Rilled clutch boss, lurch/chatter on take-off

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barncat

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I haven't pulled the basket off yet to check the dampening springs, but by the KISS principle it's probably not their fault.

'73 TX. developed a recent nasty grabby clutch on take-off only from a dead stop. otherwise works perfectly. can be defeated with low RPM feathered clutch release. new 7/6 discs and socket screw mod. no broken friction plates.

the clutch boss is rilled (not so much on basket) and possibly even fractured at intervals at the high stress concentration at the axial wall. I think the boss splines the likely culprit, preventing the disc pack from compressing quickly and evenly. some recent aggressive riding is likely taking a toll on the aluminum, that and the stamping burrs on the steel discs.

i'm thinking get another clutch hub and deburr all the steel plates. and fresh heavier 20-50 oil. lotta good clutch info here, figured i'd consult for concurrent diagnosis.
 
yes, using 6 thinner steels. less than 1K miles on new discs. stackup height should be in spec but will double check.

clutch was working perfectly, this is a new problem. will have to get the hub nut off to check everything else. thought perhaps it would be a snapped friction disc or two but they are like new.

did a recent oil change with Rotella 15-40. oil was dirtier than i'd expect when drained for only 300 miles or so but no debris in pan. good compression, no smoke out exhausts. it seems unlikely that oil alone would be causing this. the inxs write up points toward bad dampening springs or rilling or incorrect assembly or such.
 
just pulled the basket. 1 1/8" socket on 1/2" drive breaker bar, tranny in 2nd gear, held the rear wheel. nice.

all 6 dampening springs are fine. flexed them a bit with a straight blade screwdriver to check. everything looks good, assembly sequence was correct, OEM needle thrust bearing perfect.

7/6 stackup height is 29.3mm right in spec.

so, i'm pretty sure dressing up the hub splines (no evidence of stress cracks on close inspection) and to a lesser extent the basket fingers, plus deburring the steel plates and fresh oil will do the trick. really nothing else it could be. cable/worm/pushrod all working smoothly...
 
Thank you sir for bringing me a new word.
Rilled: indented with small grooves.
 
If you look real close at the steel plate engagement fingers, you may find that their contact faces are NOT 90°, maybe more like 85°, with a sharp edge that you mentioned. That alone is unsettling for me.

With 29 splines, and 6 steels, that's 174 contacts, of which maybe only 1/3-1/2 are in actual contact.
Long story short, I think that I calculated worst case load per finger edge at 4-5 lbs. Combined with the off-angle sharp edge, that's enuff to cause the digging-in rilling you're reporting.

After you round-off those corners, other options would be:
- File/smooth the hub splines.
- Flip the plates over, repositions those off-angle contact edges.
- Replace the 1st (deepest) steel with a 1.6mm thicker steel. (Offsets the steels from existing wear positions)

Just upgrading from the 5/6 clutchpack to the 6/7 clutchpack is enuff to misalign the new steels from any former wear notches on the hub splines. This can be both good and bad. Fingers now hitting fresh/unworn surfaces, but also close enuff to old wear notches to make them try to sink into or climb outta those notches.

...Brain drain off...
 
More random thoughts:

The early pressure plates have some differences to later pressure plates, possibly enhancements to address issues like yours, or others not posted here.

The early pressure plates delivered their torque thru the hub's spring standoffs. Later pressure plates deliver that torque thru the splines, hence requiring the 'indexing'. Simply re-indexing your early pressure plate would change-up the contact pattern. 6 combos here.

The early pressure plates, from what I've been able to determine in this forum, don't have a steel disc in the center, where the 'mushroom' pushrod fits. I've seen some early pressure plates here where that aluminum facing has been damaged, and may cause a wobbly pressure plate. 5twins may know more about when the steel disc was used in the later pressure plates.

You could chuck-up the 'mushroom' pushrod, check face runout, face-off if necessary.

Could try spreading the clutch springs. Measure their lengths, select 3 longest, fit those in a staggering 120° pattern. Then the shorters in the remaining positions.

With the left cover installed, clutch assembled and adjusted, check for pressure plate runout/wobbling while spinning the hub. Hub spin can be done with trans in gear and spin rear wheel.

*gasp*
 
2Many- as always your in depth XS insights are appreciated.

as you point out, it's somewhat surprising that the steels don't routinely saw into the soft aluminum hub splines and kill these clutches in short order. to make them quickly, the steels must be stamped at the factory which leads to deformation and sharp one-side burrs of course. plus, the thinner steels are a double edged sword: much more surface area in the pack but higher notch wear at the hub.

the only "easy" way to resurface the hub splines would be on some sort of rotary fixture on a mill. trying to offhand file these is going to be a real pain since there is a dead end preventing any semblance of a good file stroke. plus the more material you remove the sloppier the engagement of the whole clutch pack... may experiment with a Dremel and mini sanding drum, but it'll be tough to maintain a flat surface.

I do have an early pressure plate sans steel insert for the mushroom. there is zero discernable runout when rotating the mushroom against the plate center, both surfaces look perfect, and there is no waviness or other indication of runout in the pressure plate perimeter.

I pretty much knew what the possible causes were prior to starting the thread. will see... 2 sharp lurch/screech/grabs off the line- can't ride it that way... actually dangerous at a crowded intersection if ya were to stall it out. I figure this exact scenario has happened before, was trolling for similar experiences.

what are you racers running for clutch setups? seems like the clutch should be able to handle some street racing without crapping out.
 
That 'lurch/chatter' can also be called 'clutch jitter'.
Ran into a fair share of that on Honda 4's, often cured with mods to hub oil feed restrictor.
Amazingly, yours is the first I've heard of on here.
 
hmmm... not happy. spent 4 hours resurfacing the hub splines, deburring the steels and dressing the basket fingers. used a straight 1/8" diameter diamond grit burr in the Dremel for the hub, turned out well. used a small tapered alum oxide bit for the steels and a flat file on the basket. all good.

put it all back in with fresh 15-40, still grabby/poor engagement with any load from a dead stop only. works/shifts perfectly otherwise even under hard acceleration.

better check the worm gear I suppose but not a likely cause since it's good otherwise. may be some not obvious stress cracks in the pressure plate that are opening up under power. baffling, and irritating. if I was at main shop i'd just swap the whole clutch out... nobody ever said owning old bikes was easy. will figure it out eventually.
 
On the earlier clutches they use a solid bottom hub. The later use a spring loaded bottom plate. This eases that rough engagement. I have tried both on my 75.
The later is smoother. The early a bit rough but only on take off.
You can swap the later clutch hub and basket into your early bike.
Leo
 
Leo- I believe you refer to the dampening springs. as mentioned above I do have those in this clutch and they checked out ok.

I ended up swapping in a different clutch entirely. I was unable to find anything physically wrong with the offending clutch unit. what I suspect is contaminated friction discs from running 15-40 Rotella oil in hot FL weather and pushing the motor fairly hard for occasional bursts, combined with the god awful 3-4 minute wait time at major intersections around here that are brutal on engine temp and oil. some gas and combustion byproducts must have been blowing past the oil rings.

I switched to basic grade 20-50 oil and changed it again after 50 miles to "flush" cases. so far so good. I thought 15-40 was enough... it usually is up north where it's a bit cooler and there is no traffic or stop lights, but not down here...

also, there might be a case for using a Mike's improved needle thrust bearing. seems like the much greater needle surface area would promote smoother flatter engagement of clutch pack at critical standing start take-off.

i'll "sterilize" the original disc pack and reuse that clutch for next build. but i'm here to second the 20-50 oil at all times for warm weather riding.
 
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Hmmmm I wonder what just swapping the plates would have done/shown.
I also wondered about your gearing, but with swapping the clutch packs solving the problem that isn't it.
How are you going to "sterilize" the disc plates?
And is there any chance contamination happen prior to you using Rotella 15-40?
 
well, when you can't zero in on a problem IMO the best approach is swap in a known good part. by the process of elimination I suspect the friction discs and oil contamination. has nothing to do with gearing... clutch operated perfectly for about 600 miles then went south, all on Rotella.

i'll wash all the discs hot soapy water, then brake cleaner, then rewash/dry, then light fresh oil film before reassembly. will probably work. no rush, next build is a while off.
 
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