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McTavish
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It's not that simple. One will need to take the following parameters into this equation: the angle change on your rear swing arm due to different preload
Basically my swing arm bolt went higher from stock, resulting in reduced rake and decrease in the wheelbase. I can't count for an additional rear wheel radius as the pivot point is a swing arm bolt now, not the rear axle.
It can be done, no questions about it. And it's not as hard as with shock linkage bikes where you need to figure out virtual pivot point
Strictly speaking, you are correct.
But it can be spooked into being simpler.

Visualize the frame in space, oriented as it would be with everything stock.
Load the formula on that basis.
Next, take a step back and re-visualize the frame in space, as reoriented in space by all the changes at the back, be it tire height and/or Rider Sag shock length, all of which do nothing more than alter the height of the frame along a datum perpendicular to the centreline of the rear axle.
Take that difference, double it, and add it to the factory tire diameter.
The results will be close enough for what we are trying to figure out.
 

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hahaha, c'mon man, that's too easy. We need to complicate things here.

BTW, for all those who is looking at gsxr forks. They should have basic free sag ~10mm due to the lack of long top out springs. Therefore if you think to go with Hohey top triple:

my measurement was 495mm from the axle to bottom of the steering head.
On 929 lower triple, top of the triple's ear at about the same level as bottom of the steering head.
Top of Hohey offset fork ears are at 182mm from the top of lower 929 ear, once triples are assembled.

Therefore if your fork is :10mm+495mm+182mm=687mm+, you should be good.
Just make sure you gsxr forks still have their 54mm section at 505mm from the axle center. (495mm+10mm) or you won't be able to bolt it into the lower triple




Strictly speaking, you are correct.
But it can be spooked into being simpler.

Visualize the frame in space, oriented as it would be with everything stock.
Load the formula on that basis.
Next, take a step back and re-visualize the frame in space, as reoriented in space by all the changes at the back, be it tire height and/or Rider Sag shock length, all of which do nothing more than alter the height of the frame along a datum perpendicular to the centreline of the rear axle.
Take that difference, double it, and add it to the factory tire diameter.
The results will be close enough for what we are trying to figure out.
 

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thanks,
mine measured like 755mm,but it's close enough. with the top triple zaq posted a link to plus the fact i WAS running the forks dropped 10mm all summer and really liked it, i think it'll be fine.
:thumbup1:

I haven't posted this up....because every time I mention it, I get flamed.

I've been slowly dropping the stock '03 919 forks in the tubes all summer..... have landed at a 10mm drop..... and GASP, I'm running 2 complete turns out of ride height on my rear Penske. (I'm not even sure how many more MM that equates to) --- Ive even tried 2 1/2 and 3 turns out..... the bike was still stable at speed, but just turned in a little tooo quick.

I can say that the bike is starting to handle like the quick turn-in sportbike it should be. I can also say that the bike isn't showing any signs of instability or wabble at 100+ mph.

From owning a Honda 954 w/o a steering damper.....owning several supermotos.....and owning a slow turning TL1000R (that I did quicken the steering on....at which time it became a Wobble Beast down the back straight of our local race-track), I can say that the 919 has a LONG way to go before it gets unstable. I haven't done the math, or even pondered the different theories....but I have taken it to the threshold of instability --- and backed it off to find the 'sweet spot'.

10, 12mm of fork drop? no problem....
 

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McTavish
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6,524 Posts
:thumbup1:

I haven't posted this up....because every time I mention it, I get flamed.

I've been slowly dropping the stock '03 919 forks in the tubes all summer..... have landed at a 10mm drop..... and GASP, I'm running 2 complete turns out of ride height on my rear Penske. (I'm not even sure how many more MM that equates to) --- Ive even tried 2 1/2 and 3 turns out..... the bike was still stable at speed, but just turned in a little tooo quick.

I can say that the bike is starting to handle like the quick turn-in sportbike it should be. I can also say that the bike isn't showing any signs of instability or wabble at 100+ mph.

From owning a Honda 954 w/o a steering damper.....owning several supermotos.....and owning a slow turning TL1000R (that I did quicken the steering on....at which time it became a Wobble Beast down the back straight of our local race-track), I can say that the 919 has a LONG way to go before it gets unstable. I haven't done the math, or even pondered the different theories....but I have taken it to the threshold of instability --- and backed it off to find the 'sweet spot'.

