Front wheel rack and castor

[RossM]

Hi
Time for a new post on the forum.
Our technical advisor, who has not been on the forum for awhile, quoted that;
......after heaps of research on the topic of nosewheel strut "rake" angle, I have come to the conclusion that the optimal rake angle is 18 degrees from the vertical.This is by far the.....
http://forum.asra.org.au/forum/misce...663-g001/page3

I wonder why Mark has come up with this figure?
another topic of interest in the past maybe 'Nose wheel strut failures'
http://forum.asra.org.au/forum/gener...strut-failures

[RossM]

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[RossM]

Ian Sganzerla front wheel

[RossM]

Rosco front wheel

[tonydenton]

I have the perfect steering slash suspension slash spring travel slash design @ 20 degrees. but hey I'm not perfect I could be 2 degrees out. I can live with that.

[RossM]

you seem to be doing a lot of slashing Tony!

[WEST OZ FLYER]

My guess is they are for the technical manual that he is writing.

[tonydenton]

yes i know.matt. I'm sure mark would have it worked out and be right. I just went to 20 degrees because it was easier to see on the bevel guage . I do love the travel and correct spring combination I have. the raked angle semed to be the only way to go with the design. as your travelling forward and faster the direction of travel suits. on my 2 seater I have been building for ever I increased it to 25 degrees but the 20 degrees just looks better .

[WEST OZ FLYER]

Tony, how much trail do you have?

[tonydenton]

zero. not sure what mark would work out how it works , better or worse, but when its raked and you turn it leans so not all steering 1: 1 abruptly.
also the suspension is absorbed in travel up the spindle king pin. the actual tricky part was finding the correct spring tension. needs more than you think. the spring I eventually found works that well that I even put a dampener on the top . was great suspension over rough ground but when the spring was returning the wheel to the fully extended position it was hammering home with a bang. so I found and fitted a short spring on the top to cushion the return stroke.
i'll see if I can find a photo that will do it justice.

[RossM]

Tony Denton front wheel

If a gyro had the front wheel not connected to the steering, castor would have to be a must I dare say.
You see a bit of dampening things set up on front wheels that are not connected to the steering, I womder if they had rake, if they would not need the dampener to take the shimmy out.

[RossM]

just posted this photo for the fun of it.

[RossM]

front wheel shimmy stop posted by Chris Toev's on American forum

[tonydenton]

thank you ross , of mighty mate of mine. [ the photo with orange parts on it is mine. ] the direct and raked and perfect suspension is the only way to go in my opinion. the other best part is that it seems to trailer better. I originally had a trailering suspension setup and was getting damage form being on the trailer.

[Tim]

It does not really matter whether the wheel is raked forward or back, as long as it has positive castor. Zero or negative castor is considered 'unstable' whilst too much positive castor is heavier to steer and prone to more violent shimmy (should it occur). For good stability the castor angle is the angle between a straight line drawn down through the centre of the steering pivot and a line from the bottom of that pivot down to the first point of tyre contact on the ground. The Rosco nose wheel pivot above has zero 'rake' (the pivot is vertical) and whilst it would visually appear that it has a lot of castor, it is in fact considerably less, but still positive enough to ensure that the wheel should track straight should the steering linkage break, and it could still be controlled with the rudder in such a situation.
Negative castor (the first point of tyre contact ahead of the pivot) would tend to go to full lock without a steering link, so it is imperative that the link be inspected at every preflight.
There are exceptions to every basic 'rule', eg bikes (and motor bikes) mostly have negative castor, but they usually involve different methods of stability and control, and believe it or not the physics involved is quite complex. Suffice to say that if you kept a bicycle vertical (like a gyro nose wheel) you would not be able to steer it without coming a buster, but that's an entirely different kettle of fish and is irrelevant to this discussion.

[RossM]

Tony Foale had carried out experiments with a motor bike, reducing the rack angle back to
zero (vertical), and the bike still handled well with no shimmy.
His article with this experiment is no longer on the web (might have to buy a book to read it now)

A motor bike turns more by leaning the bike
Gyros are of coarse different to a 2 wheel bike, and a trike is something different again I presume, I have never ridden a [motor] trike (maybe a peddle one as a toddler).