I'm fiddling around. When I have the tank, frame, fork, and wheels together I'll start posting info. All I'll say is at this point is "it will be comprehensive". Lot's of design flipping through my head.
Lurker's V-twin build
- Thread starter Lurker
- Start date
To clarify our conversation with John,
I would like to explain a few things: the angle of the steering head, whatever the number would be, has to match the fork type being used!
Leading link forks need more rake because the wheel is further forward
then on a trailing link fork. The only "magic number" is that the
two lines drawn - following the center line head tube to the ground and
the front axle straight down - are crossing at some point. The distance
where the two lines hit the ground is the trail. We are running about 2.5 inches on our bikes.
The dynamics while driving are not as easy.
Factors are resistance of steering bearings, weight of front fork (cobrafreak has a very heavy front fork which helps him getting higher
speeds before getting into trouble) tire pressure,tire size (patch)
and weight distribution front to rear wheel with rider on it.
Over the years 1910 and up manufactures have increased the rake
to give the bikes more stability and allow the driver to enjoy the ride
instead to manhandle the bike constantly. Not to mention the affect
using the front brake or uneven road surface.......!
Lurker, you build a very very nice bike! It's not painted and you still
could make a change to it! You have lots of power ..... think about it!
I hope you don't take this the wrong way!!! It's an honest concern!
Roland
Thanks for the info, im clueless when it comes to this sort of thing. I dont suppose you or someone else could start a thread to have this information at our fingertips. Not only would it be very helpful to any of us wanting to build our own frame, or like you said to match the style of fork we run...... not only would this info be helpful, but as we start using bigger engines you could help prevent some serious accidents.
I am with Harry on this one. I am hit and miss guy when building frames, had to cut the head tube out a few times so this would be helpful.
I remember my conversation at the bike show with Matt and it got me thinking of how much I missed the stability of the Indian. I used to be able to sit up and remove my hands from the bars and it would be laser strait at any speed. When I made the leaf spring function able I added three inches of lead to the fork and it worked to take shock out but resulted it a wobble if I let go of the bar. I now today reverted to the original fork design. The leaf spring is still there for looks it is non functioning. I rode around today and I realize it was the right choice. It rides like a dart now. I just need to watch for ruts more 
rake and trail can make or brake a ride. look at speedway bikes almost 0 deg rake helps controlling sideways slides but makes for great tank slappers. with a chopper like 38 deg they could not bring the rear of the bike around because instead of turning the front wheel it will just lay over. trail is a different bird but is the reason modern choppers have such a large gap between the motor and the neck. to get the trail right. people have fought over this for years but is one of the reasons why dirt oval race frames make lousey street bikes. just my 2 cents and i would not rely on it. dennis
Big boy, I appreciate your concern and your insight into this issue. I value your knowledge being a professional bike builder who does phenomenal work. However I am failing to see what is wrong with my design.
I went and rechecked my measurements, to that of the 1923 HD from which I took a lot of inspiration from.
MY BIKE
Rake:18 deg.
Triple Clamp Offset: 2"
Fork Length: 21.5
TRALE IN INCHES 2.82
1923 HD
Rake:20 deg.
Triple Clamp Offset: 1"
Fork Length: 21.5 assumed not fact checked.
TRALE IN INCHES 3.85
I am failing to see a cause for concern. I know mine is about 1" less but I could always change the linkage to make up the difference, if I feel that there is a need for it. As it currently sits I do not see a need to go back and change the head tube angle.
Do you have any numbers for your bikes? I would like to compare and contrast. I know the styles and goals of our bikes are totally different, but I am still interested in seeing how different the numbers are.
I realize you guys are building a bike for consumers who may or may not have previous motorcycle experience and safety is a huge factor for you. On the other hand I am building a bike for me. I want it to have the same look, feel, handling, and sound as the originals, with my own unique take on it.
Please do not take any of this the wrong way. I value your opinion and hope we can all learn something from each other.
I went and rechecked my measurements, to that of the 1923 HD from which I took a lot of inspiration from.
MY BIKE
Rake:18 deg.
Triple Clamp Offset: 2"
Fork Length: 21.5
TRALE IN INCHES 2.82
1923 HD
Rake:20 deg.
Triple Clamp Offset: 1"
Fork Length: 21.5 assumed not fact checked.
TRALE IN INCHES 3.85
I am failing to see a cause for concern. I know mine is about 1" less but I could always change the linkage to make up the difference, if I feel that there is a need for it. As it currently sits I do not see a need to go back and change the head tube angle.
Do you have any numbers for your bikes? I would like to compare and contrast. I know the styles and goals of our bikes are totally different, but I am still interested in seeing how different the numbers are.
I realize you guys are building a bike for consumers who may or may not have previous motorcycle experience and safety is a huge factor for you. On the other hand I am building a bike for me. I want it to have the same look, feel, handling, and sound as the originals, with my own unique take on it.
Please do not take any of this the wrong way. I value your opinion and hope we can all learn something from each other.
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I think you are going to be OK with it Lurker. I think it's similar to a lot of other fork angles out there but it looks steeper because it is a taller bike. Just optical illusion. All you would need to do to fix any undesirable handling attribute is shorten the leading link slightly, I.E., move the axle in a hair. The reason I don't think it will be any problem the way it is is because you have low bars and this is forcing more weight on the front tire. This is good by a handling standpoint. Fork head angle, degree of rake, center of gravity, weight of front end, all have to be considered at the same time. I love your fork btw. It makes me want to do a HD build.
