Suspension tuning tech question

[Flows]

Hi folks, can someone explain the mechanics of rear sag's impact on cornering neutrality?

I changed the rear dynamic sag from 29mm to 25mm and lowered the front and rear tyre pressures by 1.5 PSI. The bike then noticeable wanted to stand up in corners. Smooth / bumpy, uphill / down hill corners were much the same and needed a good amount of low bar push to keep the bike in the turn. Throttling on helped alleviate it.

I set the rear sag back to 29 and that fixed 90% of the issue. I then set the front tyre back to where it was and that improved turn in a bit as well and got me back to where I was. It was a surprise to me how much of an impact these changes had. I understand the front tyre dynamics but what is the physics involved with the rear sag / cornering neutrality relationship?

Here are the rest of the data for reference.

Rider weight	95.5kg / 210 lbs with gear / full tank + 2.5kg / 5.5lbs tank bag
Suspension Oil	Factory - 5000k / 3100 Miles
Tyres	Factory - 5000k / 3100 Miles
Riding Style	Twisties

Setting	Value	Notes
Front Preload / Static Sag / Dynamic Sag	0 Turns Out / 36mm Static / 50mm Dynamic	Max preload / 89mm / 75mm (dust cap to bottom bracket - 125mm unloaded) Stiction: 6mm static / 2mm dynamic
Front Rebound	6 Clicks Out	1/4 turn = one click
Front Compression	4 Clicks Out	1/4 turn = one click
Rear Preload / Sag	~4.5 Turns In / 9mm Static / 29mm Dynamic	Turns measured from min rather than max. / 420mm / 400mm (bottom of hump key to bottom center of axle internal diameter - 429mm unloaded) ~0.5mm sag per preload ring notch. Stiction: 1mm
Rear Rebound	4 Clicks Out	1/4 turn = one click
Rear Compression	6 Clicks Out	1/4 turn = one click
Front Tyre Pressure	36.5 PSI Cold
Rear Tyre Pressure	40.5 PSI Cold

[Chris0053]

You mention ‘clicks’ on front suspension, the manual just says ‘turns’, when I adjusted mine they didn’t click. Manual says 2 turns out on the front compression damping, you are only at 1 (using 4 clicks, or 1/4 turns) I set mine at 3 1/2 turns out total, for the best handling for me. The rest of my settings are still at stock. In the rear I only adjusted preload softer to set sag for my weight. Also my front tire pressure is set at 34 psi, 40 seems awful high to me. Hope any of this helps you.

[Flows]

Thank Chris. see "1/4 turn = one click" in notes for an explanation on that. Admin / Mods can you re-enable editing for the OP. I have the front and rear tyre pressures backwards. The question is what is the physics involved and why would reducing the rear sag make the bike stand up in corners and setting it back bring it back to neutral?

[Chris0053]

Softening the front compression damping allows quicker lean in, cornering. Harder front compression damping will cause the bike to run wide on corner entry. When you soften the rear, the front will drift wide on corner exits, hope this helps

[Flows]

Thanks Chris, still trying to understand the OP question, that is what is the physics involved and why would reducing the rear sag (raising the back end) make the bike stand up in corners (having to hold a push on the inside bar to keep it in the corner) and setting it back (lowering the rear end back to where it was) restore cornering neutrality?

[Chris0053]

Thanks Chris, still trying to understand the OP question, that is what is the physics involved and why would reducing the rear sag (raising the back end) make the bike stand up in corners (having to hold a push on the inside bar to keep it in the corner) and setting it back (lowering the rear end back to where it was) restore cornering neutrality?

Sounds like you need to soften your compression damping

[Flows]

Can you explain the underlying physics? Not seeing how compression dampening could affect the way rear sag impacts cornering neutrality as yet.

[sixpack577]

it is simply how much the spring loads and unloads over bumps, how quickly it does that, and how much of that gets transferred into the frame.
Raising the rear puts more weight on the front, which gives better control, and the rear is basically just following the front anyway, the more in unison the better.

[Chris0053]

Can you explain the underlying physics? Not seeing how compression dampening could affect the way rear sag impacts cornering neutrality as yet.

