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Battle Mountain 2019

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russellbridge View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote russellbridge Quote  Post ReplyReply Direct Link To This Post Posted: 10 September 2019 at 5:58pm
I can't edit the last post. Was going to add that the chain tensioner is spring loaded.
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legs_larry View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote legs_larry Quote  Post ReplyReply Direct Link To This Post Posted: 10 September 2019 at 9:46pm
While the AIM93 gang have been contemplating all that - and rather them than me - some of the rest of us went racing.  Variable wind as usual,  Tokyo failed to launch again, as usual.  Andrew Sourk tried to qualify his repaired trike Triage, but pulled u on the course with a nasty rubbery-burny smell threatening to make him boak.  Toronto ran their tandem Titan (aka DERP) for the first time and did a wind-legal 58.  Yasmin Tredell is getting closer to the Women's multitrack mark with a 52.9, but that was before Ken Buckley ran it in the next heat and blew the rear tyre right at the start of the timing traps.  Rolling ARION5 into the scenery.  Some scrapes on the machine and the camera pod needs nailing back on, Ken undamaged, PSOs hard at work fixing it.

For once Andrea Gallo was not the fastest of the session, that honour falling to Fabien Canal in Altaïr 6 with a wind-legal 80.63 mph and a spiffy new Hat.  Right now the Small People are in from the various skools poking their little jammy fingers into everyone's bikes so I've come back to the Super 8 to upload photos, do some washing and rest my poor achey feets.
====================

a bit ov a lyv wyr by slof standirds
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 10 September 2019 at 11:20pm
Hi Russ, following our conversation yesterday, I've found the thread you started about a hub (off) centre steered bike. This is what I wrote (9th Feb 2017):

Hi Russ, your design looks light and simple. I remember Mike Burrows built a faired camera trike in the late 80s using this principle. I think it was called 'Cyclops' and appeared on Tomorrow's World! However, it was only designed for straight lines - I think you will struggle to get more than a few degrees lock with this arrangement but who knows? Maybe try playing with a rear wheel and a chain to get the feel of it?

I can see no steering mechanism in your design. As Brian suggests, Mike's design had the steering axis offset to coincide with the chainline, to remove torque steer, but introduce bump steer! Here is a radical idea that may well be disastrous, so right up your street Russ Wink: Why not have an inclined steering axis (viewed from the front) that passes through the top chainline and also through the tyre contact patch, thereby removing torque-steer and bump-steer? The mechanism would be easy to incorporate into your design, with an upper pivot on the lefthand chainstay and the lower pivot on a swan neck, placing it close to the spokes. It would be neater with a cantilevered wheel axle. What it would do to the handling, I have no idea, but it might work.

By 'torque steer' I mean a steering torque caused by the chain tension.


Edited by GeoffBird - 10 September 2019 at 11:26pm
Right Time - Right Place - Wrong Speed
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russellbridge View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote russellbridge Quote  Post ReplyReply Direct Link To This Post Posted: 11 September 2019 at 12:10am
Thanks Geoff! Sorry I’d forgotten you’d written that, all coming back to me now. :) Yes that’s exactly the problem we’re having...

Just driving back from Elco now with a new bike to hack up and use the forks off to jam into the front of the shell, with a fair wind and some midnight oil we may have a new bike ready for thurs am. We are basically now in a Nevada version of scrapheap challenge....

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Post Options Post Options   Thanks (0) Thanks(0)   Quote legs_larry Quote  Post ReplyReply Direct Link To This Post Posted: 11 September 2019 at 4:36am
I believe the USAnians refer to said TV show as "Junkyard Wars", m'lud.

Anyway, you didn't miss much on Tuesday night.  Wind.  Two runners out of four in heat one.  Josh Gieschen in Cal Poly's bike Ambition ran 63.99 mph; ARION5 ate another rear tyre and flipped Yasmin onto her side.  There was much Scouse Swearing when I left the Civic Center just now.

In heat two everyone scratched.  Wind.

Wind.  Heat three saw Andrea potter down the road at a steady 40 mph until the last kilometre, when he slowed down and did 33 mph dead.  Wind.  Ishtey Amminger, being young and thus lacking in imagination, did 51.29.  Wind.

Wind.
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a bit ov a lyv wyr by slof standirds
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Post Options Post Options   Thanks (0) Thanks(0)   Quote atlas_shrugged Quote  Post ReplyReply Direct Link To This Post Posted: 11 September 2019 at 7:58am
Below is another post from Brucey which I think agrees with the last post from Geoff. My take on Bruceys post is that because of hub offset centre steering the steering axis is offset from the point of contact with the ground. This on a bumpy road can result in the steering being knocked around (bump steer). The solution in the classic car is to incline the front wheels as the picture shows. Evo K owners are only too familiar with this issue because the steering plates appear not to have placed the virtual steering point in the right place - resulting in unstable steering at high speeds. Brucys post:
 
fwiw this picture shows the positive camber on some car wheels

https://n7.alamy.com/zooms/27172191819d44ed95be107f45c7f929/mg-tc-vintage-car-during-the-british-classic-car-meeting-2011-stmoritz-ec9bmx.jpg

the idea being that the wheel plane and the kingpin (steering) axis
almost coincide at ground level, thus reducing the scrub radius. {this
is achieved by less visually obvious means in more modern cars BTW].
Four-wheelers (and tadpole three wheelers) tend to tramline horribly
if there is too much scrub radius. Bicycles with any scrub radius (or
a changing scrub radius because something is flexing)  soon become
very difficult to steer.

