Negative Trail ? ? ?

What is trail ?

Quote from CCjon on December 8, 2022, 2:55 pm

Reading another thread about building a leading link, reducing trail, etc., got me thinking. 

With sidecar rigs; what happens to handling when trail is reduced to zero?  

What about negative trail handling?

Would additional negative trail make things worse or revert to a stable handling?

Has anyone actually experimented with various trail to determine the ideal measurement?

Does the front wheel size affect trail optimization?

Curious minds want to learn...

P.S. After thought: the choppers of the sixties, think Easy Rider, did they have negative trail or was the steering head altered so radically they still had some trail?

CCjon--I saw the same thread where the guy said his trail was 0" but the rig was nice and steady even at speed. My immediate reaction was "they measured wrong."  The closer trail gets to 0", whether with LL's or conventional forks, the steering gets lighter and lighter until it becomes unstable.  With negative trail (bearing in mind that I'm a common idiot not a sidecar engineer) the wheel will immediately go to full lock one way or the other if you take your hands off the grips.  That's always what I've been told anyway, and when I picture the vector forces in my brain it seems like that's what I'd expect to happen too.

I don't think the size of the front wheel matters much.  If you have a rig with a 21" front wheel and 4" of trail, and then swapped in a 17" front wheel and finagled the forks and suspension to keep the trail at 4", I'd expect the steering effort to be about the same.  No evidence here to back me up -- just stating an opinion.

Quote from CCjon on December 8, 2022, 2:55 pm

...With sidecar rigs; what happens to handling when trail is reduced to zero?  

What about negative trail handling?

Would additional negative trail make things worse or revert to a stable handling?....

Quote from DRONE on December 8, 2022, 11:56 pm

....The closer trail gets to 0", whether with LL's or conventional forks, the steering gets lighter and lighter until it becomes unstable.  With negative trail (bearing in mind that I'm a common idiot not a sidecar engineer) the wheel will immediately go to full lock one way or the other if you take your hands off the grips....

As I understand, extreme examples of negative trail are shopping carts and zero turn mowers, both designed to spin in their own space, but neither are designed to steer, nor does either offer directional stability.  Also, think of a shopping cart with an out-of-round or unbalanced front wheel.

Quote from Mick Boon on December 8, 2022, 6:08 pm

What is trail ?

Does someone have the diagram explaining trail, I cannot find mine. 

Here ya go --

 And most often people trying the measure it themself get it wrong.

With coppers how they got the rake becomes important. Often the neck was cut frame bent and weld back in place. The effect was different than using a raked tree. As most do now to get some rake.

Just adding long tubes or extensions was just plain nuts all that did was stick the front end up in air.

  The bikes you see with wild rake and long tubes had a habit of laying on their side turning coming out of a gas station.

If you rake a front tree without lengthening the tubes the front end drops When you rake it and lengthen tubes to keep front height the wheelbase increases.

All of that effects how the bike handles sidecar or not.

What works well on one bike may not on another Harley Touring bikes for the most part have come with a factory Rake of 26 degrees. Other manufactures may have more or less.

It seems most are able to achieve light stable steering with only a raked tree and small tube extension . Well made raked trees were not that common in the 60's.  Today they are all over the place and adorable.

Using the drawing that Drone graciously provided,  added some information...

Looking at the drawing above: The theory and practice is that reducing Trail by either installing different Triple Trees or by Leading Link, or Leading Leg, or any other means, reduces steering effort on sidecar rigs. The question is, at what point does reducing Trail or even going to a negative Trail become counter productive?

For example, the stock Trail above is 101 mm.  Reduce that by half to 50 mm, is that good or too much?

Reduce Trail to zero, too much? What happens?

How about at negative, a -50 mm of Trail?

Or a -101 mm of Trail?

All of this is possible by using longer and longer leading Links.  

What is the sweet spot for Trail reduction? Is there one?

I've read a ton about it and there doesn't seem to be a hard and fast number.

In Hal Kendall's book, Sidecar Operators Manual in the tech section he says about 2" or 50mm.

I've also read 35mm to 45mm.  And some of the European manufacturers were saying they used up to 60mm.

When I had my leading links built, I had them built for two inches of trail. 

With a regular motorcycle tire I had some head shake which disappeared when I put a sidecar tire on it.  I'm always in search for easier steering but I'm afraid if I reduced trail more I'd have high speed instability and more shake.

The guy that built them came up with a neat idea to make a test swing arm so that you could try various trails before welding on the connecting brackets. Unfortunately we live on opposite sides of the country so it was impractical to follow up and I was pretty happy at 2".

  The effects of changing trail will be different depending on the bike.

If you a dealing with a mid size BMW and a small car it will be much different than a Harley touring bike with a twin size sidecar.

My 2017 had a rake of close to 6 degrees the 2020 has 4 1/2. Both had tube to match the change. Both worked well in all cases.

The 2020 seems to steer a little lighter at slow speed than the 2017 did.

My bike ready to roll weights about 940 LBS. the sidecar about 300. I doubt anyone here would have any issue steering it with  the 41/2 degree rake.

No shake or wobble with sidecar on or off.

If I push my Rocket III rig backwards without holding the handle bar the front wheel will immediately flop to one side or another, this is without a steering damper on it. Pushing it forward it rolls straight, could be smoking crack again but this would be the effect of negative trail to me. Riding my Ural backwards would also have the same effect, exacerbated with speed.

 No. When you Push the bike backward it becomes rear steering.. If you have ever run any farm equipment or heavy equipment that use rear steering it takes some getting use to.

We lean a two wheeled bike that is how they are designed. With a sidecar we tune them into something like a car. If you run the rake and tube up and out to far the front wheel wants to lay over on the side. Watch a long forked Heavy raked bike try to make a slow turn some time. better yet if you can get on one. But watch out . Many have laid them over.

You will hear that if you rake it you can not ride it without sidecar. That is not true. But if you loss to much trail it could become dangerous.

Again depending on the bike. 4 to 6 degree rake will not make the bike impossible to ride as a two wheeler. It will handle a little different.

It will have a longer turning radius . It will have a tendency to nose dive in tight real slow turns. Going straight down the road  at high speed it will be even more stable.

  Had it not been for two shoulder surgeries a major collar bone break and a life time of beating this body up . I would have had no need to rake the 2017 or 2020.

  My 2015 Harley Street 750 has no front end changes at all.

Am learning from the comments here, keep them coming. Thank you.