Rake and trail for dummies

[barnett468]

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Billy, hope you have a happy Thanksgiving and here's a couple of the movies I mentioned to you.

1985 AMA 500 cc road race with Kenny Roberts, Randy Mamola and Mike Baldwin doing wheelies all the way down the front straight at over 100 mph after about the 4th lap.

https://www.youtube.com/watch?v=J7_WAeCe8oE


This is the Isle Of Man road race which is the fastest road race in the world thru city streets and country highways and reaching speeds of 200 mph . . A box stock Kawasaki H2R recently broke the track top speed record there during an exhibition lap at 206 mph.

https://www.youtube.com/watch?v=iRWp9rhfS_0

[El Camexican]

By no means am I claiming to know anything about this topic, but a few things have come up while trying to do the math on my KTM conversion that I thought were worth sharing on this old thread.

In no particular order...

- The "race" trikes from the 80's that used leading axle forks are the Tigers, Tecates, 350X and the Cagivas. The 250R & Tri Z used trailing axles

- According to RB Racing that provide a sweet trail calculator on their website https://www.rbracing-rsr.com/rakeandtrail.html a leading axle needs to be considered in determining your rake. In other words, if you have a 0 degree rake in your forks and a steering head that is 25 degrees when parked, then the distance of the center of the axle from the centerline of the fork tubes should be considered as additional rake. In the case of my KTM frame it adds about 1.5 degrees to the rake.

- A leading axle makes your front wheel look smaller to a trail calculation as it moves the contact patch away from the centerline of the steering head. In the case of a 22" tire with a 36" long fork it reduces the value by .558" (if I understand them correctly)

So, the stock trail for my KTM bike is 4.40" If I shorten the leading axle forks 1", use a 22" tire, 2.5" offset triple clamps with zero rake built into them and don't modify the stock 25 degree steering head I will end up with 2.29" of trail. Considering that all the stock trikes Barns listed earlier in this thread were under 1.8" (Tiger 250 specs say 1.7" of trail) I guess that's not bad for something meant for sand and it can always be altered with rear end height and fork length.

I should also post this up https://www.customtripletrees.com/MA...AGES/tech.html

"A growing number of motorcycles, domestic and imported, are being converted into a 3 wheel chassis or are installing a side car to the frame. These motorcycles are similar in style and looks, but create a whole new challenge in steering. Adding the third wheel changes the standard trail measurements. Trail reduction is needed to offset handling irregularities created by using a third wheel. A growing number of trike and side car riders are calling on us to solve their handling problems. In most stock frame applications, a 4 to 7 degree raked tree set solves the majority of handling complaints, including steering wobble and slow speed wheel flop. With the wide variety of rake angles offered in our triple trees, a set can be built to solve any of your steering problems."

Disclaimer: I know NOTHING about this topic, but I thought I should share my confusion on this topic with others

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1984 ATC250R
rake ............................ 69 mm . This is measured from horizontal instead of vertical so using current methods, the rake is actually 31 degrees but it is unknown if this is the forks or the steering head.
rake forks ....................... ?
trail ............................. 45 mm . . 1.8 in
w.b .......................... 1197 mm . 47.1 in
dry weight ......... ..... 302 lbs
dry bias ..................... ft 117 . rr 185 lbs
bias percentage ........ ft 38.7 . rr 61.3
axle position ............. trailing
tripple trees .............. unknown but look staggered which would make sense


1985 ATC250R
rake .............................. 21 deg
rake forks ........................ ?
trail ............................... 37 mm . . 1.45 in
w/b ........................... 1295 mm . 51.00 in
dry weight .................. 291 lbs
dry bias ...................... ft 117 . rr 174 lbs
bias percentage ......... ft 51.3 . rr 48.7
axle position .............. trailing
triple trees ................. unknown but im guessing they are staggered the same as the 86'


1986 ATC250R
rake .............................. 21 deg
rake forks ..................... 30 deg ? . . if this is the fork rake then these tripple trees are staggered 9 mm
trail ............................... 38 mm . 1.5 in
w/b ........................... 1905 mm . 75 in
dry weight .................. 289 lbs
dry bias .................... ft 116 . rr 173
bias percentage ......... ft 50.9 . rr 49.1
axle position ............... leading This might not be correct. Oh wait, the edit button thing.
tripple trees ................ staggered 9 mm?


