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Understanding Chassis a little more "part deux"

35K views 140 replies 26 participants last post by  jtdbsr 
#1 · (Edited)
I wanted to write this for you guys as I receive many questions regarding the differences when you make modifications to your chassis, from adjusting the stock parts to full chassis modifications and installing aftermarket performance chassis parts. What I am writing here for you guys is showing the differences between the stock set up of my bike to the currently modified position. Though the actual modifications will vary with everyone and every bike, the following will give you a basic idea on how your bike will change with modifications to your bike's chassis and suspension. Remember, chassis AND suspension are DIFFERENT, and should be treated as such. I will not be putting suspension adjustments, as everyone's settings will be different due to tires, pressure, feel, rider, weight, aggression, etc. The only suspension setting I will list is SAG, as it tends to relate to chassis as much as suspension.


When I bought my '03 999, aside from the full system, it was virtually stock. At the time, the only thing I did was set the preload, rebound and compression for street riding(I have my track bike for being stupid). The bike handled OK at best, and I knew I would have to make changes immediately. Below are the stock numbers of the bike, out of the crate. Keep in mind, my bikes' fore/aft position was in the middle, so the weight bias was closer to 50.3%/49.7%. The rake position was set at 24.5°, making the trail at 97mm.


Rake 23,5° - 24,5°
Trail 91 - 97 mm
Swingarm Length 486mm
Swingarm Angle 9.2°
Wheelbase 1420 mm
Weight Distribution 50.7% front/49.3% rear Monoposto at the front of the fore/aft position


Knowing the basic numbers(and that it wasn't touched before me), the first thing I did was brake out my handy dandy ride height tool, knowing that a basic improvement to the bike is simply by raising the rear ride height. This will reduce the rake angle and trail numbers, however apply more weight to the front of the bike so you won't lose any feel of the front end. The benefits of course is the faster steering of the bike, with less force needed to initiate the turn. So being a svelt 6' & 180lbs without gear, I tend to like a taller bike. I raised the rear up 25mm(It took several set & rides to determine where I was comfortable), but I was kind of cheating as I already had my numbers from my 996. I also raised the front end by increasing fork length from 705mm to 710mm, returning some of the trail lost by the rear ride height increase. The biggest benefit is the CofG being raised, giving the ability to turn in and transition faster. The biggest misconception is that lowering the bike will make the bike turn faster, which could not be any farther from the truth. I changed my rear sprocket by 2, then increased the swingarm length by approximately 19mm. So the following is the numbers after my initial set up.


Rake 23,6°
Trail 92.1mm
Swingarm Length 505mm
Swingarm Angle 10.9°
Wheelbase 1433 mm
Weight Distribution 50.77% front 49.23% rear


Notice the substantial increase in swingarm angle, even though the length was increased a full 19mm. When you raise the front, this increases the angle along with the rear ride height being increased. The effect to this is the weight bias being sent backward a bit, though the rear ride height made more than enough of a change to give a net forward increase in bias %.


The next step was to add our adjustable triple clamps to my 999. Since I wanted to play around I decided to make some minor changes to fork length and add the triples(27mm). If you are asking yourself “Why 27mm?” Simple, the previous trail numbers were too low, so I wanted to fix it in the way it should be. Lesser offsets would work fine, just not as well, remember trail is the “most” important factor in a bikes' geometry, and my analogy with Ducati's is, “you don't put a bandaid on a cut that needs stitches.”


I swapped the forks to a set of 1198 units I received for a great price, and installed them at the same time. When I added these units, I had the fork length measured at 695mm. I did this only to see what I was going to feel with the rake being further reduced, while increasing my trail through the triples. I also knew with the shorter wheelbase, I would not have to lean the bike over as far, plus the higher CofG combination, yielding a faster transition overall. What I found was better feel up front, (along with the previous sentences' statement) but noticed some occasional minor oversteer. It was reduced with some added preload and compression, but not entirely cured. What I found out was that the front tire was just rubbing on the front V-chin of the bodywork when loaded up. The extra preload and compression kept the front from compressing as much. FYI, the front was a bit soft from the beginning. I knew I had to make some changes, but here is the updated numbers below.