10, 12mm of fork drop? no problem....
Experimental results prevails over theoretical explorations.
No flames from this corner !:halo:
 

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blur
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1,372 Posts
Discussion Starter · #85 ·
:thumbup1:

I haven't posted this up....because every time I mention it, I get flamed.

I've been slowly dropping the stock '03 919 forks in the tubes all summer..... have landed at a 10mm drop..... and GASP, I'm running 2 complete turns out of ride height on my rear Penske. (I'm not even sure how many more MM that equates to) --- Ive even tried 2 1/2 and 3 turns out..... the bike was still stable at speed, but just turned in a little tooo quick.

I can say that the bike is starting to handle like the quick turn-in sportbike it should be. I can also say that the bike isn't showing any signs of instability or wabble at 100+ mph.

From owning a Honda 954 w/o a steering damper.....owning several supermotos.....and owning a slow turning TL1000R (that I did quicken the steering on....at which time it became a Wobble Beast down the back straight of our local race-track), I can say that the 919 has a LONG way to go before it gets unstable. I haven't done the math, or even pondered the different theories....but I have taken it to the threshold of instability --- and backed it off to find the 'sweet spot'.

10, 12mm of fork drop? no problem....

:blowup::blowup:didn't want you to feel neglected.:whistling: j/k

thanks to everyone for your help
 

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McTavish
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6,524 Posts
:blowup::blowup:didn't want you to feel neglected.:whistling: j/k

thanks to everyone for your help
I really enjoyed this thread.
Some really good content accrued.
Especially in terms of what different people are doing, have done, or are going to do.
I think the GSXR front end idea is a good one although it corrupts an "all Honda" approach should anyone have angst over that.
LDH must be on vacation skiing .........................:beehive:
 

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blur
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1,372 Posts
Discussion Starter · #87 · (Edited)
I really enjoyed this thread.
Some really good content accrued.
Especially in terms of what different people are doing, have done, or are going to do.
I think the GSXR front end idea is a good one although it corrupts an "all Honda" approach should anyone have angst over that.
LDH must be on vacation skiing .........................:beehive:
you know what tho, i'm doing this for me and only me.

i enjoyed this too, thanks for your help and i'm sure i'll need more of it:thumbup1:
 

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Back in the day
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270 Posts
chassis software

Thought you guys might be interested in this software

TONY FOALE DESIGNS - Home page

also available on Ebay sometimes

Please becarefull about jacking up the rear and lowering the front of these bikes to get them to 'turn easier'. A 919 is not a big arch bike. It is a point and shot bike. Think of a 919 as somewhere between a supermoto an a RR. Also if you are runing a lower front sprocket and/or a bigger rear sprocket you have some serious squat and chain pull issuies to deal with.

Has anyone put there 919 on some scales to see what the fore and aft weight bias is?

I see a lot of guys take there RC's and raise the rear an inch or more and lower the front and wonder why the front folds. A RC has 23.5 rake and 97 trail. Both at the low side of the spectrum any changes to the negative and things go bad really quick.
 

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McTavish
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6,524 Posts
Thought you guys might be interested in this software

TONY FOALE DESIGNS - Home page

also available on Ebay sometimes

Please becarefull about jacking up the rear and lowering the front of these bikes to get them to 'turn easier'. A 919 is not a big arch bike. It is a point and shot bike. Think of a 919 as somewhere between a supermoto an a RR. Also if you are runing a lower front sprocket and/or a bigger rear sprocket you have some serious squat and chain pull issuies to deal with.

Has anyone put there 919 on some scales to see what the fore and aft weight bias is?

I see a lot of guys take there RC's and raise the rear an inch or more and lower the front and wonder why the front folds. A RC has 23.5 rake and 97 trail. Both at the low side of the spectrum any changes to the negative and things go bad really quick.
I forgot about that software.
It came on a CD that came with the book when I bought it a few years ago.
I've referred to the book a number of times, but have never even installed the software on my computer. I should remedy that !

Once the riding season is upon us again here where I am, I intend to get front and rear axle weights done. I'm expecting a noteworthy rearward bias, a 919 is no SS style bike in terms of mass distribution.

A 919 is a squat machine, very torquey, geared short (to match HP with aero drag) which multiplies the torque even more, and has a low swing arm angle. There are things that can be done to deal with this, at least to a certain degree.

Personally, I'm after trail at the front, but I am going to experiment this season with some front end lowering. Likely a 5 mm experiment using the ride height adjusters.
 

