Clutch backplate lugs / straps / etc. setup / build
Turbo Mini Forums

I built up my clutch assembly today in the correct manner (as per TM) of machining the backplate lugs.

First measurements showed me I needed 1.4mm off the lugs to get the spring flat, which is pretty close to the usual TM figure of 1.5mm, so I machined off 1.6mm !!! - My logic is that the driven plate will lose 0.1-0.2mm very quickly and from then on the setting is against me anyway.

Assembled after machining


Spring is just a tiny bit over centre as intended.

So then I went to measure up the spacers I'll need under the straps to flywheel (it's a lightened one so thinner)



and the first thing that worries me is that the shouldered bit of the bolt will no longer be carrying the load - assuming (simplistically) that the primary loading on the bolt is shear, it's now going to be carried on the root diameter of the thread, about 1/4 of the CSA of the shouldered bit.
OK, that is simplistic, but the spacer won't be much help as the bolts are (by the book) done up to a very low torque.

Surely this should be a problem on a high torque motor ???

The bolts are at a large radius from the crank so, if I did the calculations, I'm sure the loadings are quite low anyway, but the original design (for much lower engine torques) was for the shoulder to carry the load.

Next - quite a surprise......

Whilst winding the bolt in and out to get an accurate spacer measurement, I noticed it kept bottoming out without the shoulder ever hitting the flywheel surface.....

Looked at the back of it and


Winding the bolt in until it touches


and measuring give me 1.3mm clearance if I make the spacers so the straps are parallel to the flywheel.

Clutch plate "official" wear limit is 1.0mm (AP plate) but in reallity it will wear more than that before hitting the rivets so, by machining the backplate lugs, the clutch life is when the backplate hits the end of those three bolts.

Or am I being really stupid and missing something obvious ???

EDIT - (first for typo), and I should add, the measurements off the bolts are withOUT the locktabs, I wasn't intending to use them because of the limited shoulder length. If I used locktabs the clearance would be slightly better.

Edited by Rod S on 15th Oct, 2008.

Schrödinger's cat - so which one am I ???

erm,

re the bolts bottoming, is this a lightened flywheel that has had a lot pof matereal taken from the surface the bolts would clamp onto? this would mean the bolt could screw in further.

Joe

In the 13's at last!.. Just

Edited by Joe C on 15th Oct, 2008.

Yes it is, but I don't have a problem with that....

I can make the spacers to suit the lack of metal on the top, but.....

I'm still not happy with the load on the bolts being shear on the thinness part, the spacers will NOT transfer load to the right part of the bolt.

and, not happy with the end of the bolt being so close to the backplate so restricting wear on the driven plate.

Does everyone else tolerate this or have I assembled mine differently ???

Schrödinger's cat - so which one am I ???

well any thread sticking out the back can just be cut off, but somthing does seem amiss, ive not noticed this issue on lightenend flywheels.

how thin is the flywheel, could the either of the friction faces have been machined excesivly?

I think you're worrying too much. The spacers have been used succesfully by many for many years.

Sure, it's not 'ideal' but you are clamping to a spacer with flat, parralell ends. If the ID of your spacer is a snug fit around the bolt, then the spacer will not 'tip' once clamped, and hence the bending of the bolt is minimal / non-existent - ie shear...

end of the day, do the math. Say 160 lb ft torque if the max you'll get out of a grey cover.
Multiply it by a bump factor of 2.0, so that's 320 lb ft.
Radius is what, 6", so that's 640 lb applied to a 6" radius.
Divide by three bolts, and that is 215 odd lbs on each bolt.

What's that? 100kg or somet? I don't have to do any more math to see that unless the bolt is made of putty-steel, you aren't going to have a problem.

That said - the wider you make your spacer, the better. A thin 'tube like' spacer is less than ideal of course as it does little to resist tipping (which induces bending). Make it 1/2" OD or so as a minimum and you'll be fine I reckon. Keep the ID of the spacer snug around the bolts as well.

Edited by turbodave16v on 15th Oct, 2008.

Yep, I worry too much

I machined up some solid spacers 20mm OD and with the ID counterdrilled to match shoulder of the bolts.
Then Loctite 270 inside the spacer on assembly to fill any remaining gaps.
I also used the locktabs, not for locking (I prefer Loctite) but to reduce the amount the bolt protrudes out the back. There's now 2.5mm clearance to the pressure plate.

Looking at the figures and the way it physicaly goes together, I think the shoulder on the bolt is probably more to do with accurate location of the straps and spreading the load over a larger area of a thin strap, rather than the shear loads at the flywheel interface.

Schrödinger's cat - so which one am I ???

Are you using the shorter bolts into the flywheel, and the longer bolts into the pressure plate? On my steel flywheel, the bolts only just come to the bottom of the hole on the flywheel and may protrude by about 1 thread, no more

Schrödinger's cat - so which one am I ???

the atraps do not hve to be paralle to the flywheel, somewhere near wiil work just fine,


carll

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

Agreed, they're not going to stay parallel anyway, as the driven plate wears.

I did intend to make them slightly off parallel to allow for the initial wear of the plate (like I took a little bit more off the lugs) but I got the maths wrong and they ended up parallel.

However, when using the "search" button for info on the subject before I did the machining, I found this



Yours I think !!!

It looks like the strap(s) buckled under reverse torque - correct me if I'm wrong but that's what it looks like in the photo - and as they are very slender, having the two clamped ends dead in line with each other will minimise their tendency to buckle under compression loadings (reverse torque).

Schrödinger's cat - so which one am I ???

that pic is of my rear clutch, i was mighty pissd off when that happened seing as i'd only fitted them as new that season, on removal i found them to be made of toffee, not heat treated as they should have been,

and yes they bent under take up, reverse torque

the supplier changed them of course,


carl

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE