MY 1978 XS chopper/ Custom

[jwhughes3]

trailing link

Aren't trailing link what was on the old Indians? Is that the one good for sidecars or is it leading links?

John

[fredintoon]

Aren't trailing link what was on the old Indians? Is that the one good for sidecars or is it leading links? John

Hi John,
those old 1930s leaf spring trailing link Indians had a suspension movement of less than 2" and a front brake that was pathetic compared to a modern one. More up to date attempts at trailing links, such as the 1960s Ariel Leader and Lambretta scooters, showed extreme fork dive on braking.
BTW, check out Wild Bill Gelbke's Roadog, that had trailing link forks too.

http://thenewcaferacersociety.blogspot.com/2008/05/wild-bill-gelbkes-road-dog.html

What sidecar tugs need is reduced steering trail.
Link style forks, be they leading or trailing, are easier to modify to do that than tele-forks are.
Sidecar or solo, trailing links give fork dive on braking, and that's a bad thing.

[crazy steve]

There's more than a bit of voodoo involved in bike suspension design, but his trail number of under 1.5" is troubling to me as that would suggest a bike with very light, very fast steering that might buy you a tank-slapper. Stock XS trail is 5.1", so that's a huge reduction. Like Fred said, reduced trail is good for a sidecar rig, but not so good otherwise. I see the shock tuning as a real problem too.

The craftmanship looks great (must be nice to have a friend with a CAD machine), but the functionality of the design looks a bit spooky...

Maybe he should have checked out this: http://www.tonyfoale.com/ This guy has forgotten more about bike suspensions than most will ever know.

[roccet]

Hi roccet,
yeah, complicated little thing, ain't it?
As to it's strength, even if offroad riding stresses are more than you see on the road, it's carrying mebbe 30% of what an XS11 weighs.
And, while your main girders look like they came off the Forth bridge, his suspension link is a U-shape while yours are single arms which puts all the flex resistance onto the wheel axle.
And your design is still a trailing link.
Trailing links suck.

I see what your saying. I had considered that fact but looked at other examples of bikes such as the Honda VTX 2011 proto, BMW protos and a couple other bikes that had a trailing arm system. The systems looked a bit different than mind but the function of it is still the same.
So what in your opinion would go wrong or the potential issues you see?..
You mention Flex.. can you expand on that theory for a second?

[roccet]

trail. 1.5 vs 5.1

There's more than a bit of voodoo involved in bike suspension design, but his trail number of under 1.5" is troubling to me as that would suggest a bike with very light, very fast steering that might buy you a tank-slapper. Stock XS trail is 5.1", so that's a huge reduction. Like Fred said, reduced trail is good for a sidecar rig, but not so good otherwise. I see the shock tuning as a real problem too.

The craftmanship looks great (must be nice to have a friend with a CAD machine), but the functionality of the design looks a bit spooky...

Maybe he should have checked out this: http://www.tonyfoale.com/ This guy has forgotten more about bike suspensions than most will ever know.

Lets address the 1.5 inchs for a second. The wheel is ahead not behind.. I wanted the wheel itself to remain in the stock position. only the Forks "look" like there raked out.. When choosing a position i went on to serveral websites.. http://www.streetchopperweb.com/tech...ail/index.html
is just one of many i checked out..
I made sure that if my wheel was to be anywhere it was ahead not behind the stock location. From the description they call it rake and trail so trail can be either side front or behind.. im infact in front. I was not aware of 5.1" of trail.. Can i get a confirmation of that??? if so thats an easy change on my end.. All i did was measure the distance of the center of the wheel before i took it from the frame. and tried to keep it in the stock location then i figured out my rake and trail from my cad.. That 5.1 will be a big difference in my ride. Thank you so much for pointing that out.. I am very greatful.. i will try to confirm that number and change my design accordingly..

[DGXSER]

Tony,

The trail can be positive or negative. Positive trail is when the line of the steering "axis", rotational centerline, extends down to be in front of the vertical plane of the axle, or the line between the center of the axle and where the wheel touches the ground. Negative trail is of course just the opposite. Your setup from what I see would be a negative trail, as the vertical plane of the axle is ahead of the point where the steering axis meets the ground.

[crazy steve]

Lets address the 1.5 inchs for a second. The wheel is ahead not behind.. I wanted the wheel itself to remain in the stock position. only the Forks "look" like there raked out.. When choosing a position i went on to serveral websites.. http://www.streetchopperweb.com/tech...ail/index.html
is just one of many i checked out..
I made sure that if my wheel was to be anywhere it was ahead not behind the stock location. From the description they call it rake and trail so trail can be either side front or behind.. im infact in front. I was not aware of 5.1" of trail.. Can i get a confirmation of that??? if so thats an easy change on my end.. All i did was measure the distance of the center of the wheel before i took it from the frame. and tried to keep it in the stock location then i figured out my rake and trail from my cad.. That 5.1 will be a big difference in my ride. Thank you so much for pointing that out.. I am very greatful.. i will try to confirm that number and change my design accordingly..

The 5.1" trail number is right out of the factory manual (look in 'specs' in the back). You're right, negative trail is downright dangerous (axle behind the steering axis). But too little positive trail can be bad too. There's a lot of factors to consider, such as total bike weight, percentage on the front, and neck rake. 'Most' street bikes have between 4 and 5 inches of trail, with some of your sportbikes creeping below 4 inches; you very rarely see trail below 3 inches. But these are lighter bikes, so controlling the 'wobbles' is much easier that it would be on a heavier machine. And many have steering dampers fitted from the factory to prevent this. Reducing trail gives light, responsive steering, but the trade-off is stability.

