Originally posted by dirt bike dave
Thank you svi. To the non-beleivers, here is a quote from the following page:
http://www.bankeperformance.com/catalog_dtu_tripleclamps.html
QUOTE As a general rule, less fork tube to steering stem offset yields more trail and hence, up to a point, more stability. More fork tube to steering offset yields less trail and less stability. These changes also change the wheelbase. ENDQUOTE
If you draw a quick diagram, it is easy to see why this is. Here is a link from Tony Foale discussing geometry; some interesting stuff.
http://www.ctv.es/USERS/softtech/motos/Articles/RakeEx/RakeEx.htm
Ok so we all have those momets when suddenly no matter what we thought we where doing we ae over in the meadian asleep in our boxvan at over 80MPH.... What thats never happened tou you? :p
This is one of those for me.. While I was out this sumer I had all kinds of revolations, and whilst board, dateless, and tired of drinking this last weekend, I was surfing around for fun things to look at from the time I was away.. When I came across what I had said, and what had been said back, I was getting all fired up over the verbal slaying I was gonna deliver and then It started to setle in like the sound of entire contents of your box van getting juggled as you casually come to from the vivid dream to realize your going 80MPH down the highway medain in your box van..
Lecture classes allways made good sence whilst i was in school, beacuse you could elimninate all your stupid ntions before you could embrass yourself in public.. I got real serious about chassis stuff some time back after doing some road race testing with Muddy in Austrilia. I know alot about valving but I was fairly ignorant to basic gemeorty stufff so I made it a point to learn more. I got Tonys book and continue to study it.. However I made the tragic error of cusnfusing what I know from exspereince with what I learned in the book and while if you can teach you know it, you may not really no it coorectly.
There should be a really obvious question poping up here... why do offroa bikes with less offset turn better? I can tell you that I know for a fact that they do. So why the heck is that posible when yes when you think about it correctly it increases trail with less offset? That should as SVI and DBD noted make the bike more stable, and harder to intiate a turn.. Really firging werid..
Well not able to come up with a answer I e-mailed Tony, I have gotten to know him a little over the last year talking about diferent things.. While I was waiting I also asked some other freinds, and I've not heard back yet.. Well heres Tony's answer...
Firstly, I assume that top and bottom yokes both have the same changed
dimension. If not for example, if the bottom yoke keeps the same offset
and the top one is increased then the trail will increase not decrease.
Secondly, assuming that the first comment doesn't apply. Reducing the offset does two things that work in opposition insofar as
steering resonse is concerned.
1. It increases trail which slows steering, but this takes a little time to
take effect.
2. It reduces the steering inertia which quickens steering during the
initial part of the turn-in. (It also shortens the wheelbase)
I imagine that you are talking about riding on dirt, in which case the
effect of a given trail is less than on tarmac in this regard and so the
second point begins to assume a greater proportional importance. Look at fig. 4.12 in the book. In particular the initial 0.05 secs (about
the first 3 or 4 mm. of the plot)
What is termed the "tyre induced torque" is the torque from the trail. The
"resultant" torque is what's left when all of the separate torques on the
steering are added together, and it is this resultant that accelerates the
steered assembly about the steering axis. The actual acceleration depends
on the steered inertia as well as this torque.
Note how during the initial 0.02 - 0.03 secs. The resultant torque just
follows the rider's applied torque until the feedback torque from the
effect of trail starts to build up.
So reducing the steered inertia will either require less effort during that
short initial period to achieve the same performance or else will perform
faster with the same rider input. This initial period is very short but
maybe it's enough to give the feeling that you are experiencing. Also the
data in fig.4.12 is only for that particular example which was a fairly
aggressive lean-in on tarmac. On dirt I would expect the build up of tyre
force to be slower which would extend that initial period.
I would be very interested to hear about these effects if you repeated the
same tests on tarmac.
Keep me informed on this, it's quite interesting. I'm fairly busy right
now but when I get a moment, I'll input some test parameters into the
simulation programme that I used to produce the curves in Chapter 4. and
see if it throws up results similar to yours. I'll let you know what I find.
Regards,
Tony