cheers

Brucey
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atlas_shrugged View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote atlas_shrugged Quote  Post ReplyReply Direct Link To This Post Posted: 11 September 2019 at 8:19am
How come Liverpool have blown two rear tyres? This sounds like the rear tyre may be rubbing against the carbon.  In photos their machine looks like a tadpole velomobile trike with chain drive to the rear wheel. This probably flexes the rear wheel when pedalled hard. If the torque is enough the rear wheel can be bent and so rub against the carbon. Just pull the rear wheel both ways to check this.
 
Check the dishing of the rear wheel and get the dremmel out to increase the space around the rear tyre.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brucey Quote  Post ReplyReply Direct Link To This Post Posted: 11 September 2019 at 11:33am
An MG picture should display below



Probably in the MG the scrub radius is not quite reduced to zero but it is pretty close.  Because the tyre is inclined from the vertical, the contact patch is an odd shape ( it will vary with absolute castor angle) and furthermore it will move around (laterally) as the tyre deforms differently. I'd expect these issues to also apply to a bicycle with an inclined wheel which gives nominally zero scrub radius (is this what you have got, in fact?). I'd also expect any flex in the wheel mountings  to cause problems too.

[not 100% relevant to current issues in this thread BTW] FWIW some of the steering problems with the Evo K are less than easy to diagnose, because the steering (kingpin) axis has a virtual not actual position; the lower swivel is a 'double-knuckle' design in which the axis is moving around with steering movement.   If this seems a bit crazy it is usually done  for a reason; that reason being hat the virtual (instantaneous) axis position can be placed where it would be physically impossible to place it otherwise.  

The double-knuckle arrangement at the bottom of a mac strut is particularly advantageous if the tyres/wheels are wide, so I first saw it (and they claimed it was their original idea I think, but I don't know if this is really the case or not) on a 1982 BMW motor car.  However it does require a lot of head scratching to preserve good Ackermann geometry, it is considerably more complicated than a standard Mac strut.

IIRC the main problem with the Evo K steering is bump steer, mainly caused because the height of the steering rods was completely wrong for the rest of the front suspension mountings.  

However I have an idea that when racing, you may not want perfect Ackermann geometry (in a tadpole trike or a four wheeler); most tyres have a 'slip angle' i.e. they don't roll in the exact direction the wheel is pointed, when subjected to a lateral load. The slip angle is liable to be dependant on the tyre construction/pressure and the tyre loading/road surface.   So if you optimise everything, you might (say) have the outer wheel turning slightly more (or less) than Ackermann suggests, and this might result in less net tyre scrubbing. However if optimised, this correction would probably only apply exactly for one tyre type/pressure/road surface.

cheers

Brucey


Edited by Brucey - 11 September 2019 at 11:52am
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GeoffBird View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 11 September 2019 at 3:53pm
Hope you win the Junkyard War Russ!

Hi Brucey, yes I made the exact same points as you in Laidback Cyclist a couple of years ago. The reason I didn't opt for virtual pivot-point steering on my Vulcan racing trike was precisely because of variations in Ackerman geometry. I know the argument about reverse Ackerman (Colin Chapman originally I believe?) but was told by Chris Parker that correct Ackerman geometry is very important on a tadpole trike.

My trike also has about 7 mm negative scrub radius (offset), because the brakes are not linked, but it may be introducing vagueness into the steering (a common issue on cars), so I would probably go for centre-point steering on the next one (zero scrub radius) as I don't seem to have to use the brakes much when racing - maybe I'm not fast enough!

Your point about camber shifting the contact patch around is extreme on a Formula One car - they have a far smaller scrub radius (positive offset) on the straights than appears as the front wheels run a lot of camber, shifting the contact patch inwards.

Anyway, we're getting a bit off topic. If anybody else is interested in the finer points of steering geometry then maybe we could start a new thread?
Right Time - Right Place - Wrong Speed
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russellbridge View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote russellbridge Quote  Post ReplyReply Direct Link To This Post Posted: 11 September 2019 at 9:37pm
So we've decided to go for it and try to change the internals of the bike in the field. It's a long shot but hopefully it'll give us the opportunity to run down the course with a bike that gets more stable with speed rather than less.  We are trying to change a front drive, hub centre steered (via push rod), rear brake bike into a bike with a conventional front fork and tiller steering, front brake, and rear drive via long chain down the 'hull' of the bike.

Photo below shows current state of play. We presently have most of the right parts, some of which are in the right order. We've also found a local resident with a machine workshop, so there's a few of the team there machining parts, Barney is currently laying up carbon around the new headset...hopefully we'll get it finished and rideable...



Edited by russellbridge - 11 September 2019 at 9:39pm
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