# Larry front: how much trail?

• 05-29-2011
Canoe
Larry front: how much trail?
When building a Fat Bike frame for a Larry front, how much trail should it have?

Do we measure trail from straight below the axle (call it the middle of the contact patch) as usual, or given the large contact patch, from the front of the contact patch? A third of the way from the front of the contact patch?

Does anyone know what steering properties a Larry gives with more or less trail?

How much trail is:
• too little?
• too much?
• just right?
• 05-29-2011
The contact patch is always assumed symmetrical - you take the vertical line from axle center and the steering axis line as the points to measure between.

Given this, it strikes me that as you lower the pressure in the tire you decrease your trail. You're moving up the vertical line, which moves you backwards along the steering axis line - theoretically making steering less stable.

I can't answer your question directly. You'll have to know head angle, fork offset and A-C length to get a number. Tire pressure will have a large impact in the field.

You need to get a copy of "Bicycling Science" - some good discussion on this topic. I always recall the man who set out to build an unrideable bicycle. He couldn't do it. You can ride most anything, but it will be your own personal preferences that matter. Just look at 29er's - some love them, others don't. All about personal preference.
• 05-29-2011
Canoe
Quote:

...
Given this, it strikes me that as you lower the pressure in the tire you decrease your trail. You're moving up the vertical line, which moves you backwards along the steering axis line...

But if the back tire pressure is also adjusted at the same time, the head angle to the horizon should remain pretty much the same. As the bike lowers some, the contact patch and vertical axis should stay the same? I can see the steering axis move back slightly toward the vertical line, but I wouldn't expect that to be too significant? Or with the fat tires grabbing the ground so far out to the side are these variances exaggerated in their affect on the steering properties?

Quote:

...
I can't answer your question directly. You'll have to know head angle, fork offset and A-C length to get a number. Tire pressure will have a large impact in the field....

As head angle, fork length, rake are all selectable by design, I assumed I could select them through whatever values to get a desirable target trail of X inches. From what you're saying, I think I understand that for a given head angle, you'd want a different rake to get a trail value that works well with that head angle? To get the steering properties I prefer?
As in:
head angle of x, select fork/rake to get a trail of y inches
head angle of m, select fork/rake to get a trail of n inches
?

As I have no idea what head angle or trail will work well, and I can't calculate it, looks like I have to prototype it with an adjustable head angle and adjustable rake so I can try different head-angle|trail combos to find out what works best for me at the tire pressure I'll likely be running in the Fat tire once complete.

I guess the best I can do is to check the head-angle and trail for the off-the-shelf Fat Bikes with Endo/Larry, and make sure my adjustable setup at least allows for matching those values.
Makes sense?
• 05-29-2011
sryanak
Canoe, If this is for your recumbant project I think you should measure* one of the bikes that is similar to what you plan to build and start from there. The "normal" fatbikes are likely to have way steeper angles and very different trail. If not nevermind!

*If you can't find one to measure maybe the manufacturers could get you a geometry table.
• 05-29-2011
Canoe
Quote:

Originally Posted by sryanak
Canoe, If this is for your recumbant project I think you should measure* one of the bikes that is similar to what you plan to build and start from there. The "normal" fatbikes are likely to have way steeper angles and very different trail. If not nevermind!
*If you can't find one to measure maybe the manufacturers could get you a geometry table.

Yup.
With the info above I found enough to read this evening to understand that it's more than just "trail". And just before your post I had read enough that I thought I had a handle on how much headangle and trail to go for for a "neutral" ride, when I ran into the steering/handing issue of where the handle bars are, as in, where your center or gravity is relative to the front axle. Then the awful realization (well, not that bad, I haven't even ordered any steel yet, let alone cut or brazed) with the semi-recumbent geometry, I'm in a completely different kettle of fish.

I got the geometry of the Pugsley and Mukluk. Have it for the Rans Hammertruck and Fusion ST. Will get it for the various cargo frames.
Then determine a required headangle|rake|height range and make an adjustable fork to try them all out for my odd-ball geometry Fat Bike. It appears it definitely going to be a "one-of".