Your BIKE
Rake:18 deg.
Triple Clamp Offset: 2"
Fork Length: 21.5
TRALE IN INCHES 2.82
Two lines drawn - following the center line head tube to the ground and
the front axle straight down - are crossing at some point (before hitting the ground). The distance where the two lines hit the ground is the POSTIVE trail.
If this measuring does indeed produce 2.82 inches you will be just fine!
From your pics it looks like the lines are NOT crossing! This would make ALL
the difference!
With negative trail (lines are not crossing) the bike will handle well at low speeds but will easily develop a dangerous wobble at high speed.
Our bikes have 2.5 inches positive trail. Your bike may have 2.82 inches
negative trail....
I hope I am wrong, but please verify your measurement.
Roland
Rake:18 deg.
Triple Clamp Offset: 2"
Fork Length: 21.5
TRALE IN INCHES 2.82
Two lines drawn - following the center line head tube to the ground and
the front axle straight down - are crossing at some point (before hitting the ground). The distance where the two lines hit the ground is the POSTIVE trail.
If this measuring does indeed produce 2.82 inches you will be just fine!
From your pics it looks like the lines are NOT crossing! This would make ALL
the difference!
With negative trail (lines are not crossing) the bike will handle well at low speeds but will easily develop a dangerous wobble at high speed.
Our bikes have 2.5 inches positive trail. Your bike may have 2.82 inches
negative trail....
I hope I am wrong, but please verify your measurement.
Roland
So when I was doing the math I forgot to take in to account that there is a linkage at the bottom of the fork. This linkage pushed the front wheel out 2.8 inches making my trail 0. However I came up with this simple solution, Just put the axle in the same holes as the truss part of the fork. Doing this gives me the 2.5 inches of trail, that the linkage was taking up. I have seen this done on a few other bikes. I just cant believe it didn't dawn on me earlier. I still have to modify the linkage, but you guys get the idea.
On a side note I love the way it look like this, it looks a lot lower and meaner. In the photo you can also see the second exhaust pipe that I finished up on sunday.
On a side note I love the way it look like this, it looks a lot lower and meaner. In the photo you can also see the second exhaust pipe that I finished up on sunday.
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Erm, stupid question alert. Does the wheel clear the front pipe? Probaly opticle illusion but it never hurts to ask.
I hope you didn't use those lines in the pic for your math. The rake line looks like it is on the fork tube, not the head tube.
Did you measure the way you did draw the lines on the picture?
You have to draw the line along the center of the STEERING head!
NOT the fork down tube!
Roland
I did, I was under the impression that if you have a triple tree you measure from the fork legs. So I did some research and it seem you are right.
If I measure from the head tube I have about 1" of trail. My wheel is now as far back as it can go with out hitting something. I don't think I have any other options left. Will 1" be enough? I know some speedway bikes have 0, so 1" must be better than 0 or negative. Right?
If I measure from the head tube I have about 1" of trail. My wheel is now as far back as it can go with out hitting something. I don't think I have any other options left. Will 1" be enough? I know some speedway bikes have 0, so 1" must be better than 0 or negative. Right?
You always use the pivot point - HEAD TUBE - !
Always measure on the bike directly! Doing math on paper
can hide flaws in the end result! (two strings, tape and a ruler!)
The range which will work is between 2.5 and 4 inches trail.
Back in the "good old days" they may had less.....but people
got hurt.......!
Roland
I think I am going to have to redo my front end, but before I do could some one please explain to me what is going on with these two bikes. they both seem to have about 1 inche of positive trail.
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Unfortunately using photos isn’t necessarily the best method of calculating frame geometry.
Variables like having the having the camera perfectly centered between the axles, the fork being turned a little, an under inflated tire, or just getting your center lines drawn true and square can effect the results.
I found this attached image with a quick google patent search, and even then this doesn’t mean it was actually used on a real production bike or my center lines are true and square...
(BTW, there is a wealth of information to be found there!!!)
My own personal experience building a heavy bike with a very heavy leaf sprung fork (perhaps 200 lbs overall) was trail of 1-3/4”
That was by seat of the pants engineering that took me 5 sets of rockers, 2 sets of front struts, and then finally changing the head tube rake.
I’m only able to reach speeds in the low 30 mph and it feels good there, at faster speeds I’m afraid I may want even more rake.
-Kirk
Variables like having the having the camera perfectly centered between the axles, the fork being turned a little, an under inflated tire, or just getting your center lines drawn true and square can effect the results.
I found this attached image with a quick google patent search, and even then this doesn’t mean it was actually used on a real production bike or my center lines are true and square...
(BTW, there is a wealth of information to be found there!!!)
My own personal experience building a heavy bike with a very heavy leaf sprung fork (perhaps 200 lbs overall) was trail of 1-3/4”
That was by seat of the pants engineering that took me 5 sets of rockers, 2 sets of front struts, and then finally changing the head tube rake.
I’m only able to reach speeds in the low 30 mph and it feels good there, at faster speeds I’m afraid I may want even more rake.
-Kirk
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The rake lines in these photos don't look accurate to me. They are out of parallel with the fork legs. If you stkie those lines parallel, the trail looks pretty good.
If it were me, I would take this oppotunity to add some rake.
I measured a bunch of different stock road motorcycles once when I wanted to design a front fork for a trike. Seems I remember most were real close to 21 degrees.
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