Caterham Pop Shaft Strength?

[caterhamnut]

We have just discovered we need to replace our prop, or at least the UJ's. Our recent vibration was traced to the UJ's being very very slack - this could easily be seen and felt by lying under the car and twisting the prop. Our car has just gone through 90,000 miles, many of them hooning, though we do not have big power.

 

HOWEVER - this wear and tear, IMO , is entirely down to poor maintainance on my behalf. The joints were very dry, and indeed the grease nipples had fallen out anyway - so check yours from time to time. I will be more than happy to replace ours with a standard Caterham part, and will be sure to keep an eye on this part as pat of general maintainance from now on.

 

www.mycaterham.com

here

88,500 miles -1st 1.6k Supersport, '95 Motor Show car

[Mike Biddle]

"surgery" to the tunnel just in front of the diff was required to fit the uprated prop to my car, but it was not major surgery, worth it to have peace of mind, and since it would not cost a great deal, probably worth considering if you are concerned.

 

Mike.

 

[I.Mupferit]

Angus beat me to it but I agree that maintenance is a key issue here. Buried away in the tunnel it isn't easy to get at and thus, I am sure, often gets overlooked during routine servicing.

 

I get under the car, grease the UJ's and generally check for wear on mine every year. Well worth doing I think and although it won't necessarily prevent a failure, it might at least give early warning of problems before they occurr, with nasty results.

 

Brent

 

2.3 DURATEC SV Reassuringly Expensive

R 417.39 😬

[Paul Gibb]

I agree strongly with all the comments made above re maintenance - often neglected.

Rob, I would be interested in the results of your enquiries, please post FI.

Peter, could you please post a little more about your current prop? how is it different to Caterham's own?

Marius, presumably yours is the CTG prop - any issues with installation? is it a straight swap?

Mike, fantastic car well done

 

L7 FUN

 

Edited by - Paul Gibb on 3 Feb 2006 10:00:46

[Alex Wong1697456877]

Paul,

 

Did yours break on acceleration or gear change?

 

Also, did you used to have P17HPC or something like that - a green car I think.

 

Alex

 

Edited by - Alex Wong on 3 Feb 2006 10:36:17

[Harry Flatters]

Alex, I think you'll find that is James Fletchers beast - green/ali.

 

*arrowright*Harry Flatters *arrowright*

AKA Steve Mell - Surrey AO and Su77on Se7ener

[Alex Wong1697456877]

I thought it might have been Paul's old car for some reason - Green/Ali sounds right. Very quick to from what I remember - I just remember being overtaken by it many times about 5 years ago, at just about every trackday I went to.

[Peter Carmichael]

Hey, its me again. The voice of theory that just doesn't stack up in the face of received wisdom.

 

Shock accelerations to the prop could come from either the engine (there has been lots of talk about props being up to a given engine torque output) or from the driving wheels. What I want to convince you of is that the principal situation of concern is when the gearbox is in 1st gear.

 

The theory I am working by is that: "The force felt by the prop relates to the relative accelerations achieved by the masses hanging onto the ends of it."

 

In my analysis (and in real life), the car is a much more significant mass than the inertia of the engine. The analysis can be simplified to looking at the accelerations demanded at the engine end of the propshaft... and there is a gearbox in between the engine and the prop. The worst case is 1st gear, by pure virtue of mechanical ratios.

 

The six speed box will give the prop an easier time because of its 2.69:1 1st as compared to the 3.36:1 1st gear on a normal fitment type 9. The final drive ratio is not an issue at all. Clutch-dumping take-offs on sticky slicks or sprint spec list 1Bs are the most significant issue. All the other gear ratios will apply proportionally smaller forces to the prop.

 

Are there any other situations that would cause propshaft shock loads? Re-examining my beginning assumption of the significance of the car's mass, we know that we can break traction and spin the wheels up by dumping the clutch in 1st gear; it is clear that the friction between the tyres and the road gives indication of an upper limit to the force experienced by the propshaft. It also turns out that we can produce this much load in the propshaft in other specific ways, for instance through indelicate clutch operation while downshifting - in such a case, we have let the revs drop to such an extent that the overall acceleration of the masses of the engine is equivalent to the 1st gear clutch-drop. It is much more difficult to achieve this in higher gears, so the guidelines relating to avoiding peak loadings in lower gears still holds true.