Rake 23,4°
Trail 100.8mm
Swingarm Length 505mm
Swingarm Angle 10.8°
Wheelbase 1423 mm
Weight Distribution 51.17% front 48.83% rear


Fast forwarding to today. I've decided to set my bike up the way it is supposed to be, using my triple clamps and putting the bike in the “zone”, for the best possibility to steer fast, great feedback, stability and roadholding capabilities. I decided to keep the 27mm offset, but changed the steering head caster to the 23.5° position. When putting the forks back on, I moved the fork length up to a full 715mm. This gave me the needed clearance at the V-chin, so there should be no rubbing issues when loading the front. Here are the numbers for the latest iteration.


Rake 23,1°
Trail 99.2mm
Swingarm Length 505mm
Swingarm Angle 12°
Wheelbase 1430.5 mm
Weight Distribution 51.18% front 48.82% rear


I rechecked my work to make sure everything is TQ'd properly, however I ran out of time, as my kids made me jump in the pool(It was 90°, after all), and I need to finish putting her back together. I should have her back together tomorrow and will give yo my subjective opinion shortly. By the way, when you factor in the weight loss of the exhaust(8lbs), the weight bias changes from what I have listed to 52.87%front/47.13%rear.


Feel free to throw your thoughts and questions out there. I wrote this timeline of chassis solely for you to observe how changes you make to a bike create a result. This was simply a cause and affect write up. Have fun with it, especially the guys that ask “what if I add this?” or “what if I change that?”.
 
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#2 ·
Do you have a magic wand that will instantly make my 848 track corners like it's on rails and be very telepathic?


Oh ya..... that's right....Its call a 1199 Panigale! :p
 
#3 ·
It may not be a wand, but stop by and we'll get your bike to corner like it had "reading Railroad" painted on the side. :moped:
 
#4 · (Edited)
Update

Put her back together this morning and just arrived back from a little test ride.

Though I haven't twisted in anger yet(won't on city streets) I will add this:

What I will say about my initial ride is that the bike feels 20lbs lighter. I say feels, as it is a subjective observation.

Where the feeling is coming from is initial turn in. It takes much less force to initiate a turn. As I enter the turn, there is no sign of over/under-steer, and I feel every movement as if the bars are on the tires. As explained before, the higher CofG makes turn in faster, but the shorter wheelbase requires less lean angle, so you are transferring much faster than before. These attributes combined with the dramatically reduced rake makes for a very fun twisty killer, as you have to work less to get the result versus before.

The benefits of the reduced rake, combined with the extra trail has transformed the bike into knowing where it is at all times. Almost telepathic.

The increased swingarm angle(I adjusted preload, compression & rebound) keeps the rear tire on the ground, and makes the bike want to go forward....and fast. Grip is just right, but there's no feeling of weight transference to the rear, as the bike almost feels as if it just stays level at all times.

Speaking of level. The bike feels level, literally. This means less weight on my wrists, though it does feel longer to the tank than before. This is due to the rake change, so expected.

In conclusion, the bike is comfortable and confident feeling, holds the road very well, turns very fast, with little effort, and exits on any line you pick(doesn't move from that line unless you want to). Changing lines midcorner was a simple task, unlike before.

I hope this helps you guys as to see the differences chassis and suspension adjustments can make, both objectively and subjectively. If you have any questions, feel free to call me or email me at anytime(well, reasonable time anyway).

Ciao
 
#7 · (Edited)
Hi Brian,

Great article!

I'm also going around the geometry of my bike!

My bike is a 1098S track preped, with a tuned engine. The chassis has a 26mm offset triple.

I'm having a problem with wobble on the straights at WOT even in 6th.

