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Post by SylvreKat on Aug 14, 2013 22:03:16 GMT -5
I've a question just 'cause I'm a curious 'Kat.
Why is it when I sit at a stop with the rear brake held, the mirrors vibrate, but when I sit with only the front brake, they don't? They vibrate when both brakes are on, too.
>'Kat
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Post by westendscooters on Aug 14, 2013 22:34:10 GMT -5
Common, the rear brake cable is mounted on the engine case.. getting vibrations from the rumbler between your legs, All scooters eventually do this  |
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Post by millsc on Aug 14, 2013 22:42:00 GMT -5
Or your idle is set to high and when you hold the brake it vibrates because the wheel would spin if on the stand not under load
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Post by rockynv on Aug 15, 2013 11:37:17 GMT -5
On your Piaggio with a hydraulic rear brake it is because it locks the rear wheel so that engine and belt vibrations that would be muted by the silent blocks on the engine cradle along with the slight movement of the rear wheel when the brake is not locked when you lock up the rear wheel the vibrations are no longer getting lost in the free play in the drive train and get transfered forward to the frame of the bike.
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Post by wheelbender6 on Aug 15, 2013 16:56:21 GMT -5
On my scoot, the rear brake rod pivot is mounted to the swing arm, and so is the engine and brake torque arm. My front brake is hydraulic, so vibrations from the front wheel are not transmitted through the frame.
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Post by SylvreKat on Aug 15, 2013 19:26:00 GMT -5
I think I understand what rocky's saying. It's almost in English ;D
So when I hold just the front brake, the rear tire shifts some which absorbs the engine's vibrations which is why the mirrors don't vibrate?
>'Kat
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Post by SylvreKat on Aug 15, 2013 19:30:12 GMT -5
Mill, I don't think the rear tire rolls on the center stand. But then it's been a while since I've had him up on it, let alone running.
western, mine's still relatively new in Piggy-miles. Plus seems like the mirrors have always vibrated at stops. It's just recently I played with switching between both on, vs rear or front only.
bender, sorry, you went too mechy for me. Why do hydraulic brakes make less vibration than whatever non-hydraulic brakes?
>'Kat, trying to learn....
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Post by shalomrider on Aug 15, 2013 20:24:54 GMT -5
howdy, i don't know if i can explain in english but the vibration is dampened by the brake rotor and pad being locked on the front wheel. you should be able to notice when the vibration is shaking the mirrors with only the back brake being held, when you add the front brake the vibration will dampen. try that at a stop and see if i am correct. if it doesn't work let me know.
lotsa miles and smiles to ya
ken
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Post by shalomrider on Aug 15, 2013 20:26:47 GMT -5
howdy, after re-reading , kat seems to negate my suggestion in her original post. i'll leave mine anyhow
lotsa miles and smiles to ya
ken
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Post by SylvreKat on Aug 15, 2013 22:32:38 GMT -5
Yeah ken, it's when I have just front brake that there's no vibrating mirrors. Front plus rear=vibration. Ditto rear only. That's when I got curious what was different about the front only and decided to ask my smart mechy scooter-buds.
>'Kat
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Post by prodigit on Aug 16, 2013 23:56:02 GMT -5
the engine vibrates and vibrates/semi-bends the rear fork by a few tenths of a degree.
Normally you won't notice it, but that bending of the rear forks, when locking the rear brake, could transfer through the wheel, and get converted to forward/backward motion of tenths of an inch. Good enough to make your bike, and mirrors shake.
The front brake doesn't work because the motor is more directly mounted to the rear fork, not the front fork.
Whatever vibrations go through the front fork get damped by the suspension, because whatever vibration from the motor on the front fork is vertical motion (and the suspension is a vertical suspension, unlike the rear wheel fork, which is a horizontal fork instead of a vertical fork.
This horizontal fork does not dampen anything, but the suspension dampens vertical motion towards the seat, not towards the wheels.
It's probably a very interesting topic with lots of mechanics on engineers level, that could keep you busy for days on end. The amount of info you can gain from just analyzing suspensions.
The reason why they don't equip scooters with a vertical fork on the rear wheel, like it is in the front, is because the engine and brakes would put too much stress on the fork (a front fork is backwards tiled (wheel more forward than handlebars), so that it will work better for braking).
A forward tiled rear suspension would work, if there's no rear wheel braking going on.
But the way things are, 2 wheelers mostly have a vertical fork, which at times can transmit engine vibrations to the wheels when the brakes are locked up.
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Post by SylvreKat on Aug 17, 2013 7:14:25 GMT -5
I actually understood all that, 'digit! Although I doubt I'll be able to repeat it. ;D
Except I need this clarification--the fork is what holds the wheel? And the suspension is the big spring I see under my seat?
>'Kat
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Post by ltdhpp on Aug 17, 2013 10:44:36 GMT -5
You are experiencing harmonic resonant vibration... a very peculiar and potentially dangerous (not in this case) physical phenomenon...
It's not that applying a brake is literally 'muting' the rattle, it is just changing one of many factors that can lead vibration to resonate and harmonize into a rattle. The vibration is still there, but conditions are no longer 'just right' to turn into a rattle.
You may have noticed that along with holding the brake, you can just touch areas around the instrument cluster to make it stop too.
If you could figure it out, you could most certainly simply attach a heavy piece of rubber to the right place and have it never rattle again. Good luck with that though ;D
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Post by SylvreKat on Aug 18, 2013 6:52:09 GMT -5
You may have noticed that along with holding the brake, you can just touch areas around the instrument cluster to make it stop too. Haven't tried that. Guess what I'll be doing at long stoplights now?.... >'Kat |
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Post by rockynv on Aug 18, 2013 8:04:32 GMT -5
Sympathetic vibration and such is why soldiers don't march in step when crossing a bridge as it builds up momentum that can become destructive causing the bridge to crash.
On the Piaggio there may come a time when the silent blocks which absorbe this vibration and that the rear suspension pivots on will become worn to the point that they will need to be replaced. The vibration will get harsher and you may notice a sloppy feeling when negotiating corners along with having to make more steering corrections at higher speeds.
Some replace the silent blocks with solid inserts as a performance enhancement however with the solid blocks you will get more engine vibration transferred to the bikes frame.
The don't call single cylinder bikes Thumpers without good reason. On the larger displacement bikes 400+ cc they will put a shaft in the engine with a counter weight to offset the vibration from the piston stroke to help minimize the thumping.
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