Maybe I can rip an adjustable seat control out of an old Lincoln and have the steering geometry tunable on the fly! ;-)
• 05-29-2011
Some more research info:

http://www.ihpva.org/projects/tstrik...ding/index.htm

And if you want a real set of tools to evaluate and compare different geometry:

Cal Poly Mechanical Engineering Dept. ME 441"Single Track Vehicle Design"
http://www.calpoly.edu/~wpatters/

"The Lords of the Chainring" is the class materials - based on aircraft design. Gives relationships between head angle, trail, steering leverage, many other etc.'s. You can plug in all the known good bikes to find what control values to look for.
• 05-30-2011
Velobike
Quote:

Originally Posted by Canoe
...As I have no idea what head angle or trail will work well, and I can't calculate it, looks like I have to prototype it with an adjustable head angle and adjustable rake so I can try different head-angle|trail combos...

Take a look at my DIY Fatbike fork project. By using an adjustable fork you can play around with headangle and to a limited extent the offset.
• 05-30-2011
Canoe
Quote:

Some more research info:...
And if you want a real set of tools to evaluate and compare different geometry:...
"The Lords of the Chainring" is the class materials - based on aircraft design. Gives relationships between head angle, trail, steering leverage, many other etc.'s. You can plug in all the known good bikes to find what control values to look for.

omg
Overwhelmed, I am.
Build myself a bike, I said.
Gotta be easier than the ultralight and the violin...
Just braze a frame with the holes where I want them and put some parts on.

In the past twelve hours It's reached the point where all the things I've read in the past year are now starting to come together with realizing their significance, and therefore I realize just how much I don't know and have a glimpse of how much there is to know.

And then there's that research that came out a month or two ago, showing "improved stability with self recovery" for designs with negative trail. How on earth does that fit in...

This project is definitely going to be prototyped in wood to test the geometry. Probably a few times. Then a test frame in CroMo. And I figured this would be a month long project...
• 05-30-2011
Canoe
Quote:

Originally Posted by Velobike
Take a look at my DIY Fatbike fork project. By using an adjustable fork you can play around with headangle and to a limited extent the offset.

Found it. Read it. Saved the page.
Good place to start, but with the semi-recumbent geometry I fear I'm going to need to cover a greater range.
• 05-30-2011
Canoe
Quote:

And from the above I got
http://www.wisil.recumbents.com/wisil/trail.asp

Seeing the theoretical trail from that calculator, with my intended semi-recumbent geometry, I get an idea what will happen when I:
• ride alone: target theoretical trail is Freight Train at 3.8" to Jet Fighter at 1.26"
• add a 150 lbs passenger on the back (weight and lower-rearward center of gravity) becomes Freight Train at 1.23" to Jet Fighter at 0.41"
• cargo instead of passenger (even lower CofG) and the values are 1.32" and 0.44"

So with just me a trail of 1.3" would be very responsive. Add a passenger and it becomes much more stable. Much better than the other way around, add a passenger and end up with the equivalent of a jet fighter's fly-by-wire unstable wing geometry, which I've heard described along the lines of "sitting on the bonnet of a British sports car while leaning forward steering a bicycle by the handlebars as it's pushed backwards while the car does 80 mph down a winding country road through the hedgerows".
Hmmm. Might be easier than what I'm trying for.

Did I mention I was intending on having an attachable sidecar for use at Burning Man?
While pulling a trailer?
• 05-30-2011
Velobike
Some of the old British motorbikes had double fork ends so you could vary the offset for when you were hauling a sidecar.

Bikes to look at: Panther Model 100 and 120, and I think Royal Enfield very briefly had a different set of lowers to do the same job.

I can probably dig up a pic of the Panther version (I had a few).
• 05-30-2011
Canoe
Quote:

Originally Posted by Velobike
Some of the old British motorbikes had double fork ends so you could vary the offset for when you were hauling a sidecar.Bikes to look at: Panther Model 100 and 120, and I think Royal Enfield very briefly had a different set of lowers to do the same job.
I can probably dig up a pic of the Panther version (I had a few).

Pic would be nice, but at this point handing for a sidecar is shooting in the dark. I was just going to go for a castering 20" third wheel, centered between the rear axle and the crank, and see what it did.

In visualizing castering effect from a fat tire trail, I only seems to make sense that delta drag comparing one side to the other on a turning input, would result in more of a delta hence more of a castering effect. But, this is speculation on my part...