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152 sounds off, as stock is 98 and 110 seems to be a current day high.
Math error or bad formula somewhere along the way perhaps ?
I deleted the post, I screwed up, I added the offset of 30mm, which I should subtract


Stock trail is 98mm, I think mine was 92mm, which still doesn't make any sense. I have to remeasure it again once I get home

It was 122mm of the fork tube if I remember correctly. 122-30mm offset = 92mm. Something doesn't make any sense

All my #s, including previous #s were with bike resting on the ground.
 

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McTavish
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919 Front End Geometry re Trail Change from Offset etc.

I deleted the post, I screwed up, I added the offset of 30mm, which I should subtract


Stock trail is 98mm, I think mine was 92mm, which still doesn't make any sense. I have to remeasure it again once I get home

It was 122mm of the fork tube if I remember correctly. 122-30mm offset = 92mm. Something doesn't make any sense

All my #s, including previous #s were with bike resting on the ground.
OK, I threw down the gauntlet in front of myself and went at it.
Besides, it's been too long since I have given LDH something else to laugh over at my expense. I mean, who else but “moi” buys a special angle gauge and lays on the floor to measure the swing arm angle and resultant changes from chassis manipulations fore and aft.

I forgot that touring919 had sold me a 919 headstock clip some time ago, so into the basement for that nice bit of kit (destined as a future graft on to an old CB750 to allow a full 919 front end clip fitment).
I removed the top triple and got the edges and rules out.
The stock 919 triple clamp stem / fork tube offset is 35 mm as best as I can tell. The 50 number we've heard before is therefore whacko out to lunch.

Next.
I did a 2x scale drawing with my 40 year old drawing kit in an effort to find out the change in trail for a 919 fitted with 929 30 mm offset triples as compared to the 919’s 35 mm (nominal) offset.
I came up with a measured 4.5 mm.
Then I went to the trig book and figured Cosine of 25 degrees would be right.
Cosine of 25 degrees is 0.9061
0.9061 x 5 = 4.5305 mm.
So my scale drawing work was only out by a mere 1/1000th of an inch. (LDH, smiling yet?)
I’d say the work has been validated and one can therefore say that using 929 triples will increase the trail by 4.5 mm and yield a 102.5 mm resultant as compared to the 919’s 98 mm of trail.

BUT
The above only holds true if the front chassis height remains the same.
And front chassis height needs to be considered by a datum line perpendicular to the horizon.
All your direct measurements are going to be on a 25 degree angle, be it fork length or sag checks.
To convert those measurements to true height change, simply use the Cosine of 25 degrees again, as in 0.9061, which is very close to “90 % of measured”. For example, if you reduce your sag by 10 mm, you have lowered the headstock by 10 X 0.9061 = 9.061 mm, which is a big change.
I have not bothered to figure out the headstock angle change as a function of ride height. At some point I will, but my 919 work has been done on the basis of raising the bike up the same amount front and rear, in an effort to :
• Increase swing arm angle to reduce Squat Force
• Maintain trail for maximized “feel” and “bump trail loss” margin.
• Elevated Centre of Gravity for increased chassis roll leverage.
• Increased ground clearance re footpegs and levers.
 

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blur
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Discussion Starter · #64 ·
I have not bothered to figure out the headstock angle change as a function of ride height.
so what are you waiting for?? your on a roll.lol i'd appreciate it if you'd finish my home work and i won't have to take your lunch money:bugeyes:

hey,really thanks for the help man:thumbup1:
 

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ok, I need to measure my exact rake angle before I can give you correct trail #s.
Any ideas how I get get it?

Helimech,

Hohey triple with 929 lower: 182mm from the top of 929 triple to the top of Hohey fork ears
 

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McTavish
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ok, I need to measure my exact rake angle before I can give you correct trail #s.
Any ideas how I get get it?
I don't have formulas to support it yet, but I have some articles in hand that referred to two different bikes each losing 0.4 degrees of steering head angle by lifting the rear 8.4 mm and 10 mm respectively.
So I think a reasonable rule of thumb for now is be it front end or rear end height change, each 5 mm of front:rear differential height change will affect the steering head angle by about 0.2 degrees.

Also, seem as I'm fishing about.
Tidbits on Trail.
A book I have says that for each nominal 4 mm of front sag change, the trail changes by a nominal 1 mm.
What I can't find is something I distinctly remember reading.
As in decent front end feel is tied to trail numbers of 95 and higher.
At somewhere around 92 mm "feel" is lost.
I have been told by an AMA ace whiz that some AMA Pro guys are using as much as 110 mm these days, and it sounded that anything less than 100 is very much avoided.
The new Moriwaki MD600 chassis comes with 101 mm.
The new MV F4 comes with 100.
The RSV4 comes with 105.
A 919 has 98.
My point is to be careful not to go too far dropping the front for quickness at the sacrifice of "feel" derived from trail.
I have a feeling that someone running stock soft 919 springs in a stock 919 front end that has been dropped by 10 or more mm by fork tube height changes, could very easily be getting below 95 mm of trail. IF such a person was also doing serious track days riding, they would be sacrificing "feel".
This is not a street riding issue, just a track one at higher lean angles in the slip stick zone of tire grip.
 

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blur
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1,372 Posts
Discussion Starter · #67 ·
ok,
here's what i've come up with.