 

Hopefully nothing too contentious there. If you want a propshaft to last: stop doing donuts and racing away from the lights; learn to heel and toe; have your fun at high speeds (kerb-hopping at high speed isn't going to have much ill effect on a prop).

[Alex Wong1697456877]

But in first gear, the prop shaft is spinning at it's slowest - even with a tall first gear, it is spinning at less than half the speed than it is flat out in 5th, and for a much shorter time. Whilst the shockloads may be greater in 1st, the sheer centripetal force coupled with the movement of the engine and gearbox must place HUGE demands at high speed as well. The inability to align everything up perfectly always upsets me. The diff may be central but is the engine central? How much does the engine and gearbox move in use (alot I suspect) and how much does this place demands on the UJ's.

 

Think of an engine - it sustains shockloads too but it's revs that usually kills engines, not shockloads. Sure there are friction issues with engines that props don't have but the point about speed of rotation and vibration surely holds true for both engines and props?

 

I accept your points but I suspect many failures occur at high speed too.

 

I worry even more as I have a much longer propshaft than the standard Caterham item.

 

To have the prop break while moving off in 1st gear must be the best time for it to fail as it isn't spinning quickly. Even if you are spinning the wheels, at least it will stop as soon as the transmission of power is lost as the wheels stop spinning.

 

Maybe 🙆🏻 but just some thoughts.

 

I once tried to press out the UJ's but could not work out how to do it. It reminded me of the 3D puzzles I couldn't do as a child!

 

Also, I've never had a prop fail (touch wood) but when my gearbox mount went, leaving the prop and gearbox free to rattle around the tunnel at 30mph, the force of it hitting the tunnel was terrifying. With the mount welded back together by some poor Moroccan who we woke up at 10pm (Elite gearbox so non standard mount) the alignment was all wrong. The gearlever was an inch lower than it was before the failure, but it seemed to drive ok, at least until the clutch failed 2 days later.

 

 

Edited by - Alex Wong on 3 Feb 2006 12:39:00

[I reply to every thread]

Peter - What are your views of LandRovers recent proclamations of having solved the

"K" HGF issue 🤔

[Paul Gibb]

Alex yes my previous Vx was the green ali & yellow P7HPC. Still deep in thought...

 

L7 FUN

[Peter Carmichael]

Hi Alex,

 

My mention of "theory" was a nod in the direction of the analysis being incomplete. I find it interesting that we have an expert opinion telling use the prop is underspecified. We also have circumstantial evidence (we all know that de dion tubes break, but prop failure is much rarer) that it is generally sort of OK (technical speak) although there are isolated occurrences.

 

We'd been getting into the territory of supposing the sorts of loads that lead to propshaft failure - kerb-hopping was mentioned; did it fail on "acceleration or gearchange?" was your question. Such questions suppose a mode of failure relating to momentary overloading. Certainly we can envisage massive overload leading to catastrophic failure and I suppose I was pointing out that this wouldn't happen in practice and kerb-hopping would not be the most likely cause of such conditions anyway. It is likely that Paul's propshaft had suffered some progressive mode of failure with the actual moment of failure giving the least clue as to the development of the failure.

 

In more general terms (and perhaps the ones that Chards might have been referring to), the mechanisms of concern would be fatigue and wear. We can at least do something about wear. We can inspect and recognise a joint that is worn. As in Angus's case, we can sense that something is vibrating and is not quite right. As you suggest, the power transmitted by the propshaft (speed and torque) might correlate closely with a "wear" failure mode.

 

Fatigue is a more difficult case. A fatigue failure of a propshaft would lead us to needing to life the unit and casual inspection would not be able to determine if the prop remains usable and safe. A regime of crack-testing would be in order.