One of the causes of my problem and a thing to watch when putting a lower offset triple is the wheelbase, as each millimetre of offset you take you roughly get out the same in the wheelbase. And that causes all the sort problems stability wise.
The problem is worse in the 1098 bike as the swing arm doesn't have the range of the 999 bikes. I'm at the longest position and have 505mm of swingarm length, that with a brutal engine, get's me the wobble.

Other cause that i will study in the next track day is a COG height, high in my case, the forks are flushed with the triples, and coupled with a very high speed, acceleration track like Estoril, this might be another cause for the wobble.

Here is a video of a bike with the same problem, mine is a little worse, but you get the point.



After the trackday i will have a idea of the problem.

I'm already considering a SF swingarm as will get me the lost wheelbase, and give more front weight.

If you have more solutions for me to try... :)
 
#14 ·
Hi Frank,

Great article!

I'm also going around the geometry of my bike!

My bike is a 1098S track preped, with a tuned engine. The chassis has a 26mm offset triple.

I'm having a problem with wobble on the straights at WOT even in 6th.

One of the causes of my problem and a thing to watch when putting a lower offset triple is the wheelbase, as each millimetre of offset you take you roughly get out the same in the wheelbase. And that causes all the sort problems stability wise.
The problem is worse in the 1098 bike as the swing arm doesn't have the range of the 999 bikes. I'm at the longest position and have 505mm of swingarm length, that with a brutal engine, get's me the wobble.

Other cause that i will study in the next track day is a COG height, high in my case, the forks are flushed with the triples, and coupled with a very high speed, acceleration track like Estoril, this might be another cause for the wobble.

Here is a video of a bike with the same problem, mine is a little worse, but you get the point.

Mugello_20120407_Session2.wmv - YouTube

After the trackday i will have a idea of the problem.

I'm already considering a SF swingarm as will get me the lost wheelbase, and give more front weight.

If you have more solutions for me to try... :)
If you are having issues with stability in straights, it would seem your problem is with load transfer, resulting in your front getting too light. It's not a wheelbase issue. If that were the case, every Japanese bike would be doing that, as they are all under 56" wheelbases.

I wouldn't worry about the 26mm offset triples. I run 27mm on my street bike. That is probably the one factor saving your skin right now.

This may be due to your swingarm being too flat. Back when the 1098 came out, everyone "assumed" that you had to lower the rear end to get it around the track. Not so much.

It could be many things. Feel free to contact me and let's see if we can sort it our for you.
 
#10 ·
26mm is a ridiculously small amount of offset. Why not 28mm and eccentric head bearings to change the rake to 23.5 degrees? You may want to turn your damper in a few clicks...:eek:
 
#16 ·
interesting discussion guys. would you mind explaining the measuring technique in a bit more detail for those of us that don't know enough about the subject. for instance, when you say the swingarm length is x, from where to where is the measurement taken, and what tools are used. also, with today's superbikes, what are good geometry settings to start off with?
 
#17 · (Edited)
I will try and explain the principal measurements.

Front

For the front you need to get 3 values.

Fork Offset - Distance between the centre of the forks and the centre of the headstock
Headstock angle
Radius of the front tyre

Rear

Swingarm angle
Swingarm length - From the pivot of the swingarm in the frame to the centre of the wheel. A bigger length improves traction and transfers weight to the front.

General Measures

Wheel base - Centre front wheel to centre back wheel
Weight distribution F an R

Some general rules

The measurements are taken without sag, with wheels off the ground, except weight distribution. :)

The geometry we are aiming is:

Trail - 100mm
Swingarm angle - 10 to 12%
Weight distribution - 52F 48R

There is also another variable that i think important , CoG position, but i don't know values for this.

Now, this values and the way to get them are very different from bike to bike and each change is diferent for each bike. Also the setting must be adaptable to each rider.

Brian will surelly complement my information, as he studied more this geometry stuff.
 
#21 ·
#24 ·
A general consensus is that most people have the best feel and control of the bike with the angle at 10-11*. That said, there are other variables to consider, swingarm length for example.

You should not look for the question, rather the answer, and the degree is just an equation. The upward force versus the TQ required to compress the spring in a "linear"(you asked about the linear link) movement is your answer, at a force amount to ensure enough anti-squat of the chassis during acceleration so that the longitudinal force doesn't transfer too much weight to the rear.

So does the "answer" require the same equation from an 848 versus the 1198?
 
#25 ·
Fair enough.... not just swingarm angle
So you are saying that the torque from the 1198 is considerably more than the 848 in causing the anti-squat under power and the swingarm length (which will also have an effect on the swingarm angle) will determine the balance in weight transfer (pro-squat) versus the anti-squat?

So.. if I was running 14/38 gearing on my 1198 (505mm swingarm length) and I had a DU714 ttx shock (shorter clavis) with a linear rocker.... what do I need to complete the equation?

Surely the subjective nature of rider style/experience getting on the gas (hard/soft) would get in the way of all of this objectivity?
 
#26 · (Edited)
Think of the TQ of either motor as the leverage necessary to create Squat Effect. However it is just one variable. Swingarm length, angle and spring rate have similar affects on the sum you are looking for.

Invariably, its in how you set the bike up that will create anti-squat though. Think as in Newtons' third law of motion regarding equal and opposite action(force) theory. You need to balance the components within their own regard to summarily control squat.

Does that mean the angle will always be the same on every bike? Will the swingarm length be identical? Even to make it more finite, how about an 848 versus and EVO 848 with different TQ characteristics? In this case you deal with increased motion versus peak TQ at a given distance, so equalled out to a lower result in TQ at any given RPM. This will affect your development too.

How about different TQ values and contact patch area between all of the bikes at lean angles and their relative cosign? Tire size and pressure levels as a variable, not to forget compound.

The subjective nature of rider style/experience getting on the gas is truly a variable, but don't forget electronics. Does the bike have TC? What level do you run it at? How does it affect your riding?

FYI, instead of running 14/38 gearing, I would recommend a change it to a 15/40, and the reasons are as follows: 1. decreasing angle between both radius' aiding in anti-squat. 2. (not as big a deal)opposite tooth counts aid in chain durability by decreasing the percentage of a given link hitting the same tooth, resulting in premature wear. 3. The difference in ratio is only .04, so not going to be a huge difference.

A 505mm swingarm length is recommended simply because the swingarm is inherently short for race purposes, otherwise Checa's or Zemke's bike would have a stock unit on it.

Having a baseline number is good, but never the answer, as you will find. You may notice the bike handles better with an angle of 9.5* and more preload but a softer spring and compression because you run the throttle very smooth through the turns. Another issue is the rod position in relation to the shock position. This will position the rocker at a specific angle causing premature radius movement of the rocker/rod/shock points of contact, which may increase or decrease your overall rate.

Just FYI, if you put that last statement together with ride height and swingarm angle, a little light should come on. Bink!

I know I'm splitting hairs here guys, and I'm not acting like I'm into loop versus string theory but I bring this out to let you guys open up to all of the variables of developing your bikes. It's not a finite answer, only an infinite question.
 
#27 ·
Oh Brian, I think I'm just going to buy a fixed frame push bike and sell the Duke!
It's doing my head in. I understand the variables at play, but am never going to be able to analyse it all in the detail you described.

I will throw a new rear tyre on and find out what my baseline measurements are and work from there.

Cheers
 
#28 ·
LMAO!

I was splitting hairs! I get a bit nerdy sometimes.

Start at around 10* unloaded swingarm angle and work from there. So get the bike off of the ground when measuring and set the ride height to get you in the area.

Measure your heights F&R to determine the angle. If you need help, give me a holler or an email.
 
#29 ·
Brian, that's a pretty awesome write up. I have a couple of questions though, as I tend to simplify things a lot.