*this is without hohey top triple*

stock head=25 deg
stock forks=762
stock offset=35
wheelbase=1460.5
trail=98

*with 738mm gsxr forks*

head=24.14 deg(cbr1krr=23.3..rc51=23.5)
forks=738
offset=30
wheelbase=1450.594mm
trail=101.346mm

it shouldn't be even as twitchy as a cbr or rc51.that is without the hohey dropped upper triple,if you add that into the mix i'll be able to dial in anywhere from near stock to the above numbers.
so i've decided to do it, all i need now is the $$$$$$$lol
 

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McTavish
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6,524 Posts
ok,
here's what i've come up with.

*this is without hohey top triple*

stock head=25 deg
stock forks=762
stock offset=35
wheelbase=1460.5
trail=98

*with 738mm gsxr forks*

head=24.14 deg(cbr1krr=23.3..rc51=23.5)
forks=738
offset=30
wheelbase=1450.594mm
trail=101.346mm

it shouldn't be even as twitchy as a cbr or rc51.that is without the hohey dropped upper triple,if you add that into the mix i'll be able to dial in anywhere from near stock to the above numbers.
so i've decided to do it, all i need now is the $$$$$$$lol
What formula did you use for the above 738 review ?
At a glance, all the numbers look reasonable except for the steering head angle change.
I've got two articles suggesting to expect over a 1 degree change. The articles do not say how they derived it, and could be wrong.
I can figure things out long hand with measurements but not until my bike is back together. I have not figured out the trig yet, man alive, it was Grade 11 when we learned that stuff !!! I still have my little metal cased geometry set !!!!!
 

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blur
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1,372 Posts
Discussion Starter · #69 · (Edited)

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blur
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Discussion Starter · #70 ·
btw here's the #'s for the 2010 cbr1krr

Wheelbase: 55.4 inches
Rake: 23.3o
Trail: 96.2mm (3.8 inches)
 

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McTavish
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6,524 Posts
this one

RB Racing Professional Rake and Trail Calculator

yours could be right but we changed offset bringing it in which pushes the head up again
I played with it a bit.
The only way I could get the stock trail of 98 mm to reconcile with the offset, is if a 38 mm offset was used.
I don't think I blew my measurement by 3 mm.
But aside from that, the other playing I did with it seems to yield reasonable looking numbers.
My bones tell me that the formula is correct.
By the way, if you run the GXSR front end on your 919 with 40 mm of Rider Sag, you'll be a hair below 24 degrees and a touch less than 100 mm of trail once the offset correction has been added in. Those two numbers go together nicely.
 

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blur
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Discussion Starter · #72 ·
yea that's what i was thinking,plus with the hohey top triple i could push it almost anywhere i like:thumbup1:
 

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blur
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Discussion Starter · #73 · (Edited)
I played with it a bit.
The only way I could get the stock trail of 98 mm to reconcile with the offset, is if a 38 mm offset was used.
I don't think I blew my measurement by 3 mm.
But aside from that, the other playing I did with it seems to yield reasonable looking numbers.
My bones tell me that the formula is correct.
By the way, if you run the GXSR front end on your 919 with 40 mm of Rider Sag, you'll be a hair below 24 degrees and a touch less than 100 mm of trail once the offset correction has been added in. Those two numbers go together nicely.
did you account for rear sag too? it's not just subtracting 40mm form fork length,correct?

yea i kept getting 3.99 in trail stock. do they measure trail with bike on ground? that changes the fork length
 

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blur
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Discussion Starter · #75 ·
I assumed a constant rear sag and simply varied the front.
Just like a hard tailed chopper !
When constant rear sag is assumed, just playing with fork length is valid.
but you fork length of 762 was with wheel up,correct?no sag. if you figure 40 mm sag for both then it's not as bad as it looks on paper.

all my numbers in the calc were free length, i figured sag w/stock and sag w/gsxr wouldn't change the numbers much:idunno:
 

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McTavish
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but you fork length of 762 was with wheel up,correct?no sag. if you figure 40 mm sag for both then it's not as bad as it looks on paper.

all my numbers in the calc were free length, i figured sag w/stock and sag w/gsxr wouldn't change the numbers much:idunno:
I used a 35 mm Rider sag for the 762 stockers, then compared that to the 738s set up with 35 and then 40 mm of Rider Sag.

The thing is, your numbers are good.
And you are quite correct, whether free length or sag lengths are used, the results don't vary much - as long as you use the same criteria for a comparison, in other words don't mix Free Length and Rider Sag numbers.
 

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McTavish
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I couldn't use this # as my rear shock is no longer stock and my forks sit a little lower than stock in triples. All the above = in lower head angle than stock
I think there is an easy way to spook the formula into working for you.
If you know how much your rear ride height has changed, double that number and add it to the rear wheel diameter.
The formula has to halve the wheel diameter as radius is the issue re chassis geometry, not diameter.
 

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McTavish
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I think there is an easy way to spook the formula into working for you.
If you know how much your rear ride height has changed, double that number and add it to the rear wheel diameter.
The formula has to halve the wheel diameter as radius is the issue re chassis geometry, not diameter.
zaq, I know you will realize it buy others may not.
So for the benefit of them, here is the following :
Load the formula data entries initially with stock numbers.
Then do an adjusted rear wheel diameter to reflect your rear shock length induced rear ride height change.
Note the resultant calculated differences.
Then move to the fork tube length change aspect and see those affects.
 

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zaq, I know you will realize it buy others may not.
So for the benefit of them, here is the following :
Load the formula data entries initially with stock numbers.
Then do an adjusted rear wheel diameter to reflect your rear shock length induced rear ride height change.
Note the resultant calculated differences.
Then move to the fork tube length change aspect and see those affects.
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
 
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