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blur
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1,372 Posts
Discussion Starter · #90 ·
yea, i saw that. but it's for a double wishbone(bmw link)front suspension. nice stuff for the rear it appears.

the 919 is pretty mellow as far as handling goes.i came from a 85 vf750 interceptor and was amazed how similar they felt. the lowering(10 mm)of the front and raising of the rear has been done here before, without any real ill effects.
if you look at the numbers i'll have with the gsxr forks,it's about halfway between the 919 and a 2010 cbr 1000. i'm confident it'll be fine,but i could always go back to stock,not!!:bugeyes:
 

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I see a lot of guys take there RC's and raise the rear an inch or more and lower the front and wonder why the front folds. A RC has 23.5 rake and 97 trail. Both at the low side of the spectrum any changes to the negative and things go bad really quick.
Ya know.... I mentioned the info below to McRomo in a PM talking about fork drop and rear ride height...thought it should be posted.


I have posted up that I have run a 10mm front fork drop on an '03 919 with the stock front forks/springs and the stock rear shock.... and when I switched to my Penske rear shock (which has a 1K spring), I had to run 2 full turns out of ride height on the rear in addition to the 10mm front fork drop to get the bike to feel like it wanted to turn-in correctly. This infomation is only half-correct.

Here's a piece of the equation that I hadn't really considered.... I've been told by Penske that the stock '03 rear spring was at around 1200 lbs. Once I got the Penske, I ended up with a 1000lb spring. The bike is in fact sitting lower in the rear shocks stroke than what it did with the stocker (the stock '03 spring has the bike completly topped out).......I believe that THIS is why I had to run more ride height to get the bike to feel the same. (Makes me wonder how the heck the OHLINS rear is working without it having a rear ride height adjuster....hopefully they made the shock longer or stiffer to compensate....)

Add to that ---- I am still running 10mm of fork drop.... BUT --- I'm also running stiffer than stock 9.25 Traxxion springs. Which, in essence is keeping the forks from diving during braking which reduces trail.

So....... even though I have the rear shock higher, and the front at 10mm. I do have more trail than someone running a completely stock setup, running a 10mm fork drop.

With that said.....if you are running stock springs, its probably a better idea to start at around 4-5mm of fork drop and go from there..

....and as far as the question, "Is it OK to run lower front forks?". The answer more should be ---- what have you done to the rear also? will the RC-51 forks be running stiffer springs?
 

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Arctic
My words were of caution. No flame implied or intended. I respect that you have made the measurements and adjusted accordingly.
It is refreshing to see a thoughtful investigation into this subject. Way to many just bolt it on and take off and more then not, an unhappy outcome.
As for me. I am running an SP1 Traxxion Dynamics front suspension, compression and rebound valves with a 1k, 270mm spring and a short top-out spring. In the rear I run a double click Penkse with a 550 spring and stock link. I had to raise the front when I changed to a 190-55 in the rear. If I go with an aftermarket link I still have some tube left to adjust the front ride height up. I have extended my kickstand an inch already, a
There is a lot more leeway, in regards to trail, on the 919 then the RC.
 

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McTavish
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6,524 Posts
Arctic,
A nominal 5 mm move to start with is a very legit way to go re adjustments.
What others might miss, is the overall picture.
It's the stack up of all the changes that yields the end result.
A change in tire height(s) results in chassis height and chassis pitch changes all on its own. (think in terms of the change in the distance from the centre of the axle to the ground, as a function of a tire diameter change)
And that's before spring rates, preload, and ride height adjustments even come into play.
Then things like the Traxxion style soft top out rear setup can complicate things even more.
Another way to look at is like this, especially for those that don't want to analyse it to death, but want some simple methodology to keep them out of trouble.
Imagine your bike sitting with you on it.
Imagine the seat height above the ground.
Imagine the steering head height above the ground.
Consider all that you have or have not done re changes, and think of all the factors involved.
Don't start beyond the following limits :
A
Do not raise the front and rear by more than 5 mm (both ends by the same amount)
B
Do not lower the front and rear by more than 5 mm (both ends by the same amount)
C
Do not raise or lower the front by more than 5 mm, with the rear unchanged.
D
Do not raise or lower the rear by more than 5 mm, with the front unchanged.
E
Do not raise or lower the front and rear, such that the combined front and rear elevation change exceeds 5 mm. (e.g. front down 2, rear up 3 = 5 total)
Think "5 mm max total, per adjustment".
919s have rather gentle geometry.
One can get away with bigger magnitudes of change that would bite you bad on a steep steering head short trail shorter wheelbase chassis.
Stick to The Rule of 5
It kinda goes from there.
 

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blur
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1,372 Posts
Discussion Starter · #95 ·
Arctic
My words were of caution. No flame implied or intended. I respect that you have made the measurements and adjusted accordingly.
It is refreshing to see a thoughtful investigation into this subject. Way to many just bolt it on and take off and more then not, an unhappy outcome.
As for me. I am running an SP1 Traxxion Dynamics front suspension, compression and rebound valves with a 1k, 270mm spring and a short top-out spring. In the rear I run a double click Penkse with a 550 spring and stock link. I had to raise the front when I changed to a 190-55 in the rear. If I go with an aftermarket link I still have some tube left to adjust the front ride height up. I have extended my kickstand an inch already, a
There is a lot more leeway, in regards to trail, on the 919 then the RC.
what bike are you talking about? rc??
 

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blur
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1,372 Posts
Discussion Starter · #96 ·
Arctic,
A nominal 5 mm move to start with is a very legit way to go re adjustments.
What others might miss, is the overall picture.
It's the stack up of all the changes that yields the end result.
A change in tire height(s) results in chassis height and chassis pitch changes all on its own. (think in terms of the change in the distance from the centre of the axle to the ground, as a function of a tire diameter change)
And that's before spring rates, preload, and ride height adjustments even come into play.
Then things like the Traxxion style soft top out rear setup can complicate things even more.
Another way to look at is like this, especially for those that don't want to analyse it to death, but want some simple methodology to keep them out of trouble.
Imagine your bike sitting with you on it.
Imagine the seat height above the ground.
Imagine the steering head height above the ground.
Consider all that you have or have not done re changes, and think of all the factors involved.
Don't start beyond the following limits :
A
Do not raise the front and rear by more than 5 mm (both ends by the same amount)
B
Do not lower the front and rear by more than 5 mm (both ends by the same amount)
C
Do not raise or lower the front by more than 5 mm, with the rear unchanged.
D
Do not raise or lower the rear by more than 5 mm, with the front unchanged.
E
Do not raise or lower the front and rear, such that the combined front and rear elevation change exceeds 5 mm. (e.g. front down 2, rear up 3 = 5 total)
Think "5 mm max total, per adjustment".
919s have rather gentle geometry.
One can get away with bigger magnitudes of change that would bite you bad on a steep steering head short trail shorter wheelbase chassis.
Stick to The Rule of 5
It kinda goes from there.
mcromo,
it's time for your meds,your ocd is in overdrive.lol j/k i can see this method on a "RR" but 5mm on the 919? i dropped the forks 10 mm, by the time i got to the end of the street(.7 mi) i had forgotten all about it. no longer felt different to me.
or is this just a derail?
 

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Back in the day
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hi,
Small thread jack. After sending my missive to Arctic. I got an email from our local brewery that they are having a tasting at a local purveyor of spirits. So I put on my perforated leather jacket, boots, helmet and gloves and went out on the RC. Beautiful weather out. Had a wee dram of double/imperial IPA and a good burger and came home. I was thinking of you guys that are house bound with inclement weather.
 

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McTavish
Joined
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6,524 Posts
mcromo,
it's time for your meds,your ocd is in overdrive.lol j/k i can see this method on a "RR" but 5mm on the 919? i dropped the forks 10 mm, by the time i got to the end of the street(.7 mi) i had forgotten all about it. no longer felt different to me.
or is this just a derail?
Yes, I need meds.......lol

Note the two lines below extracted from my list:
One can get away with bigger magnitudes of change that would bite you bad on a steep steering head short trail shorter wheelbase chassis.
Stick to The Rule of 5

The intent was to give someone not "into it" and maybe changing bikes or having two bikes, out of trouble.
That's all.
 

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McTavish
Joined
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6,524 Posts
hi,
Small thread jack. After sending my missive to Arctic. I got an email from our local brewery that they are having a tasting at a local purveyor of spirits. So I put on my perforated leather jacket, boots, helmet and gloves and went out on the RC. Beautiful weather out. Had a wee dram of double/imperial IPA and a good burger and came home. I was thinking of you guys that are house bound with inclement weather.
As long as it was a WEEEEEEEEEE dram on top of a good feed I'm ok with your venturing back home on your RC51. I'd hate to see anything bad happen to you, even if it was just reduced reaction to some road looney.
 
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