40 replies to this topic

[Spider]

So it it worth reusing an old crank damper? I have a couple but some have slight degradation to the rubber. Can the rubber be replaced with some new sikaflex or similar?

 

There is some new ones getting about that are a disaster, they are steel in appearance (not cast) and the 'rubber' appears to be Urethane of some time. They do come apart in use.

 

I've used second hand ones and quite often, mind you, not on competition stuff though. I know the rubber can crack, as any rubber will and often these are discarded as a 'fail' but they are usually OK. Grab them and have a good feel of the Outer rim relative to the hub. By hand, you won't move a good one at all, flexing or twisting. Using a file, I mark them before fitting, so I make a line across the inner and outer, then they can be reliably checked.

 

There is a number of firms who recondition them, I haven't checked lately, but Repco used to do them on an exchange basis. For a stock replacement, this is where I'd be going. I wouldn't recommend any home reconditioning methods. Sikaflex or anything like that is way too soft.

[mini13]

Good points by moke above,

 

Taking it case by case, the 1275 engine, apparently (alegedly) had its problem frequency at approx 6k, ( this is aciording to the vizard book), so presumably the std damper  is tuned for that frequency, so once you start wedging etc it all goes out the window a bit.

 

The S engine, had a totally dofferent crank... smaller bigends, stiffer material, so presumably the crank resonance would be higher ( asssuming 1275) so the damper "presumably" was engineered to this, but hang on... whaty about the 970 and 1071 S...  the 1071 came first so was the damper made for that and then just used on the 1275 and 970? were there different dampers? not as far as im aware. ( just saw Moke says about different S dampers above)

 

As the MED damper is prety much a copy of the S one, how suitable is it really? no idea...  but almost certainly better than nothing.

 

Moke, on your graph,  what is the left hand scale, is it degrees or somthing else?  if its degree's then thats going to be transfered to your cam timing, scattering that, and also if you are running an ECU scattering the Ign timing, which is a good argument for using a flywheel mounted trigger.

 

 

personally on my next engine I'm looking at using the Kad fluid damper, and a flywheel mounted trigger

[nicklouse]

fly wheel or pulley mounted trigger make little difference though as the pulley is smaller there is less chance for any movement. and who mounts the trigger to the damper?

[grizzler73]

Thanks for all the replies, as usual I ask what I think is a simple question and get back so much more! Cheers chap's.

[nicklouse]

true, as we all have different reasons and constraints that lead us to do what we did.

[Spider]

Thanks for all the replies, as usual I ask what I think is a simple question and get back so much more! Cheers chap's.

 

I don't think any part can be looked at or considered in isolation of any other parts it works with and there's always much more going on than meets the eye eg, " it's just a pulley after all" , so a broad spectrum of answers and people's experiences can be helpful.

[Spider]

Good points by moke above,

 

Taking it case by case, the 1275 engine, apparently (alegedly) had its problem frequency at approx 6k, ( this is aciording to the vizard book), so presumably the std damper  is tuned for that frequency, so once you start wedging etc it all goes out the window a bit.

 

The S engine, had a totally dofferent crank... smaller bigends, stiffer material, so presumably the crank resonance would be higher ( asssuming 1275) so the damper "presumably" was engineered to this, but hang on... whaty about the 970 and 1071 S...  the 1071 came first so was the damper made for that and then just used on the 1275 and 970? were there different dampers? not as far as im aware. ( just saw Moke says about different S dampers above)

 

As the MED damper is prety much a copy of the S one, how suitable is it really? no idea...  but almost certainly better than nothing.

 

Moke, on your graph,  what is the left hand scale, is it degrees or somthing else?  if its degree's then thats going to be transfered to your cam timing, scattering that, and also if you are running an ECU scattering the Ign timing, which is a good argument for using a flywheel mounted trigger.

 

 

personally on my next engine I'm looking at using the Kad fluid damper, and a flywheel mounted trigger

 

mini13, the scale on the left is Torsional Orders of Angle.

 

I too recall that on a 1275 critical RPMs with respect to Harmonics occur in orders of around 3000. I say 'around 3000' as this figure will vary with each individual engine. This being the case, critical speeds on these are 3000, 6000 and at 9000 it's potentially a 'killer' without the right damper. A discussion I had on this with Dave Rosenthal a few years back, we both came to similar numbers on this.

 

The 970 S Engine, as it came from the factory could well get away without a damper, the 1071 too, was one that you could just about get away without one. I'm only guessing they fitted one to allow the end user to carry out further tuning of the engine with little risk in this regards. The big S damper is apparent well suited to the EN40B Cranks and S rods fitted to the 1275 engines.

 

I really can't offer any comment on the MED Dampers, I haven't seen one, but as an off hand comment, coming from them, I think it would be OK.

 

Cranks sensors is something I've been looking at and like you, wish to try a flywheel mounted sensor, however my reasons here are quite different, though Torsional Harmonics was definitely a consideration.

[nicklouse]

dont forget that there were different later dampers. of different mass

 IMG_0531.JPG   58.63K   0 downloads

Edited by nicklouse, 21 March 2018 - 11:49 PM.

[timmy850]

Junk or ok? Its nice and solid when mounted to the crank with no flex. It would be for a slightly tuned 1100

Edited by timmy850, 22 March 2018 - 12:08 AM.

[nicklouse]

mmm have a close look at the Keyway

but i have seen worse that were ok

[Spider]

@ Nick - yes but i) there's not a lot of difference in rotating mass in the outer rings between them and ii) again, IMO, shows that - from the factory - they were not overly sensitive to their tuning needs.

 

The latter photos with the spit keyway - yeap, seen quite a few of though and it does well illustrate that harmonics are real and what they can do. I think the factory torque figure for that bolt is way too low (75 ft/lb from memory) and with the 'rattle gun' effect of the harmonics, it just loosens them well off. I do these bolts up well tighter than the recommended figure on any engine and use loctite.

 

@ Tim - hard to say from appearance alone but nothing there pokes me in the eye. Check the keyway and also where the Oil Seal runs isn't grooved too. I also like to run a tap through the holes in case I later need to fit the slide hammer on it to remove it.