks (directly inline) with each other? This distance is the offset, the offset of the steering stem to the fork center.
So how does it all relate?
Remember when I said motorcycles are a series of trade offs? A lot of customers have questions of what offset they need for a bike, and really all I can say is "Depends..." Its totally dependent on the requirements of the bike and how you feel the bike handles best. Testing is really the only way to know for sure what you feel is better FOR YOU.
We can manipulate trail numbers by moving the tire with fork offset in the triple clamps (Did I mention I make those?). Generally speaking, smaller offset numbers will increase the trail, larger will decrease trail. This is why just using OEM clamps for another bike, while the might fit, might not do the best things for trying to get your bike set up... but man that bike looks sweet with the USD forks on the crash truck. That bike probably didn't have the same rake to start with.
Another benefit is while you are able to adjust trail with fork height its pretty limited and offset will have no effect on the rear of the bike. Height stayed the same you're just moving contact patch forward or backward.
Trail generally has a range people will feel is acceptable. For simplicity lets skip numbers and just talk about it quantitatively larger or smaller. All things equal the bike with a smaller of trail is going to turn easier and have less resistance to change direction than the bike with a numerically larger trail number. So handles better, right? Not so fast... However, the bike with larger trail number is going to be more stable at speed and in the corners. It just comes down to how you're transitioning force into the tire. A bike with lower trail number is going to be more nervous on harder brakes also. Totally depends on what you need or want the bike to do.
So how do you determine what you need? The best advice I can give is work with a suspension tuner. It all comes down to pace, personal preference, and the bikes overall characteristics. In the last year I've read several books, made hours of phone calls and had bikes professionally measured to work on different set ups.
A great tool I've found super helpful is MotoSpec Chasis Software (http://www.motospec.ca/) . It allows you to see what the effect changes have on set up numbers on the bike as a complete system. Say you want more swingarm angle, well what did that do to trail numbers or my bikes anti-squat? Need to change gearing but bike is handling great and afraid to have it effect set up? It has a table for that. I've worked with Jeff on a couple projects and found him and his software to be a great resource.
I've also worked a lot on setup with Eric Vallarta at Velocity Calibrations (http://www.velocitycalibrations.com/) Worst life decision Eric ever made was give me his personal cell phone number. Eric has helped me set up several bikes, both my own and several of the bikes in the MotoAmerica Paddock.
Its a deep rabbit hole and if you're just tooling around on track maybe just simply setting sag will get you to a place you can turn some laps with friends, in which case save your money, you don't need my clamps. Use that money for track time. If you're planning to build a dedicated track bike or a bike for a racing series I'd love to help and if I can't maybe I can point you to who can.