Front end wiggle when downshifting

[Igor900]

Hey friends. I got a '98 M900. He's got about 64k miles on him.
I've noticed that when I downshift, especially when he's rev'd up pretty high, I get a pretty good wiggle in the front end. I don't wiggle any other time, I can do 75+mph and he stays nice and smooth, can take my hands off and everything. Just when I downshift and let off the clutch, he's definitely got a good wiggle to him.
I've replaced the fork oil/seals earlier this season, just put on new front rotors, and I've been trying to adjust the steering stem nut (the bearing was replaced about 25k miles or 5 years ago). Any other things I should think about checking? Anybody else experience this before?

[Buhgaboo]

Start with checking your suspension sag numbers

[Charlie98]

How is the condition of your front tire?

My old CBR1000F was very sensitive to front tire profile, it would headshake just like you describe when the front tire was worn.

[koko64]

You need to run a close eye over all those components that can hurt you if they mis behave, because that don't sound right. I mean you ain't stomping on the shifter with both heels and upsetting the bike, so check the following:

Chain slack for a whiplash effect
Tyres for uneven wear and correct pressure (or damage)
Steering head bearings
Wheel bearings and rim condition
Axel nuts
Suspension sag and bolts, etc as DP said
Swing arm pivot area for case cracks, loose bolts, anything looking wonky
Anything else our brethren here can think of.

Also are you letting the revs fall enough before downshifting? It's not an in line 4, so allow for that as you need to allow a greater drop in revs before clicking it down or you'll be asking a lot of your slipper clutch if you have one. Your model came with a regular clutch and those big pistons can chirp the rear sending a shudder through the front if you change down too early. You come off a 4 cylinder bike previously?

This is most likely but giving the old girl a going over with a keen eye over a few beers is worth it.

[Igor900]

I’ve checked mostly everything that you all have said.
The front rim is used, had to replace after a near accident last season (someone cut out in front of me I had to brake hard and the wheel bearing which was worn and needed replacing pushed into the rim enough to replace the rim) so I don’t know the history of the replacement rim (eBay find)
The tire itself looks clean and even. Only have two season on it.
I have been pushing the bike more in the last couple of years, and probably always had the wiggle when downshifting, but I’ve been doing it from a higher rev. I’ll be sure to slow him down and not push him so hard.

I started with my Monster. He’s been my first and only bike for the last 6 years.
During quarantine I picked up an old Ninja 500 to build for my boyfriend, and a 1985 Suzuki gs700es to restore back to original form.

I’ve experienced a rear wheel wiggle before based off of what is the usual suspects, but never the front wheel/handlebars until this year.
Just wanted some good starting points to look into.
Thanks guys

[Charlie98]

What kind of tires are you running?

[S21FOLGORE]

,SNIP>
Also are you letting the revs fall enough before downshifting? It's not an in line 4, so allow for that as you need to allow a greater drop in revs before clicking it down or you'll be asking a lot of your slipper clutch if you have one. Your model came with a regular clutch and those big pistons can chirp the rear sending a shudder through the front if you change down too early. <SNIP>

This.

To OP,

Can you downshift WITHOUT clutch?
(Yes, you can shift up AND down WITHOUT clutch, if you know how the transmission works, and know how to shift.)

My guess is, (assuming that you don't have problems with the bike itself) your riding style is causing front end wiggle mostly, or entirely.

So, ... let me ask a few questions.

Can you shift up WITHOUT clutch ?
(Meaning, you can do that any time you want, you can do that all day long. NOT that you can do 1 out of 10 tries.)

Can you shift DOWN WITHOUT clutch ?

If the answers are no, ...

Are you pulling in the clutch lever completely?




If your "hard downshift" goes like this ...

