Lotus Elan and Lotus Elan Plus 2 Forums

Hi all,

I'm interested to hear from anyone who has experience with crankshaft damper on their Twin Cam. I suspect with FIA requiring Elan's to run old school cranks and flywheels that vibration will be an issue with rev's now used regularly and more solutions will be available.

On my 26R which has original Cosworth engine fitted, 4 bolt steel crank, rods etc I had vicious vibration, which due to accident we never solved. I suspected it is in the drive train, so we had checked and adjusted gearbox to diff alignment and angles, rephrased uni's on propshaft to follow modern motorsport thinking and it still remained. It appeared around 4800rpm and got progressively worse, was not apparent in 1/2nd gear. the engine was stripped, refreshed with new pistons bearings etc by reputable Victorian Engine builder for me who frankly were a bit slapdash.

I was running Dunlop CR65's we check new TTR rims for run out, they were fine, then balanced with new tyres. I did not shave tyres to ensure they were not out of round as someone suggested was a mandatory approach, is this true? I'm planning on using Avons in future.

My 26R is late S2, so it has the S2 26R drive train, we stripped and checked hub faces for run out, all fine, due to the accident we did not get a chance to test the sliding yoke half shafts for balance.

I finally stripped the damaged chassis yesterday and for a 190kph rear hit into concrete pit wall the damage was very localised, mainly due to the custom built welded roll cage. I've a replacement 26R S2 chassis which we're taking to panel shop to put on aligning jig to check its true and to take key measurements off in event of a minor accident. I'll post some pictures in due course of the damage, I had thinwall chrome moly wishbones, they bent and absorbed a lot of energy and rear upright cracked, shock tube bent and shock shaft!

Anyway, I'm not intending running with original engine again, its been oiled up, cam capped eased off and put on a stand for posterity, the new engine has DKE 8 counterweight 12 bolt crank, arrows rods, etc. I'm taking no chances as its easier to deal with now. I may try out initially on an 1860 tall block TC I've got which has standard GT crank etc to see what difference it makes with series of dyno runs.

I've been researching options and have found a few, the traditional crank damper which in an Elan maybe a little hard to fit due to chassis cross member, another option is flywheel damper and lastly I've noted reference to clutch cover damper assembly which has been developed due to space constraints for Porsche engines.

I'm taking no chances and will source a damper while the car is being rebuilt. I'm interested in Vibration Free's TCR Rattler type and either flywheel or clutch cover type.

I look forward to your responses.

If anyone's had similar vibration issues

Vaughan

Hi Vaughan

i have never seen an engine with a vibration issue around 4800 rpm and getting worse as revs increase unless there is an engine or drive line balance issue or issue with engine and gear box mounts or clearance issue with the chassis of some component. I have seen pictures of a mini front pulley with integral damper used in a race engine but never seen it done in real life.

Generally engine vibration only becomes an issue around 8000+rpm in race engines and that is around the nose of the engine and results in cracking to oil pumps and alternators and front covers overtime.

cheers
Rohan

The mini with the Crankshaft Pulley with a Damper were the Austin/Morris Cooper S models 970, 1071 and 1275.
Only the "S" models

Alan

I would suggest looking at "Fluidampers". They have a very good/responsive technical department in the US. Fluidampers use silicone as the damping agent and work on a different principal to the usual rubber/steel dampers. They are fitted to the E Type engines that are running to 7500 rpm (!) at the Goodwood Revival etc and also other front running engines-Aston Martins etc. My understanding is that Fluidampers are "amplitude sensitive" whereas rubber/steel dampers are "frequency sensitive". This means that the rubber/steel damper (ATI etc) must be tuned to the particular crank/rod/piston/flywheel combination to work. Fluidampers do not need this critical step.
The front cross member of the Elan would have to be notched to accommodate it. My (limited) knowledge of crankshaft dampers tells me that if the damper isn't at the opposite end of the crank to the flywheel it is not going to have maximum effect.

Edit: http://www.fluidampr.com/

I have no connection to this company whatsoever!

I doubt you have an engine problem:

1. You say it doesn't occur in 1/2nd gear - An engine problem would manifest itself regardless of gear position.
2. You say it starts at 4,800 RPM and gets progressively worse. This does not seem to be characteristic of a torsional vibration problem. Usually the vibration would peak at a certain RPM then diminish and then peak again at another RPM that is a multiple of the first. These vibration periods correspond to the natural frequency of resonance of the crankshaft assembly.

Hi,
Regarding the vibration damper I think Keith of Weber carb fame fitted one to his Elan - might be wrong it was a long time a go but anyway should be easy to contact him.

Regards
Steve

Any crankshaft damper needs to be designed to match the type of crank and flywheel you are going to use. You?d be extremely lucky to find one designed for another engine that happened to work on a twink.
A four cylinder engine can never be perfectly balanced unlike a straight six so it?s all about minimising the vibration and keeping the natural frequency out of the normal working rev range.

Bigbaldybloke wrote:Any crankshaft damper needs to be designed to match the type of crank and flywheel you are going to use. You?d be extremely lucky to find one designed for another engine that happened to work on a twink.
A four cylinder engine can never be perfectly balanced unlike a straight six so it?s all about minimising the vibration and keeping the natural frequency out of the normal working rev range.

Most crank dampers are designed to damp torsional vibrations so you see them most on straight sixes which while well balanced for reciprocating loads tend to suffer from crank torsional vibrations due to the long crank length.

Torsional vibration is rarely a problem on the shorter 4 cylinder crank

cheers
Rohan

I agree Rohan, I think we are both saying the same thing - fitting a crankshaft damper on this engine is unlikely to cure the problem mentioned. The example above about the success of crank dampers on E types and Aston?s is a good example of their use to damp torsional vibrations on a straight 6 engine

rgh0 wrote:

Bigbaldybloke wrote:Any crankshaft damper needs to be designed to match the type of crank and flywheel you are going to use. You?d be extremely lucky to find one designed for another engine that happened to work on a twink.
A four cylinder engine can never be perfectly balanced unlike a straight six so it?s all about minimising the vibration and keeping the natural frequency out of the normal working rev range.

Most crank dampers are designed to damp torsional vibrations so you see them most on straight sixes which while well balanced for reciprocating loads tend to suffer from crank torsional vibrations due to the long crank length.

Torsional vibration is rarely a problem on the shorter 4 cylinder crank

cheers
Rohan

they offer a small one for the Honda B series (4 cyl 7500+ rpm) so I've send them a query about a possible adaptation - I'd like to find out more about their "free floating" damping mass (a bit like a torque convertor, but hooked up to nothing ?) - even with the rubber type there is some lateral motion/damping in addition to the rotational effect but to what extent (what amount of free play?), and the fluid type seems of a better design to me (graceful degradation)...

Something designed for a 7500rpm Honda 4 cylinder is probably getting nearer for what you need for a racing twin cam

cheers
Rohan

Many later 4 cylinder engines have a decoupled "bi-mass" front pulley where the inner and outer annuli make up a rubber to metal bonded assembly, the early 1.8 Mazda MX5 has one I know for sure.

I had a 2.6 litre in line 4 pot shogun that had a balance shaft, it may even have been contra-rotating, a very smooth engine which would otherwise have been very agricultural.

I had a V6 Omega years ago. The rubber in the front damper pulley came unbonded, the pulley moved forward, took the cam belt with it and the pistons and valves had a bit of a get together! Expensive! No pre warning, car regularly serviced and only about 50k miles on it. Not keen on them.

I was going to write that the pulley had de-bonded and the outer moved relative to the inner putting the timing mark out, that is what I recalled and thats what I thought had happened but then I recalled that in fact it was a DPO bodge up timing belt replacement allowing the woodruff key to chewing away at the crankshaft keyway.

rgh0 wrote:Something designed for a 7500rpm Honda 4 cylinder is probably getting nearer for what you need for a racing twin cam

cheers
Rohan

I have one of those sitting beside me as I type this. It would not easily, if at all, adapt to a Lotus Twin Cam. The best solution is to send a crankshaft pulley to Fluidamper and have them make one.

4800 rpm is a strange rpm for harmonic vibration in a small 4 cylinder engine-it does sound rather like some form of sympathetic vibration.

As I tried to point out earlier, the rubber/steel dampers as made and sold by ATI and others, must be designed for a specific engine condition-changing the weight of any of the crank/rods/pistons/flywheel and even clutch can negatively effect the performance of the damper. Also, the dampers can by their nature come to pieces as mentioned above.

V8 engines also suffer from harmonic vibration-the Cosworth DFV was very prone to failure early in its life due to this.

Balance shafts first appeared before 1920 with the Lanchester Harmonic Balancer. I used to have a 1922 Vauxhall that had been fitted with one-it still had the hole in the crankcase where one of the balance weights tried to escape!

Woodruff keys are for location only-the tightness of the component on the shaft is what transmits the drive.

Unlike some here, I am not an engineer-so treat the above with due caution!