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Author Topic: Trail, stability, and what really matters  (Read 5542 times)

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Offline Blue

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Trail, stability, and what really matters
« on: December 02, 2007, 05:49:05 PM »
I am re-posting this here from a build diary thread.  Take it for what it's worth, I expect a great deal of disagreement.

...Our team did a lot of analysis on stability and control because of the Sonic Arrow 1996 roll.  The most applicable data came from motorcycle stability and design studies.  These studies included tests that isolated rake, trail, wheelbase, axis of rotation (flexability in torsion and bending is all axis), etc.  This isn't theoretical, they actually built bikes with obscene ratios of rake and trail that we would never consider.  The data surprised me at first, but explained everything I had ever felt in 5 years of motorcycle road racing.

The most important factor in steering stability is trail.  It is TOTALLY independent of rake.  The studies included a bike with 15 degrees of rake and trail between 2 and 20% of wheel base.  Anything over 5% led to almost no wobble; trail greater than 10% of wheelbase made steering force very high.  The tendency of wobble to couple into weave (tank slapper) was directly proportional to rake due to the long moment arm of the forks associated with high rake angles.  Weave was related to how flexible the frame was laterally (left to right bending).  This is important: NOT torsion, NOT vertical bending; 

Lateral stiffness dominates weave.

It looks like your existing bike has ~10" of trail on a ~75" wheel base.  This is a LOT of trail, which seems fine as long as you can still steer.  It's also a LOT of rake, which only matters since the lateral flexion of the fork adds to the frame.  The frame is of excellent construction, and needs more lateral stiffness to avoid weave.  It's a double-down-tube, single -spine design instead of a perimeter or split-spine as most modern race bikes are.  I raced superbikes during this changeover from 1980 through 85, and these reports explained every wobble and weave I ever had.  Any steering instability will likely couple completely to the longitudinal bending and create weave.  Steering dampers take wobble and feed it into the frame to create weave.  They only work if the frame is laterally stiff.

I know you're very far along in this and have had lots of success.  As we found out, the time to see if the vehicle wobbles or weaves is BEFORE an upset, not after as Craig did at 675 mph.  The car was fine to 636 without a crosswind.  So we have added as much lateral stiffness as we can and will test wobble and weave at every speed increment in our test program.  Everyone needs to do that same.  From my own experience on superbikes, we could go very fast with bad steering instability as long as we only went straight and didn't hit any bumps.  Once upset, the bike could tank slap fatally. 

This suggestion is only for future bikes:
Trail: 5 to 10% of wheelbase;
Rake: as little as possible;
Lateral stiffness:  as much as you can get;
Longitudinal stiffness:  enough to support your weight.

Good luck, and all the best.
Eric Ahlstrom
Program Manager
Fossett LSR
« Last Edit: December 03, 2007, 12:32:01 PM by Blue »
"Doing the same thing as everyone else insures the same result", Shawn Fischer
"Extraordinary ideas do not come from ordinary thinking", Dan Bond
"Don't compromise, optimize", Eric Ahlstrom

Offline Sumner

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Re: Trail, stability, and what really matters
« Reply #1 on: December 02, 2007, 10:51:06 PM »
Great info.  I had forgot what the trail actually was and found the following site that kind of compliments what you are saying:

http://www.msgroup.org/forums/mtt/topic.asp?TOPIC_ID=76

c ya,

Sum

Offline willieworld

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Re: Trail, stability, and what really matters
« Reply #2 on: December 02, 2007, 11:40:46 PM »
thanks eric----willie buchta
« Last Edit: December 03, 2007, 04:45:36 PM by willieworld »
willie-dpombatmir-buchta

Offline willieworld

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Re: Trail, stability, and what really matters
« Reply #3 on: December 03, 2007, 10:55:37 AM »
sumner you also have rake and trail on your car --  please remember that trail (caster) is your friend at high speed do the research  --go back and read the postings again     thanks  willie buchta
           
« Last Edit: December 07, 2007, 12:37:42 AM by willieworld »
willie-dpombatmir-buchta

bak189

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Re: Trail, stability, and what really matters
« Reply #4 on: December 03, 2007, 11:12:53 AM »
All this talk about "rake and trail" is very interesting and correct.......but most of this information does not pertain to a sidecar......
A sidecar is a "car" with 3 wheels....with a 1to1 steering ratio......in order to be able to corner a sidecar.....one has to reduce the trail.....down to around 11/2 to 2 inches otherwise the steering will be to "heavy"...........I have roadracing several sidecar outfits that had 0 trail (like power steering) however, as the late great Eric Oliver noted " 0 trail makes it difficult
to keep the outfit between the hedges) he was talking about racing sidecars at the Isle of Man races............NOW all of the above one can throw out the window when racing LSR sidecars.....we are going only in a straight line and a certain amount of trail is beneficial in order to keep the outfit on the straight and narrow.....too much trail
and not enough rake will at times create a "speed wobble"......................................................
In the past most of the LSR sidecars were roadracing outfits...........However, some of us build our sidecar just for LSR................and over the years we have found by trial and error that
30 degree rake and 3 inches of trail is a happy medium.

bak189

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Re: Trail, stability, and what really matters
« Reply #5 on: December 03, 2007, 11:21:10 AM »
P.S. to the above post.....................now if one is talking about the "new type" roadracing sidecars
that use A-arm suspension and a kingpin......it is again a whole other story........................................

Offline Blue

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Re: Trail, stability, and what really matters
« Reply #6 on: December 03, 2007, 12:41:19 PM »
I edited my earlier post to the correct usage of wobble and weave.

All of this does apply to a tricycle, it even applies to front suspensions other than forks.  We have a forward swing arm that is independently adjustable in trail and roll (rake doesn't apply) and it has zero kingpin. 

Looking at our roll, it was clear that we went through 1 1/2 cycles of a "tank slapper".  The frequency matched the lateral bending frequency of the car.  We figured this out by timing the tank slapper on the video in frames per second and going up to the middle of the car and giving it a good kick in the side.  The frequencies matched.  Once a tricycle goes unstable, it becomes a two wheeled vehicle unless there's lots of independent travel in the rear wheels, or the rear wheel and the sidecar. 

Getting it up on two wheels was a separate issue that we solved and will put on our web site as soon as it's up.
"Doing the same thing as everyone else insures the same result", Shawn Fischer
"Extraordinary ideas do not come from ordinary thinking", Dan Bond
"Don't compromise, optimize", Eric Ahlstrom

Offline Sumner

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Re: Trail, stability, and what really matters
« Reply #7 on: December 03, 2007, 05:24:43 PM »
sumner you also have rake and trail on your car its called kingpin inclination and caster  please remember that trail (caster) is your friend at high speed do the research  --go back and read the postings again     thanks  willie buchta
           

Yea I know all about the car stuff.  I have 18 degrees of caster and did as much as I could to get the scrub radius right ( http://purplesagetradingpost.com/sumner/bvillecar/construction%20page-21.html ).

I use to think that caster and how it worked was some kind of black magic.  All that happens with more caster and the the car turning is when you turn it starts lifting the front and the weight of the front pushes back down and for the front to return to ride height the wheel has to go back straight. Exactly what happens when you make a turn and the steering wheel will return to center.

c ya,

Sum