I’m thinking your rear compression is too hard which can also cause you to drift wide, add to that increasing your preload via sag. Remember when fine tuning your suspension only adjust one area at a time, easier to diagnose

[Flows]

This is an odd single variable and repeatable exercise. I asked a suspension tuner here and they couldn't explain it either.

Raise the rear 4mm and the bike wants to stand up in corners. Lower it back down and she turns in and sits on rails like before. I would have thought the impact to the rake and trail would have made it want to turn more rather than less. Wouldn't have also thought there would not be a meaningful increase in weight on the front from a 4mm increase in tail height and that doesn't explain why it did it going up or down hills as well... ?

[zeepopo2]

This is an odd single variable and repeatable exercise. I asked a suspension tuner here and they couldn't explain it either.

Raise the rear 4mm and the bike wants to stand up in corners. Lower it back down and she turns in and sits on rails like before. I would have thought the impact to the rake and trail would have made it want to turn more rather than less. Wouldn't have also thought there would not be a meaningful increase in weight on the front from a 4mm increase in tail height and that doesn't explain why it did it going up or down hills as well... ?

Standing up in corners is not on preload. It's a lack of rebound speed in balance to the other settings. Preload increases the reaction time of the suspension.
Every other setting has to have a a force applied to it before there is a reaction. Preload is stored up force. Every setting is balance to preload. Change preload? Settings have to change accordingly. You raised the rear and lower the front. That means your weight bias is now much higher on the front tire. But your tire pressure up front is too high. Your rebound speed has to come up, on the front. Open her up a few clicks at a time. But now that you changed front rebound rear has to match as well to adjust one then the other to match the forces. Faster front rebound will throw the force towards the rear to the rear now has to be faster to balance the force and keep that front tire tracking properly. That front tire pressure is too high with that much front load it's a crash waiting to happen. Keep front and rear pressures even at 36 cold. 40.5 cold is probaly around 44 hot or even higher. Front tire can't do much for you at those pressures. Bike stands up because that front tire can't track well and you are basically turning with the rear. So more throttle removes weight from the front and takes away some of the symptoms. In reference I run 15 to 18 dynamic rear sag. Been there done that lol.

I just realized something. Is your front sag correct? 50mm of dynamic? Should be in the 32 to 37 range. That is way too low on your suspension travel. Wouldn't be surprised if you are bottoming out at times with such high dynamic sag. If correct with 0 preload out on the front you are outriding that spring. Need a new spring. Your front ain't doing much for you if that's all the sag you can get out of the front.

[Flows]

Thanks Zee, I am trying to get the admins to edit it, I entered the front and back pressures around the wrong way. All understood on what you are talking about but the fact is when I change nothing else and raise the rear by 5mm I need to hold push pressure on the inside bar. When I set it back down 5 mm that goes away. I don't think it is weight transfer as up hill and down hill corners are the same, holding push on the inside bar when the rear is high and neutral when I lower it back down again.

One thing I would gently push back on is preload changing the stored force. I would offer that only changing the weight or weight distribution of the bike will change the tension stored in the springs. Preload simply moves the same tension to a different part of the fork or shock stroke. Essentially it is for setting sag rather than "pre-load" as such. Preload could only "pre-load" if we had 0 static sag. My 2c, and happy to be proven wrong.

[zeepopo2]

Thanks Zee, I am trying to get the admins to edit it, I entered the front and back pressures around the wrong way. All understood on what you are talking about but the fact is when I change nothing else and raise the rear by 5mm I need to hold push pressure on the inside bar. When I set it back down 5 mm that goes away. I don't think it is weight transfer as up hill and down hill corners are the same, holding push on the inside bar when the rear is high and neutral when I lower it back down again.

One thing I would gently push back on is preload changing the stored force. I would offer that only changing the weight or weight distribution of the bike will change the tension stored in the springs. Preload simply moves the same tension to a different part of the fork or shock stroke. Essentially it is for setting sag rather than "pre-load" as such. Preload could only "pre-load" if we had 0 static sag. My 2c, and happy to be proven wrong.

