Trail Calculation

[Staffy]

Help, Help, Help PLEASE.

I would like to install an American Suspension Springer (Dragon) on my '08 Mastiff. Have been in touch with Ted from AS forks, who has been a GREAT help and plan to speak to him further in trying to weigh my options. Here's where I'm stuck and need some help and advise.

If I were to work with the "stock" frame (NOT re-doing the neck/rake), I have two triple tree options - The Batwing (which has a 0 offset) and will give a trail of 3.5" or the Decapitator which will give me a trail of 1.2" (I'm not comfortable with either; from what I read reasonable/preferred trail is between 4"-6")

Now based on BDM's spec's the "stock" trail on a Mastiff is 4.81". Using the trail calculator at RB Racing - I come up with 7.73" - what am I doing wrong (have I got the offset numbers messed up?). RB Racing Rake and Trail Calculator

If the stock numbers do not add up then I'm probably feeding Ted from AS forks the incorrect info and getting back incorrect results of (3.5 and 1.2)

I do realize the calculating trail on a Spring is different, but if I can't get it right with Stock, I have to be wrong to begin with. Any and all help is appreciated. Thanks

 

[Staffy]

Update

Just got off the phone with Ted, one thing he did mention that these were "true" trail numbers. (Still NOT sure what that means) however he says border line true trail is 1.5" and anywhere around 2.5" is great. He also mentioned that raising the fork tube by about a 1/2 - 1" helps by putting another degree in the neck; bringing our rake to 40 degrees which is just about where they recommend using the Decapitator trees.

Am having him send me a cutout of the Batwing trees. If they do not interfear with the tank I will use them.

That aside would still like to solve the mystery of our stock trail numers - lol.

Thanks

 

[montana]

My best friend works for RB Racing. I'll pass this post to him and see if he can help.

 

[Staffy]

My best friend works for RB Racing. I'll pass this post to him and see if he can help.

Thanks a ton..:2thumbs::2thumbs:

 

[lee]

don't know if this helps at all. True trail seems to have a line intersected at 90 degrees. Not an easy diagram to follow. The 30 degree rake appears to have no rake in the trees. Also remember that most springers to my knowledge have no rake in the trees and that part is dealt with by the rockers. Not sure if springers have offset trees or not whereas the ones in the diagram do if I'm reading it right.

 

[alhall88]

Calculating springer trail is very difficult because of the rocker offset. I'm surprised Ted is telling you that you can use the decapitator trees for anything under 40 deg. When I put that springer on my bike there was no way around using the batwing trees because the actual neck angle was 36 on my 04 chopper. The batwing trees worked on my bike but I did have to add 1 inch spacers under the handlebars to clear the speedo and then I had to shorten the turning radius due to the fact that the trees would have smacked my. Rake is the angle of the steering stem that holds the frame and front end together as a pivot point. Trail is the relationship of steering stem rake and front axle location.

When rake is added to a motorcycle, trail is often increased causing steering to become heavier. Using "raked" triple trees move the axle further forward and shorten trail to a lower amount. Although the angle of the forks is changed, the rake of the steering stem is not changed.

A springer works the same way in the trail relationship of the steering stem and front axle location. Adding length to the axle rockers moves the axle forward similar to using raked trees or tree offset. Some of the more outragous appearing springer rockers not only improve trail handling but can also position geometry to reduce flop that comes from increased rake.

 

[Staffy]

Calculating springer trail is very difficult because of the rocker offset. I'm surprised Ted is telling you that you can use the decapitator trees for anything under 40 deg. When I put that springer on my bike there was no way around using the batwing trees because the actual neck angle was 36 on my 04 chopper. The batwing trees worked on my bike but I did have to add 1 inch spacers under the handlebars to clear the speedo and then I had to shorten the turning radius due to the fact that the trees would have smacked my. Rake is the angle of the steering stem that holds the frame and front end together as a pivot point. Trail is the relationship of steering stem rake and front axle location.

When rake is added to a motorcycle, trail is often increased causing steering to become heavier. Using "raked" triple trees move the axle further forward and shorten trail to a lower amount. Although the angle of the forks is changed, the rake of the steering stem is not changed.

A springer works the same way in the trail relationship of the steering stem and front axle location. Adding length to the axle rockers moves the axle forward similar to using raked trees or tree offset. Some of the more outragous appearing springer rockers not only improve trail handling but can also position geometry to reduce flop that comes from increased rake.

Thanks Al, use your "how to" as a bible as I go through this adventure. The reason I might be able to use the Decapitator is the Rake on the Mastiff is 39, which is border line, Ted only recommends them at 40 and over. He did mention by adding 1/2-1" high to the fork tube makes the rake "mimic" an additional degree - taking the 39 to 40, hence being able to use those trees. Like I mentioned he is sending me a cut out of the batwing trees, if they work well and good else I'm back to the drawing board.

The REAL CONFUSING part is these trail numbers, and what is this "TRUE" trail they keep refering to.....:angry::angry:

I read acceptable trail is between 4 to 6". The Mastiff claims to have 4.81" I cannot calculate that number from the info I have. Then Ted says 1.5 - 2.5 "true" trail is recommended....
(To top that all I have a 360 brake coming in for this setup, which is going to be an engineering feat by itself)....

This is not a suicide mission – just want it to be SAFE after all is said and done, hopefully I end up doing the right thing.

Am anxious to see what Montana's buddy comes up with .

 

[montana]

Am anxious to see what Montana's buddy comes up with .

I sent the info to him this morning. It may take a few days to get an answer. I don't know how much my friend knows about calculating trail but I'm sure he'll be happy to assist if he knows.

BTW, I tested the prototype for the improved RB Racing Black Hole Spyder pipe a few weeks ago (on my Harley). These guys make a great product and the know their shit! I'm hoping to put one of their pipes on my Mastiff.

 

[alhall88]

Staffy read this link from the Vince Costa the owner of American Suspension. It explains the true trail a bit clearer.
Custom Motorcycle Suspension Tuning - Rake And Trail - Street Chopper Magazine Online

 

[Staffy]

Staffy read this link from the Vince Costa the owner of American Suspension. It explains the true trail a bit clearer.
Custom Motorcycle Suspension Tuning - Rake And Trail - Street Chopper Magazine Online

THANKS AL
That's IT...:2thumbs::2thumbs: I now know what TRUE TRAIL IS :2thumbs::2thumbs:

From the article -
A lot of builders will measure trail from the tire contact point to the point where the steering axis intersects the ground. We call this false trail. To determine vehicle dynamics, you need to measure the leverage that the tire has over the steering axis. The correct measurement is the perpendicular distance from the steering axis to the tire contact point. To clarify things, we often call this measurement true trail.

 

[Staffy]

Intresting....

This has been bugging me so in an attempt to solve the mystry, I kept staring at the bikes only to realize that the OFFSET is DIFFERENT – I remember reading somewhere on the forum that our bikes have a 1” offset, while may be true but, NOT in all models/years. A quick “approximate” measure shows the’07 K9 at 2” and the ’08 Mastiff at 3” – based on that the calculations come pretty “close” to the BDM trail spec 4.81" (Mastiff) – hope that helps especially for anyone working on forks. Offset does have a significant impact on trail – bottom line measure all specs independently where ever possible. :2thumbs: