Theory behind rekluse clutch

[rhw125]

Im not sure i fully understand the physics behind how this clutch works. I know it must use centripetal force to engage/disengage the clutch, but if the engine is always turning, wouldn't this force cause the clutch to always engage? I'm also seriously considering the purchase of a rekluse clutch but I wanted to know of any game changing differences between the exp 2.0, the core exp 2.0, and the z start pro.

 

 

[uuhhhh]

http://www.youtube.com/watch?v=TdBNR3HjURw

 

You can see how these things work in the vid. You have centripetal force which acts toward the axis of rotation and keeps things moving in a circle, and a reactionary force (centrifugal) that acts in the opposite direction. When you spin the weights fast enough to overcome the forces due to spring pressure, they will get forced up the ramps and expand the exp unit. That is what puts pressure on, and engages the clutch pack. The z-start, although its a different design, works in the same way. There's also the style, like on a go-kart, where a set of shoes are forced outward against the inside of a drum.

[rhw125]

so the z start is kind of like a clutch on a weed wacker? it seems like the weights and ball bearings wouldn't generate a whole lot of force, at least enough to compress the clutch pack enough to make the bike go forward. Which system do you think is more reliable? the z start or the exp? Thanks for the reply and the video

[uuhhhh]

Here's a z-start. Its a different design from the exp, but works the same.

 

Your weed whacker clutch probably looks more like this.

 

When you get the weights whippin around that fast, then combine it with the motion ratio of the ramps it will generate quite a bit of force. Most guys can one finger a manual clutch, so it isn't a huge amount of pressure that is needed to stop slipping.

 

I haven't heard of any reliability concerns with Rekluse products, at least over the last few years. I have no idea whats going to more reliable in the end.

[idahojoe]

There are two concepts that both Z-start Pro and EXP share.  One is the springs, in both clutches they hold the device closed untill a high enough RPM is reached to start the bike moving foreward. There are different springs available to tune the engagement RPM.  The second is the weights, different masses of weights can be used to determine how hard or soft the clutch engages. Z-start pro activates from the outside in and EXP activates from the inside out. I could go on and on, but call Rekluse if you want to learn more.

[grayracer513]

Boiling it down, the clutch does use centripetal force to engage the clutch, and if that were the whole story, then you'd be right; the clutch would be engaged whenever the engine ran.  But the centripetal force is opposed by a set of springs that are positioned so as to disengage the clutch.  These forces are adjusted so that the spring pressure will overcome the centripetal force at a speed near idle and disengage the clutch below that point.

 

As to the clamping pressure generated, let me assure you that it's entirely adequate to hold a YZ450 running on any surface.  The force generated by the weights is exponentially relative to the speed of the clutch, and with the wedging that takes place, it actually exceeds the pressure created by the original clutch springs at a fairly early point.

[bg10459]

The Z-Start Pro essentially has no wear parts since it uses your existing clutch pack (though, it does come with a few special thickness steel plates to adjust the installed gap).  The only parts that wear out are your standard, available everywhere, clutch frictions and steel discs.  The rest of the parts of the auto clutch kit should last a long time.  The EXP clutches come with an expanding pack that replaces a bunch of your existing frictions and steels (the number it replaces varies between the EXP and the Core).  So these clutches do have wear parts; the surfaces of the expanding clutch packs.  These surfaces are replaceable, but not with any standard stock or aftermarket clutch parts.  You would need to get the new parts from Rekluse.  In all of their auto clutches, you can alter the engagement rate and lockup rpm by changing the weights (balls for the Z-Start or wedges for the EXP) and the springs.

 

I had a Z-Start on my WR450 and personally, I didn't like it.  I never tried an EXP, so I can't say.  Don't get me wrong, it's cool as hell being able to carry a gas can (or whatever) in your left hand while taking off, or standing on dry ground and reaching in with just one hand to goose the throttle and having the bike ride itself out of the muck, but when it mattered most, it seemed to fail on me.  I pushed my bike out of the woods twice due to a slipping clutch, one of which was a DNF in an enduro, and I almost DNF'ed another.  I tried different weight steel balls, but too much wasn't stall proof, too little slipped too much.  I ruined a brand new clutch in less than three rides.  In the end, I decided my left hand was more reliable.  Even if I did stall it a few times, I still finished the ride/race.

[grayracer513]

The Z-Start Pro uses Rekluse proprietary steel plates that are compatible with their clutch boss for that unit, and NOT interchangeable with OEM Yamaha.

 

I've been running the ZSP for two years in a desert racing/recreational environment, no such problems.  Yours was probably a setup issue if you had trouble like that.

[bg10459]

I had the standard Z-Start, not the Pro, but I was under the impression that the only difference was the perch adjuster on the Pro.  I guess they updated the kit because now I see it comes with the boss and plates, as you mentioned.

 

Maybe the new setup works better, but I definitely had mine set up right.  Installed gap was perfect, lightest spring for early engagement, but the standard set of balls slipped too much.  I put in 5 tungsten balls and it didn't slip, but it stalled when I was quickly on/off the gas for a log crossing or similar obstacle.  I tried raising the idle to no avail.  I tried 3 tungsten balls but it started slipping too much again.  In two rides the installed gap would be out of tolerance.  That told me the frictions were wearing.  I adjusted the gap and in two more rides the clutch was shot.  It seemed to me that .012" wasn't enough tolerance between good and bad; working and slipping.  Is that still the range on the new ZSP?

[grayracer513]

No, it's considerably less critical.  I started with used steels and the 5 year old OEM friction plates and ran them for over a year.  The only reason I replaced the plates was that they weren't releasing as cleanly as I thought they should.  They never did really show any wear, nor did I notice any sign of slipping under a load.  The new plates have been in there since, and I haven't had to adjust the gap once.  Manual override with the OEM lever and cable is a standard feature on the Pro model, too.

[tonkaman]

I'm running the Z start pro and love the way it reacts. My question is why would you need to upgrade to the core 2.0? I'm currently installing a 523cc stroker in my bike and am unsure how this clutch will hold

Edited November 8, 2013 by tonkaman

[grayracer513]

The advantage of the Core EXP and EXP 2.0 (two VERY different things using the same operating principle) over the Z-Start Pro is that it retains normal lever feel and can be temporarily set up to be capable of bump starting by working with the cable adjuster.

 

The Core EXP is a complete clutch, replacing the basket, boss, pressure plate, springs...the whole deal, usually along with a deeper cover to clear the whole thing.  The is mostly so that the EXP expanding ring that drives the design can be included in the plate stack without reducing the total number of plates in the set.  The EXP 2.0 is just the expanding ring and a couple of other pieces that go into the OEM clutch, making it necessary to leave 3-4 plates out and reduce the load carrying capacity of the clutch by that amount.

 

Except for the bump start thing, there really is no reason I can see for upgrading from a ZSP to the Core EXP.  The EXP 2.0 would not be, IMO, an upgrade at all over the ZSP.

[tonkaman]

Ok great info thank you

[A13]

The advantage of the Core EXP and EXP 2.0 (two VERY different things using the same operating principle) over the Z-Start Pro is that it retains normal lever feel and can be temporarily set up to be capable of bump starting by working with the cable adjuster.

 

The Core EXP is a complete clutch, replacing the basket, boss, pressure plate, springs...the whole deal, usually along with a deeper cover to clear the whole thing.  The is mostly so that the EXP expanding ring that drives the design can be included in the plate stack without reducing the total number of plates in the set.  The EXP 2.0 is just the expanding ring and a couple of other pieces that go into the OEM clutch, making it necessary to leave 3-4 plates out and reduce the load carrying capacity of the clutch by that amount.

 

Except for the bump start thing, there really is no reason I can see for upgrading from a ZSP to the Core EXP.  The EXP 2.0 would not be, IMO, an upgrade at all over the ZSP.

Sorry for the bump, But are you saying that using the clutch lever on the ZSP is more difficult to pull in? Would you spend $300 on a ZSP? I can get one for that, just not sure if I'm sold on it yet. Haven't used a clutch like that since my PW50 days! 

[grayracer513]

The lever effort on the ZSP is never more than a stiff one finger pull at high RPM.  At speeds below the engagement point, the only pull on the lever is from the return spring on the release arm.  The EXP clutches have a normal lever feel.

 

$300? Absolutely.  Try to get the original installation instructions and all the kit parts with it (alternative springs and whatnot).  The loose parts and docs are available, though, too.   If you don't like it, you can sell it for $300 pretty easily.

[tonkaman]

Sorry for the bump, But are you saying that using the clutch lever on the ZSP is more difficult to pull in? Would you spend $300 on a ZSP? I can get one for that, just not sure if I'm sold on it yet. Haven't used a clutch like that since my PW50 days!

It's just that if your not giving it throttle the clutch lever has zero resistance. Some people just can't get used to it, but if your using the auto clutch and not pulling the lever then you will never notice. DEFINITELY BUY IT

[A13]

It's just that if your not giving it throttle the clutch lever has zero resistance. Some people just can't get used to it, but if your using the auto clutch and not pulling the lever then you will never notice. DEFINITELY BUY IT

Thanks guys, I probably am going to try an get it....Just as long as I still have some spare change for new boots, an oversized tank and a few other of the 'mandatory things'. When the plates wear out, you you have to use rekluses parts?

[tonkaman]

Thanks guys, I probably am going to try an get it....Just as long as I still have some spare change for new boots, an oversized tank and a few other of the 'mandatory things'. When the plates wear out, you you have to use rekluses parts?

The ZSP uses stock plates

[grayracer513]

The ZSP uses OEM Yamaha friction plates only.  The steels are proprietary Rekluse parts.

[jar944]

Im not sure i fully understand the physics behind how this clutch works. I know it must use centripetal force to engage/disengage the clutch, but if the engine is always turning, wouldn't this force cause the clutch to always engage? I'm also seriously considering the purchase of a rekluse clutch but I wanted to know of any game changing differences between the exp 2.0, the core exp 2.0, and the z start pro.

 

Some math/measurements behind it. If you assume the opposing spring force to be between 75 and 100N you can calculate how it all works (and for some bikes why is does not)

 

measurements from the exp "ring":

Total wedge weight 159g

wedge spacing diameter (centers) min 124mm, max 131mm

ramp angle 18.5 deg

spring force ? (not yet measured)

engine parameters

primary reduction 2.52:1

idle RPM 1500 rpm

 

F_c = m v²/r

= m (n 2 ? r / 60)²/r

= 0.01097 m r n²

where

n = revolution per minute

Or in this case

.01097*m*r*(n*n)

m = .0159kg

r = .062m

rpm = 595

.01097*.0159*.062*(595*595) = 38.3N

which when the 18.5 degree ramp angle is included (3:1) the clamping force would be 114.9N