Once you have installed your components, eg: triples, shocks, springs etc - then set your sags up to your weights, then aren't things such as swing arm angles and rakes going to be determined by any changes you make to fork height and ride height anyway? Why is it really necessary to know these dimensions? If you treat your trail measurement as the starting point target measurement, and for argument sake lets call it 100mm, then by starting at your fork height, the only way to then change your trail is by adjusting ride height, correct? This will change all the other dimensions, but practically, do you really need to know them?

If you are not happy with the result of your starting point fork height, you can either raise or lower them and adjust your trail back by re-adjusting ride height, correct?

Please forgive my ignorance here, I am trying to understand if there is something I am missing myself.
 
#35 ·
Hi Soz,

My write up was more a description of the relation of the set up to the variables associated with it, thus realizing that there is no "one" magic number for all bikes. Just had a few extra words in it. :blah:

Inevitably, you develop the bike objectively to give a rider the ability to make changes based on his subjectivity. Equal and opposite.
 
#30 ·
Hey Soz
I was working on the same principal initially, then I started to realise that you need to start at the back of the bike and then move forward. If you get your swingarm angle/wheelbase etc correct (thru your ride height adjustment) then you will get the bike driving forward out of the turns. Then you approach the front end for turn in/stability etc.

I found this really long link from Dave Moss who explains it in basic detail.


Measuring swing arm angle, interrelationships of shock length, and axle position, How to measure swing arm angle cheaply and quickly, how shock length and ...
 
#32 ·
Hey Soz
I was working on the same principal initially, then I started to realise that you need to start at the back of the bike and then move forward. If you get your swingarm angle/wheelbase etc correct (thru your ride height adjustment) then you will get the bike driving forward out of the turns. Then you approach the front end for turn in/stability etc.

I found this really long link from Dave Moss who explains it in basic detail.


Measuring swing arm angle, interrelationships of shock length, and axle position, How to measure swing arm angle cheaply and quickly, how shock length and ...
Hey woopy I couldn't get that link to work....
 
#36 ·
Interesting how taller race tyres have seen many aftermarket products hit the market for raising swingarm pivot points (triumph/bmw etc)
Check out Dan Kyles website - they make up to 4mm increased inserts for the swingarm pivot to get more angle on the bmw 1000rr.

Also notice that they are now machining new fork caps for ohlins cartridges, to make the fork longer.......... to increase trail!!!!!

Very interesting to see how much geometry is getting played with.
 
#37 ·
It's all about the MM with geometry. When the new S1000RR came out, the did such little tweaks on paper, but made a mile of difference.

IIRC, BMW increased the 2012 SA pivot for better feedback at all lean angles and to assist the shock in it's actuation.

This was the equal part of the opposite when they increased fork length, reduced offset and rake up front.
 
#41 ·
and with this I have 300 posts!
 

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#43 ·
Let's bring this thread back from the dead..

Took some measurements on my 848 EVO as I had run it on track with the 180/60 rear tire:

Swingarm Angle: 9*
Swingarm length: 495mm
Trail: 88.9mm

Raised the rear to set swingarm angle to 10* and after dropping the forks all the way, trail came in at 90mm. Based on this thread, a 25mm offset triple would get the trail to 101mm, which means I could raise the rear a bit more to get it back to 100, correct?

Haven't touched the eccentric yet to play with the SA length. Right now it's at the 6 o'clock position. I'm guessing I'll have to try a couple of different rear sprockets to get the correct chain tension as close to the far end as possible.
 
#44 ·
Hi Steve, a couple of things I want to address to help you out. Don't fixate on the exact trail number yet. One of the equally important things you mentioned is the swingarm length. It's length will also have an affect on squat, weight bias, plus aid in rear trail. Concentrate on getting the gearing positioned so as to allow the swingarm length to increase(try to get it to 500mm).

This will be changing everything, so when the kit arrives, we will go over everything you discussed. If I didn't give you my cell phone yet, PM me and I will shoot that to you brother. That is when the fun begins. ;)
 
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