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Prop catcher location
steve m - 19/11/15 at 06:20 PM

As my car is in a winter strip down, and good access to the tunnel,
I may fit a prop catcher, but where ?
front middle or back

I was planning on the middle, as best access for welding

steve

prawnabie - 19/11/15 at 06:24 PM

I have 2, one at front and one at back

Smoking Frog - 19/11/15 at 07:07 PM

I only fitted one and decided on the front.

designer - 19/11/15 at 07:09 PM

I always have mine at the front.

If the UJ's are the same the one at the front will always fail first because thats where power is applied first.

joneh - 19/11/15 at 07:09 PM

Front is probably the most critical but I'd want and have one at both ends. The rear one is a bit bigger to allow for suspension movement. Front one is a fair bit tighter.

scudderfish - 19/11/15 at 07:25 PM

If the front UJ lets go, the prop will flail until the car comes to a halt (whilst beating the hell out of the leg you need to stop the car), if the rear goes the prop will flail until you put the clutch in. If you can only fit one, fit it at the front.

Slimy38 - 19/11/15 at 07:57 PM

quote:
Originally posted by scudderfish
If the front UJ lets go, the prop will flail until the car comes to a halt (whilst beating the hell out of the leg you need to stop the car), if the rear goes the prop will flail until you put the clutch in. If you can only fit one, fit it at the front.


I remember someone saying that putting the clutch in is probably going to be difficult in that situation (and probably not the first thing on your mind), however I do agree that if it's just one then it should be at the front, and for the same reason you describe IE the alternative is driven by the cars momentum.

myke pocock - 19/11/15 at 08:34 PM

Why all the concern over prop catchers? Is there a lot of evidence that props fail and if so, why? Poor design , poor construction, UJ's not replaced when needed etc. Will I regret asking?

steve m - 19/11/15 at 08:46 PM

Thanks for the answers, front seem to be the preferred,

I will revise my plans

steve

joneh - 19/11/15 at 08:48 PM

I've seen reports of some breaking and there tends not to be much between your leg and the prop in a kit compared to a commercial vehicle. Probably a slim chance of it happening but worth protecting against. I want a roll bar and I reckon my prop stands a better chance of letting go than I do of rolling the car!

geoff shep - 20/11/15 at 09:14 AM

A number of early Sierra Robin Hood propshafts broke as they had used the original Sierra rubber doughnut. In the Sierra this acts as a damper, not a UJ - the prop run is almost dead straight and there diff movement at the back end (unlike a live axle). The RH prop run was not straight and the donut ended up acting like a UJ and promptly failed.

I saw the results of a failure where the prop had beaten the square tunnel out into a circular shape, at about hip position. Considering that was in a steel monocoque tunnel, the damage it could create ally tunnel sides would be alarming.

Not sure what the likelihood of a proper propshaft breaking is - maybe it is down to the quality of fabrication, specially where it has been made by shortening a longer donor prop - a sort of 'cut-and-shut'. That said, you are sitting very close to it.

I see the rationale for front rather than rear but I would have thought that on a IRS car there would be zero movement at the diff end and 'some' movement at the front end, as the engine moves on its rubber mounts? But then I haven't seen one of these 'prop catchers' so don't how they fit or what the tolerances are.

HowardB - 20/11/15 at 09:22 AM

I have yet to fit one, however it's on the list, when I crashed my kit the engine lurched forward and the front spline on the prop dropped out of the gearbox. The car carried on rolling and the inside of the tunnel was damaged where the prop had bashed into it. Not quite the same as full chat but still a handy reminder to look after ones legs.

I have since fitted a tie bar to stop the engine moving too far on it's mounts, and will fit a prop catcher over winter

loggyboy - 20/11/15 at 11:11 AM

2 IMO -
Unless you have one centrally that is a good fit and will stop the prop from reaching the tunnel.
1 at 1 end is silly, just cutting the chances of it working by 50%

[Edited on 20-11-15 by loggyboy]

DW100 - 20/11/15 at 11:28 AM

If the prop fails at the front it is possible for the prop on some cars to dig into the road and pole vault the car.

Angel Acevedo - 20/11/15 at 04:58 PM

quote:
Originally posted by DW100
If the prop fails at the front it is possible for the prop on some cars to dig into the road and pole vault the car.


