trenace
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posted October 28, 2005 10:49 PM
Thoughts on trail... counterintuitive or am I wrong?
I wasn't really particularly interested in an adjustable triple tree for the 12 because I thought the steering geometry seemed pretty good as is. However in getting new Marzocchi forks I had to get an adjustable triple tree to go with it (though neither has arrived yet.)
So, I was considering, since it's adjustable why not make it like for example the 10R, perhaps it would steer better, and with a steering damper a little decrease in stability would be OK.
So, I started looking into it.
Actually the 12R has pretty steep rake, 23.5 degrees, which is as steep or steeper than many supersports 600's. No issue of rake being too cruiser-like.
The trail is also pretty much the same as the 10R already, 99mm vs 102mm with the 10R being the longer of the two. Other literbikes such as the Gixxer or R1 are a little shorter than this but not much.
Hmm.
Anyway, reading the Geiger Sport (sp12) site, their recommendation is to adjust the triple tree for MORE trail, by adjusting the offset. Running it through a calculator indicated that they were saying to add about 1/4 to 1/2 inch of trail.
Now, what I was thinking is it seems to me that while designing a bike from scratch, the trail can be adjusted totally independent of everything else, and so for example shorter trail could just mean faster steering, lesser stability and nothing else, no change on wheelbase or weight distribution etc. But when adjusting an already-existing bike this is NOT so: changing the offset also moves the wheel forward or back, and thus changes weight on the front wheel and wheelbase.
And adjusting to LONGER trail, while you would think is the last thing you'd want to do for a racebike's steering (sp12 roadracers their bikes) just from what one knows about trail and steering, will move the front wheel back, thus putting more weight on the front wheel and shortening the wheelbase.
For example, it seems to me that lengthening the trail by 1/2 inch by changing offset would bring the front wheel back half an inch, thus tightening the wheelbase that much and changing weight distribution significantly.
Not that one always wants to reduce wheelbase or put more weight on the front but sometimes that's the case. So maybe one might increase trail to an apparently "tourer-like" value so as to gain those benefits, with the cost maybe really only being more muscle required for turn-in.
Does that make sense? I have never seen an article put it this way. But anyway the sp12 guys are recommending adjusting the offset for longer trail for roadracing, so there has to be some reason and stability alone should not be it as the bike is hardly that unstable in the first place. Do these effects I'm theorizing occur? Any thoughts?
  
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psycho1122
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posted October 29, 2005 05:37 AM
To my understanding, Trail does not have a large effect on "turn in". Rake does.
Trail effects stability, but at a cost of wheelbase.
New Literbikes can increase trail to keep the steep rakes in check. This is possible by haveing more compact engines and shorter chassis. Shorter chassis alows for longer swingarms and increased trail. Thus keeping the wheelbase nice and short!
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You say PSYCHO like it's a BAD thing!!
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trenace
Needs a job
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posted October 29, 2005 12:00 PM
I'm not sure which way you meant with "at cost of wheelbase" but actually more trail means shorter wheelbase, at least if accomplished by less offset.
To visualize it, trail is that distance the middle of the contact patch is behind a point projected from the steering head, so the further back the tire is (shorter wheelbase) the longer the trail is.
I agree with you (within my limited knowledge) that the rake is the bigger factor in steering.
Anyhow, unless I'm mistaken in that above reasoning, it seems to me that increasing the trail, for example by half an inch, by reducing the offset as Geiger Sport recommends, would move the front tire back by half an inch. On a wheelbase of for example 56 inches (to have a round number and also I'm shortening the swingarm) the center point would be 28 inches, so if 50/50 in the first place (the center of gravity being at 28/28) that would change to almost 51/49. So it's a pretty significant weight distribution change as well as a significant wheelbase shortening.
What I may do, since the weight of the bike is being changed so much and plus that, the rear wheel is being moved forward, is determine the weight distribution at stock trail while on it... and based on that, if the front end is light then increase the trail (decrease the offset) to give a reasonable weight distribution for the "first guess" on where trail should be, unless of course this theory is just wrong.
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238mph
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posted October 29, 2005 06:35 PM
Tren, a bit of info you might like to know...
I lowered my 12 long ago, a lot, for good hole shots.... and it has been that way for 2
years... one thing I noticed (or didn't until later) was that with the rear end down, more
rake, the bike was extremely stable at wide open speeds... 180+ .... no twitchiness, no
need for a steering dampner...
But recently I raised the rear back to stock height thinking that it would speed up my
turn ins up on the mountain twisties... well, 1st of all the bike sucks with the rear way
up there... high speeds are almost scary, front end is waaay to twitchy with steering
suppression... and it's almost impossible to put a knee down in the turns... I can't
reach the ground even with the bike way down.
I found ( at the roadrace schools) that you gain much more stability in the turns when your
knee is on the ground simply because you have three points of contact, like a tripod.
If the rear end starts to slide out, all you need to do is push down on your knee and tilt
the bike up a bit, which loads the rear end and it hooks back up... works great.