1. going fast on the straight line

2. see the corner coming, wait as late as you can, then brake HARD ! (all the weight (of the bike and you) is pitched forward, loading the front forks to near its max capability)

3. pulling the clutch lever in, completely

4. Twist the throttle, revving up the engine high, let the clutch lever go ...


... then, try to shift without the clutch, both up AND down.
That (clutch-less shift) will force you to shift properly (and ride smoothly).
Then, most likely, the problem would be solved.
(And no, clutchless shift won't hurt your transmission.
You can cause damage to constant mesh transmission in 2 ways.
One is by shifting slowly. This will damage dogs.
Another is force shifting while dogs are strongly engaged by the torque or back torque.
This, you'll end up bending shift fork.
You don't damage anything by not using clutch.)

Also, try to shift with pulling in the clutch lever only 2 - 3mm, not all the way in. (Both shift up & down).


Another thing you can do, is
to flip the shift lever upside down, to make shift pattern 1 up 5 down.
Adjust the shift lever height (and clutch, brake lever) at the same time.

[koko64]

You can ride these bikes hard for sure, just let the revs drop down a bit more before downshifting. You can also try riding in a gear taller when going hard and use the torque , you'll be amazed how much quicker you can go on tight twisty roads when using the "grunt".

It's a great sport, you never stop learning new things even when you're older and experienced

[S21FOLGORE]

You can ride these bikes hard for sure, just let the revs drop down a bit more before downshifting. You can also try riding in a gear taller when going hard and use the torque , you'll be amazed how much quicker you can go on tight twisty roads when using the "grunt".

It's a great sport, you never stop learning new things even when you're older and experienced

This is also a very good point.

Wider ratio transmission, more reciprocating mass make downshifting more difficult.

And, using "lower RPM" allows you to go round the corner faster ...



Typical air-cooled 2V Ducati's (like 900SS, M900) power output is like this ...



The peak of power output is just around 8K rpm.
So, you may think you need to rev up your engine that high, but ...




look at the torque curve


If you are revving your engine high, using 6 - 8k rpm during the cornering,
you are actually slower than someone on the same bike but using 4 - 6k prm.

Yes, using 4 to 6K rpm allows you to go round the corner(s) faster.

So, before going out for a ride next time, think about what RPM range to use when going through the corners, in which gear.

[DuciD03]

Hey friends. I got a '98 M900. He's got about 64k miles on him.
I've noticed that when I downshift, especially when he's rev'd up pretty high, I get a pretty good wiggle in the front end. I don't wiggle any other time, , he's definitely got a good wiggle to him.
I've replaced the fork oil/seals earlier this season, just put on new front rotors, and I've been trying to adjust the steering stem nut (the bearing was replaced about 25k miles or 5 years ago

yep doesn't seem right; lets think about what's being shifted around here … there engine torque is slowing bike and rider mass with engine breaking ; rear wheel is doing it all, from sprocket slowed by chain and drive sprocket; clutch to piston pressure; loading the front of the bike with the weight of rider and bike … unless your also using the front break then break pistons, rotors, ft wheel bearings and steering head may be more at play …
are you mistaking clutch slip for front end wobble?

You need to run a close eye over all

Chain slack for a whiplash effect
Tyres for uneven wear and correct pressure (or damage)
Steering head bearings
Wheel bearings and rim condition
Axel nuts
Suspension sag and bolts, etc as DP said
Swing arm pivot area for case cracks, loose bolts, anything looking wonky
Anything else our brethren here can think of.

This is most likely but giving the old girl a going over with a keen eye over a few beers is worth it.

Interesting problem-o;
wise to think this one through.

Also older bike, with older technology,  900 with dry clutch; good proven old school moto tec … I was surprised by how much more torque and snatchy the dry clutch on a s2r 1k was coming off a nice forgiving oil apc slipper on an s2r 800... the 4 stroke duc engines are a totally different to ride that most motos …

post back on the solution; Im curious

 

[Igor900]