I saw a chapter of Mithbusters where the had a very hard time trying to accomplish that.
IIRC.

Slimy38 - 20/11/15 at 05:28 PM

quote:
Originally posted by Angel Acevedo
quote:
Originally posted by DW100
If the prop fails at the front it is possible for the prop on some cars to dig into the road and pole vault the car.


I saw a chapter of Mithbusters where the had a very hard time trying to accomplish that.
IIRC.


From memory they had a harder time actually getting it to fail on demand, I'm sure at one point they'd cut almost all the way through and it still wouldn't fail!!

But it is used quite successfully to somersault a bus in Terminator Genesys so it must be true...

The Black Flash - 22/11/15 at 10:58 PM

quote:
Originally posted by myke pocock
Why all the concern over prop catchers? Is there a lot of evidence that props fail and if so, why? Poor design , poor construction, UJ's not replaced when needed etc. Will I regret asking?


I collected reports during my build because I like to have something to worry about:

http://buroz888.com/racing/racing.htmlloose propshaft bolts caused the shaft to come off and rupture the fuel lines in the tunnel, car burns. Later rebuilt.
Prop off at 50mph - Prop fails at engine end, collects wiring loom, pulls all wires out of dashboard and engine bay, taking out brake cylinders, coolant pipes, gear linkage...
Prop Snapped at HIGH Speed - Prop sheared, and "...tried to beat its way out of the tunnel. did alot of damage, cut thro brake pipe, fuel pipe, wiring etc. bent the chassis big time in the tunnel, lucky it wanted to come out thro the passenger side."
Propshaft Failure - Snapped prop, only minor damage here as it seems to have had a prop catcher in place.
My car is no more - Snapped prop, ruptures fuel line, car burns and is utterly destroyed. Photos have gone now, but I was totally shocked when I saw this one. There was almost nothing left.

Scary, scary stuff. I put a catcher on mine. tbh it's not great due to lack of space in the tunnel, but is hopefully enough to limit the damage/prevent a fuel line getting smashed.

sam919 - 23/11/15 at 08:54 AM

No one likes buying a fire extinguisher, and 99.999% they're never used, but........

hobbsy - 23/11/15 at 09:27 AM

quote:
Originally posted by sam919
No one likes buying a fire extinguisher, and 99.999% they're never used, but........


True. However I saw one fire (not prop fail related) which started small and despite 4 (!) Extinguishers (2 plumbed in and 2 handheld) the car still burnt from bulkhead back, melted alloys. Engine saved though and car rebuilt

mcerd1 - 23/11/15 at 12:53 PM

quote:
Originally posted by myke pocock
Why all the concern over prop catchers? Is there a lot of evidence that props fail and if so, why? Poor design , poor construction, UJ's not replaced when needed etc. Will I regret asking?
^^ as the others have said its not that common a fail if you've got a well made and well maintained propshaft...

the point is that if it goes it can be nasty (fuel lines, wiring loom and your legs are all potential targets in a lot of cars)
but fitting a pair of catchers that limit the damage costs next to nothing - so why wouldn't you do it ?

it could be the difference between just fitting a new propshaft and rebuilding the whole car or your legs




other things like lightened cast iron flywheels are probably just as dangerous, but much more expensive to sort (my steel flywheel cost me nearly £200 )

[Edited on 23/11/2015 by mcerd1]

Paul Turner - 23/11/15 at 05:17 PM

Way back in the early 1990's my Caterham started to make a loud banging noise from the tunnel when taking LH bends with some enthusiasm, fine on RH bends. The instant cure was to drive slower but why have a Caterham and drive slowly. Carefully inspected the car but could find no evidence of the issue until I took the seats and tunnel carpet out. The aluminium tunnel at the side of my left thigh was well battered but since the car had a full floor covering the underside of the transmission tunnel it was not visible during a normal check. Once the tunnel cover and gearlever were removed the problem was obvious, the gearbox mount rubber had failed probably due to oil contamination. Mount replaced and a breather tube brazed to the gearbox lid with a pipe to the catch tank and no more issues.

But seeing the damage that was caused by a prop that was still intact focused my mind for a few weeks.