Preload doesn't know where in the stroke it's at. All it knows is how much energy is stored against the ground. More energy equals faster tracking when the road moves away from the tire. It's the suspensions response to a free fall. You see preload as a single adjustment, when in reality it moves the median suspension curve left or right, impacting each and every movement and as such needs readjustment.

Why run such sag in the front though? Where you get that number from? I would start there. Use the zip tie method on the forks. You will see that your fork is bottoming out basically leaving you without a suspension in the front, more of a pogo stick. Change the sag up front. That rear ride height is just showing you that your front end ain't working well when under load. It's just more obvious now that you raised the rear.

[Flows]

Preload doesn't know where in the stroke it's at. All it knows is how much energy is stored against the ground. More energy equals faster tracking when the road moves away from the tire. It's the suspensions response to a free fall. You see preload as a single adjustment, when in reality it moves the median suspension curve left or right, impacting each and every movement and as such needs readjustment.

At the great risk of learning something entirely new, I must first disagree my learned colleague.

Spring preload is one of the most misunderstood concepts when discussing suspension. Often we hear riders talk about adjusting their motorcycle’s spring preload to make the spring stiffer or softer.
This is a misconception: changing spring preload does not change the spring rate at all. The spring has the same rate regardless of how the preload adjustment is set. Let’s look at what really happens... It is important to note that all types of forks can have the preload adjusted internally by changing spring spacer length...
With a given amount of preload force on the spring, it will take that same force to initiate suspension movement when the suspension is fully extended. As preload is increased, it takes more force to cause the fork or shock to begin to compress. When preload force is decreased, less force is required to cause movement. Keep in mind that when the motorcycle is resting on the ground with the rider on board, the suspension is compressed. When preload is changed the sprung mass is held higher or lower. This means more preload does not require more force to initiate movement after the weight of the bike and rider has compressed the suspension.

Parks, L. (2015) Total Control - Second Edition, Section 1, Chapter 3, p. 11

I would add that the more fork preload (less sag) the less force is required to initiate movement due to the progressive air spring curve.

I am maxed out on fork preload FYI. Zero turns out. I could always add a bit of oil until I get the Ohlins or Bitubos lined up. Or put luggage in the back and get rid of the tank bag...

[kml]

[sixpack577]

You are 'maxed out on fork preload'
How much do you weigh?
220 lbs+ ?

[sixpack577]

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What book is that?

[kml]

What book is that?

[sixpack577]

View attachment 1676555

I have that,
I need to dig it up and try out this Motool Slacker I still have new in the box.
lol

[zeepopo2]

At the great risk of learning something entirely new, I must first disagree my learned colleague.


Parks, L. (2015) Total Control - Second Edition, Section 1, Chapter 3, p. 11

I would add that the more fork preload (less sag) the less force is required to initiate movement due to the progressive air spring curve.

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I am maxed out on fork preload FYI. Zero turns out. I could always add a bit of oil until I get the Ohlins or Bitubos lined up. Or put luggage in the back and get rid of the tank bag...

Yeah I saw you maxed out the preload up front. That's why you need a new spring. You gave it all it can and that's all the sag you can get. I'm 210+ gear and In order not to bottom out at the track while heavy breaking at triple digits I'm running 1.4 springs. Whatever spring is there it's not enough. Total fork travel is around 120mm and you are using close to half of that and ain't even moving. You are making sag adjustment to get a specific geometry. Change your geometry then adjust suspension to match. Zip tie method is the best for knowing where you suspension is.

On preload don't forget most roads ain't flat. When the road occilates away from you it's stored energy that allows the front tire to follow the road. Extension is most important for this, otherwise as you say it takes less to initiate movement. But that range of movement then becomes huge, instead of oscillating in a narrow range after bumps you are moving massive ammounts in the fork travel range. Which is fine for dirt bike but on a sportsbike that leads to a lack of responsiveness. We are talking ms here, but those ms is what keeps you safe on the road at very low speeds and stupidly high speeds. Lots of videos out of Cali with people taking easy corners and wiping out at low speeds. Those roads have areas that appear flat but actually are not, front end floats, tire cant extend fast enough when it encounters the declination and down they go.