I will fully admit to pushing him much harder in the last year or so than I had in the beginning.
My hard downshift was using more engine braking to slowdown, knowing that I have to rebuild my front calipers (they're starting to stick). Most of that wiggle is probably just from engine braking at a much higher rpm than I usually do.
There is still more of a wiggle than I'm happy with, and I'm not sure if it has to do with my forks (I replaced my oil and seals earlier this year, but did put in a lighter weight oil than I normally do) or maybe should replace my wheel bearing because I can feel a slight wiggle when I'm stopped and gently wiggle my handlebars back and forth.
The tires aren't all that old. They are Mich Road Pilots, I can't remember which number and I'm too lazy to go outside and look right now lol

I just wasn't sure if my engine brake wiggle, that I'm definitely feeling out of my front wheel and handlebars, was something was a "Oh I definitely know what that is" or a "hey check out these few points first before replacing your entire front end" lol.

[koko64]

I would double check the head stem bearings for free play and adjust or replace. Good idea about checking the wheel bearings.

You definitely sound like a good candidate for a slipper clutch if you don't want to change your style.

[ducpainter]

Also, don't discount some rider input from the rapid decel on a high rev downshift.

[koko64]

Edit.
True, and the heavier you get the smoother you have to be with your inputs and movements on the bike. If you're light then you are probably outpacing your suspension package. When I raced I was 155 lbs and could slam down gears while braking (rear tyre in the air) and moving to hang off. Now at 200 I smoothly hang off first then brake and downshift as smoothly as possible or else I will overwhelm the mass of the bike with my own. You can substitute heavier for faster and more aggressive. I have needed heavier springs over time too as I've got heavier. Also if you're strong and counter steer aggressively you may have to learn to finesse it a bit. As you get quicker the need for pro suspension work becomes more important than what most of us can do. My suspension guy told me that I needed harder fork springs as I either got faster or heavier.

I think you should be relying on your brakes more to brake than engine braking as it will help you set your corner entry speed with better stability.

[S21FOLGORE]

The "speed" plays bigger roll.


Anything that has mass and in motion posses Kinetic Energy.

 Kinetic Energy = 1/2 mass x velocity squared
                   Ek=1/2 m × v∧2

Assuming that both Koko and myself are on the same bike , S4R 200kg wet, traveling at 30mph.

Koko =200lb=90.7kg
S4R = 441lb wet = 200kg
Velocity = 30mph = 13.4 m/s (meters per second)

Ek(k) =  0.5 x (200 + 90.7) x 13.4∧2 = 26,099.046 Joules

S21FOLGORE = 125lb=57kg
S4R = 441lb wet = 200kg
Velocity = 30mph = 13.4 m/s

Ek(s) = 0.5 x (200 + 57) x 13.4∧2 = 23,073.46 Joules

When you change the velocity, you have to deal with this kinetic energy.

BTW,
Velocity is a vector (magnitude AND direction),
Speed is a scalar (magnitude only).

So, if you are changing the direction of the bike, even though you maintain the same speed ,
you are accelerating.



Now ...
 
If both of us are moving at 60mph (26.8 m/s)

Koko
Ek(k) =  0.5 x (200 + 90.7) x 26.8∧2 = 104,396.18 Joules

S21
Ek(s) = 0.5 x (200 + 57) x 26.8∧2 = 92.293.84 Joules

Notice how much Ek increased ?
Increase the speed twice, it gets 4 times more.
If you go three times faster, it gets 9 times bigger.

If I hit a solid object (such as parked car, guard rail, tire wall, etc) from 60mph, my body gets that much energy,
which is equivalent of me falling from 165m (541 feet) high.

(How to calculate it
Ep (potential energy) = mass x gravitational field strength (which is 9.8 N/kg on the surface of earth) x height of the object
92,293.84 = 57 x 9.8 x h
h = 92,293.84 ÷ (57 × 9. = 165.22 m)

To put it very simple,
being bantam weight is a pretty big advantage, in accelerating and decelerating.
HOWEVER,
if I go twice as fast as Koko, despite being 37% lighter (about 2/3 of the weight),
the cornering & braking becomes much more difficult, as I would be carrying 3.5 times more kinetic energy.

Think twice, before you go fast on the street.