Rake and Trail

[panic]

Much of the early work regarding increased rake angle (with trail following as consequence) was simply an attempt to cure wandering and head-shaking at speed due to lack of stiffness at the steering head. It was a crutch, and when they finally figured out how much 3-dimensional area was needed to insure sufficient rigidity, the rake requirement went down.
Testing an early (and many current) drag bike by inserting a bar through the steering head, and another through the swing-arm pivot would show that the bars can be rotated by far less effort than your nerves can stand.
Foale (paraphrasing) views trail as more important, but the large numbers frequently seen are not necessary if, again, the frame works. Not an accident that 200 mph road racers do quite well with 24° & 3".

Regarding "flop": in order to reduce flop to zero (viz., where the front wheel will not "fall over" to either side under vertical load) the rake angle must be zero. With any caster angle at all, the axle at 0° steering angle is elevated above its vertical position at any other angle. The axle climbs when the wheel is centered.
The relevant question is, IMHO, "how much angle produces sufficient axle drop (at a steering angle that will be in use) that stability is compromised?". I don't know the answer, and this is more complex that it may appear since large diameter wheels are less affected.

More comments: http://victorylibrary.com/brit/chassis.htm

I have a related question: omitting the effect on frontal area and streamlining in general, I'm puzzled about the effect of CG height in a bike.
A low CG is normally considered a good thing for most purposes, but in terms of degrees of camber needed to transfer weight for steering correction isn't a high CG an advantage here? A bike with a CG at ground level would need to incline to horizontal to make any corrections.
It seems to me that the less disturbance to the camber angle the more stable the bike will be?

[Stainless1]

Thus my point "better yet leave it alone"  We have some very fast "busa's" running with
stock rake and trail and they appear to be fairly stable. 

I think we will find most of the fast fast Busas are running an extra 5 degrees.  I wish I was smart enough to know what was best...    But as I read all this stuff, I am following links and learning, and that is what if fun here.   Glad we have a lot of old farts to learn from...   

[JackD]

"For every bit of straight you want in a bike, you have to remove a bit of nimble, build it like that and make sure it stays that way." (me)

[isiahstites]

Thanks for all of the info to this point! This is what I was looking for to better educate me on the subject and along with my own research it has done just that, so thank you.

A lot of people have stated to leave it alone because of the R&D done by the big name companys. My thoughts are that have done a lot of R&D, however is it not mainly for the use of street applications? Most LSR bikes never see the pavement and will never see the twisties where good handling would be of the utmost importance.

I decided to change mine for several reasons, I wanted to decrease frontal area, more stability due to the chassis being a few inches longer and to allow me to be more comfortable on th bike instead of balled up when in the riding position.

Here are a few pics of the old neck, downtubes and backbone and what will be the new. I have not figured the number yet but I left the backbone long until I decide hom much stretch I want.

Scott


Old



New

[isiahstites]

Thanks to everyones help on this. I finished up the backbone and neck today and came out with 38 degrees of rake with 7.5" of positive trail.

Scott

[willieworld]

what made you decide on that     willie buchta

[isiahstites]

what made you decide on that     willie buchta

Based on the info I have found during my research and the different trail lengths above 38 degrees were not where I wanted to be. I talked it over with Randy and we agreed that what I came up with would work well.
I could of went with less rake and it would of been fine, however I did not want to purchase or modify my forks, by kicking it out a few degrees it will allow me to use the exisitng front end.

Scott

[willieworld]

good choice     willie buchta

[tomsmith]

Here is a site that has control and stability papers:

http://www3.imperial.ac.uk/controlandpower/research/portfoliopartnership/projects/motorcycles/publications

The guys that wrote them do work for Ducati on their factory road racing bikes, and are motorcycle riders themselves including a Suzuki GSX 1000.  They said that the papers are oriented toward academic types so they are not the easiest for non-techs to understand or wade through.  One paper is on steering dampers, where the rest are on handling.  Handling is complex - there is no easy solution except to get it rigid to start with, then work from there.  For example, steering dampers can cause wobbles in some circumstances, and excess trail is bad.  Study and have experts give you advice.  Too bad I don't know much or I'd help.

[1212FBGS]

It looks like a lot of those papers were authored by Sharp...the guy I mentioned earlier… he’s is world renowned and has been at it for a long time...not as well known as Fole but more analytical… he mostly does analysis study of design in accident cases...Willie that means he was hired to analyze the cause of an accident… the after fact of poor design…. no horror stories, just the scientific answer of what killed the rider...Any one who is involved in accident reconstruction like Randy Nelson, Rus Odaily, Jon Mckibben, even Scott Guthrie have read at least one  Sharp report….good reading...dry, but good info… I wish whom ever authorized the new rule that limits our steering lock to 15deg had read some of sharps papers, he would have realized that limiting steering lock would not prevent or reduce tank slappers... I voted no….Sharp also wrote a nice paper about chassis oscillations and steering dampers...maybe someone should do a compilation of his work into a book...
Kent

[panic]

"he would have realized that limiting steering lock would not prevent or reduce tank slappers"

Please, please tell me that was not offered as the reason...?
If anything, bouncing off a stop at full velocity may make it worse, with none of the self-centering that trail might add.

[JackD]

As is the case with many of the LSR rules, they reflect a lack of understanding on the part of the authors, and allowable participation by a number of people that know better .

[1212FBGS]

panic...oh yes.... you should have heard my eyes roll at the meeting.... passed anyhow...
kent

[JackD]

The Wizzzzard srikes again.

[willieworld]

eric ross and i were informed about the rules committee meeting about the time it started --we were also told that we could go but had to be quiet-which we did--maybe its just me but every meeting i have attended seems like everyone is in a big hurry to get out of there except the drivers meeting and that one seems like it lasts forever  just some thoughts  and i think a lot of you are confusing road racing with land speed racing when it comes to the set ups on your bikes      willie buchta