Anyway, my point is that the 12 needs MORE rake if you plan on doing high speed stuff...
stock is way to unstable compared to how it handles with the rear end lowered.
Also, with the rear end up, my stand up wheelies went all to shit... sometimes the rear
end simply spins the back wheel instead of lifting the front... and once up, the bike
tends to load.unload the rear suspension and never really settles down.
What I'm trying to say is that I would add rake and lower the rear... just my 2 cents...
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Pilot Powers took me to the grave, and brought me back...
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trenace
Needs a job
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posted October 29, 2005 07:34 PM
 Edited By: trenace on 29 Oct 2005 20:35
I am not sure (as the site is in German) but I think the rake is nonadjustable.
However, that's technically speaking... which I think refers to the front and rear being at nominal ride heights.
As you are talking about, as you change the angle of the chassis this FUNCTIONALLY changes the rake even though the spec sheet would not say so... but if those were the ride heights the manufacturer had established as nominal then they would give that different rake figure.
So indeed raising the rear but not the front (or moreso than the front) increases the steepness of the rake, and vice versa.
I've never had a stability problem but I've never been flat out on the bike either.
I believe you are right: for Maxton it would be wise do everything you're talking about and including 1) run the maximum trail this adjustable set up allows, 2) lower the bike for the reasons you say, and 3) lower the rear a little more than the front for the reason you say.
For street riding though I'm trying to get it to become so to speak a "1290 literbike." So it's possible I might do the opposite and raise the rear slightly (either by going to 190/55 or increasing ride height with the Penske by 0.2 or 0.3 inch) and maybe go to 120/65 on the front. That would be unwise without a steering damper but I'm getting one... I'll definitely start out only with the lower profile front tire and then if that works, raising the rear that little bit as well. (The lower profile front tire would also get trail back to about the stock value while using the shorter offset pulling the front wheel in a little, if weight balance indicates that, which I tend to think it will because it seems like more weight is coming off the back than the front, though I'm doing my best at the front.)
I agree with your conclusion completely for Maxton. While I certainly hope something won't happen to me as happened to Mr Hill's son, in a situation like that the last thing that's needed is a bike that is on the edge of instability at speed.
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psycho1122
Pro
Posts: 1608
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posted October 30, 2005 07:30 AM
My Bad..
Manufacturers are steepening rakes too make up for added trail. Shorter chassis and higher in chassis engine mounting also help.
It is a fine balance between quick steering and stability.
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You say PSYCHO like it's a BAD thing!!
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flite leader
Zone Head
Posts: 651
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posted November 18, 2005 01:22 PM
you are not wrong
you simply cant be that cerebral.......which may mean counter intuitive
youve left out many other factors
especially when it comes to road racing....... such as
changing lines while cornering
accelerating off the corner
no less the swing arm pivot has been changed on the 12..... at least once
all of which according to style & feel has a direct effect when cornering
thats also a reason why many bikes during developement
in collaboration with a tire manufacturer......use a Specific Brand & Size
going to the Same size in a different Brand
or the Same Size in the Same Brand can have dramatic results
has absolutely nothing to do with the stickiness or compound
at least not in road racing at all
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or break your ass...!!
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238mph
Pro
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posted November 18, 2005 01:43 PM
That's why no matter how much analysis you do on a computer model... you still
need a rider for feedback.... 
Thank God we're still needed for our 2 cents.... 
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Pilot Powers took me to the grave, and brought me back...
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trenace
Needs a job
Posts: 3056
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posted November 18, 2005 05:07 PM
Edited By: trenace on 18 Nov 2005 17:11
Yup, it's just well preferable to have a starting point, once bolting everything back together, that's not too far off in anything.
Easy enough to change one thing and then adjust that even drastically if need be... but when removing 90 lb or so from the bike and not necessarily equally front to rear, changing swingarm length, changing rear shock and spring, AND changing forks, if several of these are "off" from where they should ultimately wind up it could be easy to adjust to a wrong local optimum... in other words, with this wrong choice of offset and trail giving wrong weight balance such and such is looking from how it rides like the correct rear ride height... but the problem wasn't the wrong rear ride height, it was the wrong offset.
So while I cannot possibly have every setting guessed right, the closer each guess is for that starting point, the easier for riding to be able to discern the correct changes needed, rather than the changes that move to a local optimum but actually further from the true optimum.
Anyway, it's seeming to me that I should look closely on what the weight balance is with the new shorter wheelbase and with all the weight taken off, probably more off the rear than the front though I've tried to be very aggressive with the front weight. If the bike is winding up rear-heavy/front-light then changing the offset for more trail (moving the front wheel back) will help correct that. I am thinking weight balance is more important than any particular trail value per se. In other words, from a clean sheet of paper maybe a given designer would want, oh, 95 mm trail. But if 95 mm trail yields an undesired weight balance but 105 mm the desired weight balance, that latter fact is probably the more important.
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