Downshifting Question
Extreme HDF Member
Joined: Feb 2015
Posts: 10,907
Likes: 8,758
From: Galt's Gulch - MI
I was told by a fellow Harley rider that downshifting in order to slow the bike and be in the right gear as you come to a stop is bad and will cause oil to be pushed out of the sump and into the air box. I am new to HD but in every vehicle or motorcycle I've ever owned I've used the motor to aid in breaking. Is this a problem?
Whoever this fellow is you're speaking with, stop talking to him. But on the subject of downshifting; brakes are cheap, engines and transmissions are expensive
On every automobile, track bike, or HD I've owned I've rev-match downshifted matching engine RPM to road speed. All it takes is a bit of practice and coordination but this is sadly becoming a lost art; too bad, it's great fun...
http://www.trackdaymag.com/categorie...for-downshifts
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Last edited by ROCKOUT Rocker Products; Mar 20, 2016 at 10:01 AM.
Road Master
Joined: Feb 2007
Posts: 781
Likes: 35
From: St Louis
Whoever this fellow is you're speaking with, stop talking to him. But on the subject of downshifting; brakes are cheap, engines and transmissions are expensive
This has always been my thought. Brakes is cheap. Normal speed matching downshifting perfectly safe and smart. Several years ago I was slowing to a stop for a car making a left turn ahead of me and downshifting as I was slowing. Glanced behind me in the mirror to see a car coming up hot and not slowing. Being in the correct gear I was able to immediately crack my throttle and shoot out to the right shoulder slightly as the car slammed on it's brakes and hit the fender of the car turning left. Not super hard but enough to damage both vehicles and my bike would have been the creamy filling of that oreo. I always keep my gearing close to my speed now.
This has always been my thought. Brakes is cheap. Normal speed matching downshifting perfectly safe and smart. Several years ago I was slowing to a stop for a car making a left turn ahead of me and downshifting as I was slowing. Glanced behind me in the mirror to see a car coming up hot and not slowing. Being in the correct gear I was able to immediately crack my throttle and shoot out to the right shoulder slightly as the car slammed on it's brakes and hit the fender of the car turning left. Not super hard but enough to damage both vehicles and my bike would have been the creamy filling of that oreo. I always keep my gearing close to my speed now.
Cruiser
Joined: Mar 2009
Posts: 167
Likes: 3
From: Cornelius OR
Any rapid change of state can ruin a TC crank.
Im surprised they dont blow up when they buck the starter.... (not spinning fast enough I guess)
In a sudden change of state the driven flywheel wants to pass the drive flywheel, that load is passed through the pin. Since a TC pin is (not locked in place by a taper and held there by a nut) merely held in place by a (cheap to assemble) press fit it can move.
Given this the most likely cause of TC crank shift is dumping the clutch at an RPM significant enough to apply enough force to separate the flywheels.
You will not accomplish that down shifting, the rear tire simply does not have enough traction to cause an instantaneous force great enough when dowshifting.
Im not sure a burnout or wheelie done at less than redline would cause a force great enough.
Most are done in lower gears and the motor has mechanical advantage.
That leaves a High gear (4-5)(maybe a 3-4) upshift at redline and performed violently as the most likely way a pilot can cause the crank to shift.
The rest is just poor engineering.
Given this I would not do a chain drive, or aftermarket clutch in a TC.
Even the installation of the SE clutch spring that comes with a 95" kit might be enough to initiate a slip.
OBTW Blipping the throttle on a downshift lessens the impact by better matching the motor speed to the gear selection.
Free reving creates very low flywheel differential forces.
Think about it, it takes a solid impact to one side of the flywheel pair to cause enough differential force to make the pin slip.
And that impact has to be made at a high enough RPM for the flywheel effect to impart enough force across the pin for a slip.
Im surprised they dont blow up when they buck the starter.... (not spinning fast enough I guess)
In a sudden change of state the driven flywheel wants to pass the drive flywheel, that load is passed through the pin. Since a TC pin is (not locked in place by a taper and held there by a nut) merely held in place by a (cheap to assemble) press fit it can move.
Given this the most likely cause of TC crank shift is dumping the clutch at an RPM significant enough to apply enough force to separate the flywheels.
You will not accomplish that down shifting, the rear tire simply does not have enough traction to cause an instantaneous force great enough when dowshifting.
Im not sure a burnout or wheelie done at less than redline would cause a force great enough.
Most are done in lower gears and the motor has mechanical advantage.
That leaves a High gear (4-5)(maybe a 3-4) upshift at redline and performed violently as the most likely way a pilot can cause the crank to shift.
The rest is just poor engineering.
Given this I would not do a chain drive, or aftermarket clutch in a TC.
Even the installation of the SE clutch spring that comes with a 95" kit might be enough to initiate a slip.
OBTW Blipping the throttle on a downshift lessens the impact by better matching the motor speed to the gear selection.
Free reving creates very low flywheel differential forces.
Think about it, it takes a solid impact to one side of the flywheel pair to cause enough differential force to make the pin slip.
And that impact has to be made at a high enough RPM for the flywheel effect to impart enough force across the pin for a slip.
Last edited by Bent Wrench; Mar 20, 2016 at 10:42 AM.
Tourer
Joined: Jul 2013
Posts: 389
Likes: 9
From: MT
So you saying power shifting is more likely to do it Leaving it pinned and then just fanning the clutch? And burnouts in 1st not creating the forces to bend or otherwise destroy the crank? Think I'm just trying to justify doing more burnouts.
HDF Community Team
Joined: Jun 2006
Posts: 12,791
Likes: 2,621
From: san antonio
A lot of mis-information in the Bent Wrench post; just don't have time to pick it apart right now.
It interesting that he starts of by saying "Any rapid change of state can ruin a TC crank." Then goes on to say that wheelies and burnouts are not likely to tweak a crank but a more likely cause would be a high rpm power shift.......
Cranks have been tweaked by hot start kick backs; emergency braking while failing to disengage the clutch; and yes, burnouts and wheelies. Not saying that dumping the clutch at high rpms won't tweak a crank but the instant the clutch is engaged, it will either slip a bit before full engagement or, the back tire immediately starts to spin.
Just use good sense when down shifting because if the down shift/rpms are not matched properly to road speed, one could twist a crank.
It interesting that he starts of by saying "Any rapid change of state can ruin a TC crank." Then goes on to say that wheelies and burnouts are not likely to tweak a crank but a more likely cause would be a high rpm power shift.......
Cranks have been tweaked by hot start kick backs; emergency braking while failing to disengage the clutch; and yes, burnouts and wheelies. Not saying that dumping the clutch at high rpms won't tweak a crank but the instant the clutch is engaged, it will either slip a bit before full engagement or, the back tire immediately starts to spin.
Just use good sense when down shifting because if the down shift/rpms are not matched properly to road speed, one could twist a crank.
Cruiser
Joined: Mar 2009
Posts: 167
Likes: 3
From: Cornelius OR
I'm standing by my statement the downshifting even if done in the most brutal fashion is not likely to impart enough energy to separate the flywheels, the rear tire simply does not have enough traction under engine breaking.
Really though it's a crappy design and destined to fail, it's best if you dont start it at all then your crank will never slip. :^)
I like the Darkhorse fix, but if that is so good why do they weld them? The weld is like an oil light, it cracks when the crank slips.
Really though it's a crappy design and destined to fail, it's best if you dont start it at all then your crank will never slip. :^)
I like the Darkhorse fix, but if that is so good why do they weld them? The weld is like an oil light, it cracks when the crank slips.
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Club Member
Joined: Oct 2011
Posts: 11,179
Likes: 2,490
From: Nor Cal
I had no idea that this thread would be so entertaining right from the "get go"...
"...rider in front of me locking up his rear and crashing because he was cut off."
"...Probably best to ignore any future advice from this individual.
Whenever I rode with him I'd downshift the crap out of mine and smile at him"
"...wannabe pundits"
Then of course the rest of the thread
Forums are GREAT!
Thank-you to all
"...rider in front of me locking up his rear and crashing because he was cut off."
"...Probably best to ignore any future advice from this individual.
Whenever I rode with him I'd downshift the crap out of mine and smile at him"
"...wannabe pundits"
Then of course the rest of the thread
Forums are GREAT!
Thank-you to all
Seasoned HDF Member
Joined: May 2012
Posts: 32,934
Likes: 36,033
From: Carlsbad, CA
Oh yeah...good point...don't downshift if the resulting RPMs over revs your engine. The rev limiter is electronic, and has no impact in mechanical over revving. For example, if you are in 4th gear at 5,500 RPMs...dos shifting to 3rd...or worse 3rd...would not be in the best interest of engine longevity.
Smooth speed matching sounds great too.