Another Trigger Wheel Question

[Chris W]

Actually more than one and sorry if they have been asked before but I did look back through the history and couldn't find an answer.  

I want, eventually to move to efi so I have been collecting the necessary hardware.  I need to mount a trigger wheel and the neatest solution I have seen is to machine the rear face of the pulley so that the trigger wheel sits flush with the remaining rear face. (CP TR6 engine).

Q1. The pulley assembly is about 6.5 inches or 165mm in diameter.  The trigger wheels I have looked at are also the same OD so the top of the 'teeth' would not be proud of the pulley but about level.  Is this a problem or do the teeth need to protrude to be effective?

Q2. If the teeth don't need to protrude, would it be advisable or necessary to machine a step so that there is say a couple of mm gap between the trigger wheel and the pulley?

As I said above, I'm just collecting hardware at the moment so I don't have any installation instructions. 

Q3. My guess is that the longer notch/missing tooth on the trigger wheel is there to tell the system where TDC is? If so, the notch presumably should be in the vicinity of the sensor when the engine is at TDC?  How critical is this, should it be in line with the start, middle or end of the notch?  

Sorry if these are numpty questions but any advice, gratefully received.

Cheers

Chris

[Nick Jones]

I'm not absolutely sure but my feeling is that "proud" by at least 3 - 4mm would be advisable to get a clean signal.  It can be a fussy area an it's a critical signal, so best not to disadvantage yourself from the start.  There's a pretty wide range of trigger wheels available and some vendors will make to your spec.  Someone sent me details recently - I'll put them up if I can remember who/where......

Something else to consider is that the part you are referring to is not solidly fixed to the crankshaft, but flexibly (albeit slightly) via the harmonic damper rubber.  This potentially causes two issues.  The first is that there could be some timing inaccuracy due torsional flexing of the harmonic damper rubber, which could vary with rpm and load.  The second is that these dampers sometimes become unbonded and the damper ring moves relative to the core of the pulley and thus the crankshaft.

Whether the first makes any practical difference is debatable.  Certainly people do do it this way and seem to get away with it.  The A-series crowd even get the notches machined directly into the circumference of the damper.  The second certainly would be a problem but is hopefully an uncommon event!

Missing tooth have to be a certain number of degrees ahead of TDC. 60º for a 6 pot.  This link explains quite well

https://wiki.autosportlabs.com/MJLJ_V4_vehicle_installation_guide

I aim for the middle of the tooth.  It's good practice to strobe the thing once you get it running and make sure the degrees you get are the degrees you ask for.  Most ECUs allows a few degrees trim many let you define the base number meaning you the position isn't mechanically critical, though I think it remains good practice to stick with 60º BTDC unless there is a good reason why not.

Hope this helps.....

[JohnD]

5 hours ago, Nick Jones said:

Something else to consider is that the part you are referring to is not solidly fixed to the crankshaft, but flexibly (albeit slightly) via the harmonic damper rubber.  This potentially causes two issues.  The first is that there could be some timing inaccuracy due torsional flexing of the harmonic damper rubber, which could vary with rpm and load.  The second is that these dampers sometimes become unbonded and the damper ring moves relative to the core of the pulley and thus the crankshaft.

I may have missed a previous conversation here, but the trigger wheel will rotate with the crank nose, if it is fixed to the hub of the pulley.   Only the "inertia ring" the plain outer rim behind the pulley itself will move to damp torsion vibration.       Again I don't know if this is a 2L or a 2.5L engine, but their damper pulleys are significantly different in this respect, as the 2.5 has the pulley AND the inertia ring as one piece, when for the 2L  they are separate.    See below:

  2.5L damper pulley

  2L pulley

Of course, the damper is there because torsion will twist the nose, a bit, but not, IMHO enough to cause mistiming.   This is a trace of the signal from the nose, showing the tiny variation in amplitude (Top) due to an imperfectly centered trigger wheel, and the variations in frequency due to torsion (below).       The latter are impossible to detect by eye, and again IMHO, will not affect timing significantly!

John 

 

 

Edited March 13, 2021 by JohnD

[Nick Jones]

We are talking about 2.5 ones here John.  Thanks for the extra info.  I agree that the torsional vibration probably doesn't cause enough movement to mess the timing up.

[iani]

As you can  see, my  trigger  wheel is effectively flush with my damper/pulley, I did have the mounting holes carefully drilled though so I could fit the missing tooth within 60 degrees as mentioned by Nick above.

[Chris W]

Thanks to all who responded. The depth of knowledge and experience never ceases to amaze.

Nick's answer kind of confirms my gut feel that the teeth should be proud although Ian's arrangement suggests it isn't critical in the real world.  In fact Ian's picture comes up on a Google search and was what I was referring to when mentioned 'neatest solution'. 

Triggerwheels website has wheels at 6.5" but the next size up is 7.25" which I feel might be too much or give clearance issues.  There is a guy on ebay who I will probably use, as he sells the standard 165mm OD but also states that he does bespoke sizes.

So Ian, your second picture shows a gap between the trigger ring and the meat of the pulley.  I wonder if that is why you can get away with the teeth being flush.  Also, your pulley looks new with moulding tags on the rubber. It is certainly different to my pulley.  Is this an aftermarket pulley or an OE pulley from a different car?

John thanks for your contribution, as I'm acutely aware that the rubber bonding on original pullies can be marginal and as Nick mentioned the proposed solution does rely on the outer keeping up with the solid inner!  I'd also be interested (from the work you did on this) whether you feel that machining out say, 25% of the mass of the pulley would change its vibration damping characteristics, or more to the point, render them ineffective?

Thanks & regards

Chris

[iani]

8 hours ago, Chris W said:

Thanks to all who responded. The depth of knowledge and experience never ceases to amaze.

Nick's answer kind of confirms my gut feel that the teeth should be proud although Ian's arrangement suggests it isn't critical in the real world.  In fact Ian's picture comes up on a Google search and was what I was referring to when mentioned 'neatest solution'. 

Triggerwheels website has wheels at 6.5" but the next size up is 7.25" which I feel might be too much or give clearance issues.  There is a guy on ebay who I will probably use, as he sells the standard 165mm OD but also states that he does bespoke sizes.

So Ian, your second picture shows a gap between the trigger ring and the meat of the pulley.  I wonder if that is why you can get away with the teeth being flush.  Also, your pulley looks new with moulding tags on the rubber. It is certainly different to my pulley.  Is this an aftermarket pulley or an OE pulley from a different car?

John thanks for your contribution, as I'm acutely aware that the rubber bonding on original pullies can be marginal and as Nick mentioned the proposed solution does rely on the outer keeping up with the solid inner!  I'd also be interested (from the work you did on this) whether you feel that machining out say, 25% of the mass of the pulley would change its vibration damping characteristics, or more to the point, render them ineffective?

Thanks & regards

Chris

My pulley is a new one from Moss Chris, it wasn't cheap having it machined so I didn't want to risk using  an already worn original pulley. 

[Nick Jones]

10 hours ago, Chris W said:

although Ian's arrangement suggests it isn't critical in the real world.

Ok, but Ian has come up with a nice work-around by cutting the outer part back further so there is a decent air-gap to give a bit of magnetic separation. Looks much better than having the teeth butting directly up against the back of the damper. Maybe talking bollocks, but it looks right to me and obviously it works for Ian.

[JohnD]

12 hours ago, Chris W said:

John thanks for your contribution, as I'm acutely aware that the rubber bonding on original pullies can be marginal and as Nick mentioned the proposed solution does rely on the outer keeping up with the solid inner!  I'd also be interested (from the work you did on this) whether you feel that machining out say, 25% of the mass of the pulley would change its vibration damping characteristics, or more to the point, render them ineffective?

Thanks & regards

Chris

Chris,

Maybe the function of the damper ring isn't clear?   It's not a matter of the ring 'keeping up'.   The crank is prone to torsion vibration, a twisting motion that potentially could damage it.      The analogy is a child's swing, where a thrust synchronising with the swing of the seat pushes the child higher and higher.      Time that push out of synchrony and the opposite happens.   The inertia ring absorbs the energy of that vibration, the thrust, and  damps it into the rubber.

Changuing the massof the inertia ring certauinly would chnage its function!     My cutaway diagrams of the two pulleys show the massive (literally) change that Triumph made between them.  They thought that it was necessary to add the entire mass of the pulley to the inertia ring,  AND made the ring itself 15mm wider to further increase that mass.   This was in response to the greater stroke and mass of the 2.5L crank.     I haven't done the calculations myself, but Triumph clearly felt that much extra mass was required.   Cutting the ring back to the same diameter as the pulley would reduce its mass by over 400grams and would IMHO make the damper non-functional.

JOhn

 

Edited March 14, 2021 by JohnD

[Nick Jones]

6 hours ago, JohnD said:

Cutting the ring back to the same diameter as the pulley would reduce its mass by over 400grams and would IMHO make the damper non-functional.

You are adding back the weight of the ring though so the end result will be much closer to original, but I agree that the weight is important.

[Chris W]

Hi John,

Sorry, my comment about the outer ring keeping up was more related to ignition timing (using a ring mounted trigger wheel) if the bonding rubber was marginal through age. The bit I was more interested in your view, was about the effect of removing mass from the ring and you answered that, thank you. Nick makes a good point that most of the mass removed would be replaced by the trigger wheel (assuming they are similar metals).

[Chris W]

15 hours ago, iani said:

My pulley is a new one from Moss Chris, it wasn't cheap having it machined so I didn't want to risk using  an already worn original pulley. 

Hi Ian,

Thanks, I had no idea replacement crank pullies were still available outside the hugely expensive (and worryingly named) 'rattler' pullies.

[JohnD]

Thank you, Nick and Chris!  Yes, I omitted the mass of the trigger wheel in my calculation, but as with intentional mass reduction, for instance on a flywheel, a rotating part's mass gains in importance by its distance from the centre.     BUT, your wheel is secured to the inertia ring, Chris.   Gosh, I can't work out if the wheel's mass 'inboard' of the securing bolts will count as if it were at the bolts!  But a mass rotating on a smaller radius than another spinning wider will not have the same rotational inertia.    

So, assuming that the trigger wheel masses about the same as the lost mass from the inertia ring, then if that mass is spread out in a disc as wide as the ring then it will not  restore the same damping function.