 

Paul's experience shows that a new propshaft can have problems. This suggests a weakness present in that particular propshaft from the point of fitment or from extraordinary interference (impact from debris, etc.). The general mode of failure will fall in the category of crack initiation and propagation. Chards might well be pointing out that there is not sufficient margin for error in the strength of the unit to cope with normal manufacturing variances and provide assured operation at such higher power levels.

 

Chards are therefore correct, in conventional terms. Colin Chapman would not have thought for a moment before fitting the cheapest/lightest/most compact (delete as appropriate) component that didn't fold within the first five minute's use. With our propshafts we probably have a direct inheritance of the Lotus tradition. If you get a good one it will not fail catastrophically before the joints wear out for any power engine normally seen in a Seven (RST-V8 excluded?).

 

We know that not many Sevens achieve high-mileage in a single owner's possession, but an ageing Seven might well wear out a propshaft. If the prop shows no signs of wear at high-mileage, I'd propose the safest course is to stick with the devil you know. If you're paranoid, with a high-powered engine, you will crack test it at 50,000 miles regardless of wear in the joints.

[caterhamnut]

Makes total sense to me Peter

It is obvious ( I think) that the highest shock loading as you describe - in my eyes logic dictates that as any loosness in the UJ joints gets taken up very violently, this is when damage is most likey to take place - damage through lack of maintanance is a more constant issue I guess - lots of very very fine 'rust' dust around out UJ's were a sign of wearing/grinding!

However - the moment of failure, or what the car is doing at the time of failure, might be totally unconnected with when the damage is most likley to occur in the first place, as you suggest Peter.

 

www.mycaterham.com

here

88,500 miles -1st 1.6k Supersport, '95 Motor Show car

[Alex Wong1697456877]

Peter, I struggled to follow your post fully but Angus' simplified interpretation for dummies makes sense to me.

 

Paul, Even in that car, you were going mightily fast. Your claim that your 280BHP is why you are quick is somewhat modest, I suspect!

[Paul Gibb]

Way OTT on the bhp there Alex.

Still deep in thought.....

 

 

L7 FUN

[Alex Wong1697456877]

I thought L7FUN was up to those sort of power levels?

[Normans_Ghost]

I'm a bit confused by the "instant" failure claims.

 

I've seen many props with worn/broken needles and some with no needles. All came to our attention because the driver heard a clonk on acceleration/de-acceleration. Looking at the design of a Hardy Spicer Joint it's difficult to understand how a joint can go with no warning. I'm excluding broken metal here. But even props I've seen with broken ears ahve been gradual breakage, not instant.

 

Peter's correct in that "wear" on U/J's is usualy fractured needle rollers and this takes place at the point of shock when drive is taken up. Also U/J needles wear/fracture more when the joint is constantly moving as with a live axle. To drive 300bhp an constant or near constant speed without the massive force generated by taking up drive will not cause a hardy Spicer joint much problem.

 

The comments made by the repairer are typical of "little knowledge being dangerous"

 

As it happens my rear joint is showing a small amount of movement which I noticed last wek when diff out to weld upper diff bracket back on. I'll replace joints when car is stripped to fit carbon body in the summer (hurry up RiF). I don't do donuts, rarley scream away from lights and drive with mechanical sympathy. The joints have done about 25k with 150bhp and 25k with 220 (about 7k with ultra low 1st and low CW&P from BGH)

 

Norman Verona, 1989 BDR 220bhp, Reg: B16BDR, Mem No 2166, the full story here

You and your seven to The French Blatting Company Limited

[Blackbirdman]

A timely thread as I took both my prop shafts to Bailey Morris here last week for a routine check and service. However it turns out that both the splines are worn so I’m in for replacement items. Bailey Morris have a rouges gallery of broken prop shafts and UJs of all sizes in their reception area and it’s a bit worrying.

 

In my case it’s probably at least partly due to a lack of maintenance/lubrication.

 

While the props are out have out I have been thinking about some form of additional protection especially after hearing Paul’s problem and seeing Peter T’s prop.

 

Worn joints can get very bad and still not fail at least on lower power cars having replaced one on an early Bookatrack 8V Vx which was very worn.

 

I won’t be putting myself on Dr .Carmichael’s ‘no donut diet’ though

 

Matt

 

 

Life begins at 10,000

 

Edited by - Blackbirdman on 5 Feb 2006 17:50:04

[SouthernBanana]

If anyon'es ever seen a dyno plot of a power run through the gears, (rather than in a single gear), you will see the rate of change of torque is prob greatest in 1st gear. I think this is what Peter C is alluding to ?!

[julians]

Is the only sign of a worn prop a detectable (by hand) degree of play in the UJ's. Or can it be worn but show no signs of play?

 

I only ask as I have a loud noise/slight vibration when on a trailing throttle at high'ish speed from around the diff/prop area, but the prop shows no sign of play, so I'm presuming its a 'normal' diff noise, its made the noise for years.

[Neill Anderson]

Just to throw my (engineer’s) tuppence worth in: one of the more fundamental design parameters for proshafts is whirling velocity, the critical speed at which the prop assumes a deflected shape mid way between the UJ centres and whirls around as a shallow bent U shape rather than simply being a rotating tube. The key to maximising the rpm at which this starts is the STIFFNESS of the prop in bending between the UJ. Thus a shorter shaft is better (hence long drivelines use centre bearings), as is a large diameter (stiffness increases massively more with diameter than with wall thickness or material properties), as is less angular misalignment. It is the angular misalignment that can start all of the nasties (harshness, whirling et al) but it is not possible to eradicate fully. The beloved sliding spline yoke in the gearbox is also nasty as it cannot be as rigid as a flanged coupling that is properly supported by real bearings.

In general the torque capacity is set initially by the size of the UJ cross spider and associated machining of the end clevises (some will be much better than others). It is unlikely that the tube receiving end of the UJ forging would be too small for a sensibly walled steel tube to transmit the max torque achievable by the UJ itself (i.e. it would take some stupidity for the tube to be weaker than the UJ) but I have seen these tubes twist when dented during fitting, or when localised stress raisers like balance weights are badly fitted.

However fatigue failures of the forgings and of the cross spiders are far more common. Sensible preventative maintenance as advocated above is far better than assumptions requiring a redesign. I personally would always strive to ensure as much engagement as possible in the sliding yoke at the gearbox, but too little will cause driveline harshness under acceleration as the differential pinion flange moves up and forward. All things being equal this should be easier to do consistently with a De Dion car!

 

Neill

 

 

New 7 Owner

1996 VX 2.0

[EFA]

This thread is overcomplicating a very simple issue.

 

Peter C correctly points out the mass of the car is the enemy here. The mass of the car gets transferred in a most ferrocious manner if a sequential box is used. Far more than any other transmission load.

 

I have had 3 or 4 prop failures with my car in its life, luckily none so severe as Pauls, from a car more powerful than most in this club, which has been around for much longer than many others and has 50k miles of hard road and some track use at over 250bhp under its belt. The failures are down to UJ issues because the nipples fall out.

 

 

The UJ's in the std prop are from a Lada and Volvo 340. You could read something into this.

 

On a powerful Seven it is critical to life the prop. I replace mine every 24 months. It is even more critical to check the condition of the UJ's - expecially that the greese nipples have not dropped out. This does happen, and has been the cause of every prop failure I have had - symptom being clonking or vibration.

 

Peter T is correct in saying material selection is critial. but one has to remember that his prop failing prop was made from seamed tubing. Its near instant demise was not a surprise. I wish he had known this is waht the numbskull who made it was using! BTW Peter, I don't think your "with no issues" point above has perhaps the validity of slightly greater mileage. Without wishing to offend you, I would enquire as to exactly how far you have driven your car in the 2 years since your first prop failure???

 

The Solution Is:

Life your prop

Service it regularly (at least check the UJ nipples are intact) every 2-3 months

 

Stop and check your prop if you get any transmission vibrations or jumping out of gear issues. Then you won't have to worry. Its not a science, its common sense.

 

K2RUM - Its bright but its not yellow......