1984 and 1985 TECATE
rake ............................. 24 deg
trail .............................. 40 mm . 1.57 in
w/b .......................... 1280 . 50.4 in
dry weight .................. 286.6 lbs
wet weight ................. 312
wet bias ..................... ft130 . rr 181
bias percentage ....... . ft 42 . rr 58
axle position ............... leading
tripple trees ................ non staggered


1986 TECATE
rake ............................. 24 deg
trail .............................. 48 mm . 1.57 in
w/b .......................... 1280 mm . 50.4 in
dry weight ................. 280 lbs
wet weight ................ 299 lbs
wet bias .................... 121 . 178 lbs
bias percentage ........ ft 40.5 . rr 59.5
axle position .............. leading
tripple trees ............... non staggered

[BOB MARLIN]

I'm still not getting how a leading axle would change the rake. Rake is a frame measurement so how does the leading axle change the rake in the frame ?. Are they saying that leading axle changes the trail so they have to change the rake to bring it back in to spec ?. I'm not trying to contradict, just trying to understand more about steering geometry. Adding "Rake" to the triples is a way of adjusting trial but the rake on the frame remains unchanged, other than the position of the front tire would move forward or backward changing the height of the of the frame neck, which would in effect ,change the rake of the neck as it relates to the ground. Do you think that's what they are getting at ?.

[El Camexican]

I'm still not getting how a leading axle would change the rake. Rake is a frame measurement so how does the leading axle change the rake in the frame ?. Are they saying that leading axle changes the trail so they have to change the rake to bring it back in to spec ?. I'm not trying to contradict, just trying to understand more about steering geometry. Adding "Rake" to the triples is a way of adjusting trial but the rake on the frame remains unchanged, other than the position of the front tire would move forward or backward changing the height of the of the frame neck, which would in effect ,change the rake of the neck as it relates to the ground. Do you think that's what they are getting at ?.

I agree with you that the rake on the frame is just that, but I think what they are trying to say is that it doesn’t matter what the frame rake is unless the trees have a zero rake and the axle is positioned at the bottom of the forks. In that scenario the frame rake is suitable for use in calculating theoretical trail.

However, my understanding of how a leading axle adds to the rake is that true rake for the purpose of calculation is the angle of the center of the top of the triple tree stem hole to the center of the axle. So while one may have a 25 degree angle on their steering head, the true rake for the purpose of calculating theoretical trail depends on whether or not they have a rake built into their clamps, as well as where the axle is located.

At the end of it all the final word is the physical measurement of the distance between a straight line through the center of the steering head to the ground and a vertical line from the center of the axle.

I’m going to speculate that when a factory like Honda, or Yamaha uses triple trees with an angle build into them its because they wanted to change the handling characteristics of the vehicle after the tooling for the frames was completed, rather than any sort of advantage that would come from having a few degrees built into the clamps.

[BOB MARLIN]

I agree with you that the rake on the frame is just that, but I think what they are trying to say is that it doesn’t matter what the frame rake is unless the trees have a zero rake and the axle is positioned at the bottom of the forks. In that scenario the frame rake is suitable for use in calculating theoretical trail.

However, my understanding of how a leading axle adds to the rake is that true rake for the purpose of calculation is the angle of the center of the top of the triple tree stem hole to the center of the axle. So while one may have a 25 degree angle on their steering head, the true rake for the purpose of calculating theoretical trail depends on whether or not they have a rake built into their clamps, as well as where the axle is located.

At the end of it all the final word is the physical measurement of the distance between a straight line through the center of the steering head to the ground and a vertical line from the center of the axle.

I’m going to speculate that when a factory like Honda, or Yamaha uses triple trees with an angle build into them its because they wanted to change the handling characteristics of the vehicle after the tooling for the frames was completed, rather than any sort of advantage that would come from having a few degrees built into the clamps.

Got it.
So they have you adding in a factor for leading (or I assume trailing also) because their formula is based on the axle being on centerline of the fork leg, because that would be the only constant for the calculation. After the bike was all together you can measure the actual rake (at the frame) and the actual trail at the centerline of the axle. At that point it doesn't matter if it is leading or trailing- it is what it is. So they are adding a variable for the leading axle only because their formula is based on a centered axle, not because it actually changes the physical rake. Does that sound logical or am I confusing everyone including myself ?.

I believe when Honda added the so called "Rake" to the 250r triples it was to shorten the trail to correct some steering issue after the frames were already set.

[ironchop]

Nevermind... Was just repeating what someone else said already

This is a good article. It's how I figured out that I was measuring trail wrong back in the day.... Check the diagrams

https://www.streetchopperweb.com/rake-and-trail#page-3

Sent from my Z958 using Tapatalk

[ironchop]

In my opinion, raked trees are to correct excessive muscle movement needed in order to turn the handlebars with the rake and trail being set at optimal locations

If you think about it, since our forks aren't straight up and down, the wheel and tire technically kind of flops from side to side when you turn the forks and it's especially notable when you have alot of rake. Long bike chopper guys have to keep ahold of those handlebars or the front end "flops" over to the side when you turn the bars as a natural tendency with that kind of geometry.

I'm pretty sure that the force required to turn the handlebars left and right are also figured into the rake and trail and that's why I think you have raked trees is to try to affect the amount of force or leverage needed to turn the bars left or right at a specific and optimal rake/trail setting

That's just my guess. I'm probably way off base but that's my best guess

Sent from my Z958 using Tapatalk

[El Camexican]

Got it.
So they have you adding in a factor for leading (or I assume trailing also) because their formula is based on the axle being on centerline of the fork leg, because that would be the only constant for the calculation. After the bike was all together you can measure the actual rake (at the frame) and the actual trail at the centerline of the axle. At that point it doesn't matter if it is leading or trailing- it is what it is. So they are adding a variable for the leading axle only because their formula is based on a centered axle, not because it actually changes the physical rake. Does that sound logical or am I confusing everyone including myself ?.

I believe when Honda added the so called "Rake" to the 250r triples it was to shorten the trail to correct some steering issue after the frames were already set.

I think I agree with everything you just wrote

[El Camexican]

In my opinion, raked trees are to correct excessive muscle movement needed in order to turn the handlebars with the rake and trail being set at optimal locations

If you think about it, since our forks aren't straight up and down, the wheel and tire technically kind of flops from side to side when you turn the forks and it's especially notable when you have alot of rake. Long bike chopper guys have to keep ahold of those handlebars or the front end "flops" over to the side when you turn the bars as a natural tendency with that kind of geometry.

I'm pretty sure that the force required to turn the handlebars left and right are also figured into the rake and trail and that's why I think you have raked trees is to try to affect the amount of force or leverage needed to turn the bars left or right at a specific and optimal rake/trail setting

That's just my guess. I'm probably way off base but that's my best guess

Sent from my Z958 using Tapatalk

Earlier on in this thread I think there was mention of a leading axle being easier to turn. I've also read that its better for sucking up bumps, hence all modern dirt bikes having leading axles.

As far as ease of steering I wouldn't be surprised if a raked triple tree provided some sort of change to the amount of force needed to move a wheel.

I do know that the front end combination I have on my stretched Suzuki is terrible for everything but straight line stability. I need to go back to it and determine if I messed up on the front end, or if it all relates back to the 8" over swing-arm. I want to say my trail came in around 6"+ and that the rake is about 37 degrees. Never looked at the off-set during the build. Sure would be nice if it steered a little lighter.