On a large, heavy bike like the XS, too little trail will probably only show up when you least want it; hitting a bump, or high-speed cornering. This can give 'head shaking' leading to a 'tank slapper' where the bars oscillate violently back and forth. I've experienced this, and it ain't fun. If you want to check your sphincter control, this is a good way to test.

I see some of the 'chopper' guys claiming low trail is fine (and that has as much to do with selling raked trees as anything else), but most of the big retailers won't go near low-trail 'conversions' with a ten foot pole, so what's that tell you...

[fredintoon]

I see what your saying. I had considered that fact but looked at other examples of bikes such as the Honda VTX 2011 proto, BMW protos and a couple other bikes that had a trailing arm system. The systems looked a bit different than mind but the function of it is still the same.
So what in your opinion would go wrong or the potential issues you see?..
You mention Flex.. can you expand on that theory for a second?

Hi roccet,
those protos you mention are just that, protos.
You don't see them in the showrooms as yet, eh?
OK, if a well engineered trailing link suspension has it's brake reaction torque transferred to the main fork beam with a pantograph linkage it won't nosedive but your sketch don't show such a linkage.
Flex?
Hold the front wheel between your knees and reef on the bars. The way the bars move and the wheel don't is flex.
The stock XS tele-forks flex like a bastard.
Try that on a BMW Earles fork or on a Greeves LL fork, they'll hardly move at all.
Some of that rigidity is due to the fork configuration and some is because the "links" the wheel bolts into is a U-shaped loop which, combined with the axle, makes a complete box.
Your two separate links are only braced by the axle.
Your use of separate links instead of a closed loop reduces the torque resistance of a Rectangular Hollow Section to that of a C-channel.

[fredintoon]

Lets address the 1.5 inchs for a second. The wheel is ahead not behind.. I wanted the wheel itself to remain in the stock position. only the Forks "look" like there raked out.. When choosing a position i went on to serveral websites.. http://www.streetchopperweb.com/tech...ail/index.html
is just one of many i checked out..
I made sure that if my wheel was to be anywhere it was ahead not behind the stock location. From the description they call it rake and trail so trail can be either side front or behind.. im infact in front. I was not aware of 5.1" of trail.. Can i get a confirmation of that??? if so thats an easy change on my end.. All i did was measure the distance of the center of the wheel before i took it from the frame. and tried to keep it in the stock location then i figured out my rake and trail from my cad.. That 5.1 will be a big difference in my ride. Thank you so much for pointing that out.. I am very greatful.. i will try to confirm that number and change my design accordingly..

Hi roccet,
quote from Clymers XS1100 manual:-
Wheelbase 60.8" (1,545mm)
Steering head angle (AKA rake) 29.5º
Trail 5.12" (130mm)
So if you have kept the stock stance, wheelbase and rake you still have the stock trail no matter what configuration the forks have.

[roccet]

trail..

Tony,

The trail can be positive or negative. Positive trail is when the line of the steering "axis", rotational centerline, extends down to be in front of the vertical plane of the axle, or the line between the center of the axle and where the wheel touches the ground. Negative trail is of course just the opposite. Your setup from what I see would be a negative trail, as the vertical plane of the axle is ahead of the point where the steering axis meets the ground.

Thank you. I understand all of that perfectly and its pretty well explained on that link i had above although like it was mentioned by someone else its geared toward choppers but the same principle and equations . No i do not have Negative trail. It is positive by 1.475 but i will be changing that to stick with the standard 5.1" that comes stock on the bike. I only need to change two parts and it brings me back to stock location. I did take all of that into consideration before drawing those forks.

[roccet]

trail and rake


Here is a better view placed at 29 degrees.
.

i can adjust from 3.125 - 4.25 just by changing my upper bracket.
Its currently sitting right at 3.125 positive.
I can also shorten my lower arm to get to 5.1"
those lower arms BTW are 1.375 thick and 1.75 wide. so flex wont be a problem.
I was just putting some numbers together and 1.25 thick 1.75 wide will start to tweek at about 500lbs at 12 inchs in length.. so it looks like im not to worried.
My lower arms are 7.5 inchs long I am thinking of making them 2.5 inches wide
for extra support just in case. .

[DGXSER]

ummm Tony, that IS Negative trail. Positive Trail would have the vertical line intersecting the ground BEHIND where the steering axis line meets the ground.

At least my eyes are not as bad as I thought they might have gotten.

[roccet]

ummm Tony, that IS Negative trail. Positive Trail would have the vertical line intersecting the ground BEHIND where the steering axis line meets the ground.

At least my eyes are not as bad as I thought they might have gotten.

I am getting mixed signals here. move the wheel forward or back?
I moved this wheel forward according to the Website i had linked.. and a couple others concidering trail and rake in that they also said 2-3 inchs of trail is what is recommended. And even showed a picture showing direction.
If i move back it will be super twitchy.

I guess i was getting my terminology backwards. So Yes i am Negative Trail at the moment.

[fredintoon]

- - - I guess i was getting my terminology backwards. So Yes i am Negative Trail at the moment.

Hi roccet,
negative trail is inherently unstable, if you ride like that you are gonna die!
The tire has to contact the road BEHIND where the steering head axis intersects it.

[DGXSER]

either that picture changed or my mind is older and more feeble than I thought. What I see now shows positive trail. Not alot, but some.