Do you need to fit a centre bearing with BEC. I could get away with a straight run but most of you guys seam to use props with 3 UJ's and centre bearings. WHY!!!!!!

Carl

to spend more! Seriously, i dont know. Id rather have a one piece one myself, to avoid adding a bearing mount.

I think the general consensus is a two piece prop is a must on a BEC.

Wouldn't be strong enought

Joao

If you do the maths on a conventionally made (ie no carbon fibre etc) single piece prop of the length required in a Locost, and also factor in how fast it needs to rotate and you'll likely find its either to large a diameter to fit down the tunnel, or it shakes itself to bits long before you hit terminal velocity

A chap I know had a single piece prop in his Fury ZX9 several years ago and from what I recall he was lucky to escape without injury when the prop self distructed.

[Edited on 6/3/06 by ChrisGamlin]

I also reckon it has something to do with the single donor concept. The Sierra propshaft is useable with little modification and will fit a multitude of engine types.

Phil

[Edited on 6-3-06 by Hellfire]

The rotation speed will be the same as a car won't it?

And Carls will be at least 12" shorter than most as he's modded the chassis and the engine is right back and the Phoenix chassis is short anyway.

Direct straight line from prop adaptor to Sierra diff as well.

Is it just the general length that's the problem?

Pat...

I was under the impression it was the issue of weight on the gearbox output shaft.
Col

Im fairly certain its mainly the length that causes the issue, a prop that long reaches its resonant frequency before the terminal speed of the car unless its quite large diameter. Obviously it doesnt rotate any faster than a car prop assuming the same diff and top speed, but without the long gearbox coming up the transmission tunnel, its nearly twice as long so has a much lower resonant frequency RPM.

If you can shorten the distance enough then I can't see any reason you couldnt use a single prop, I can't see the weight of it on the output shaft being significantly more than a long two piece prop, but might be wrong.

Probably best to give someone like Bailey Morris a call and ask them what length you could safely fit.

[Edited on 6/3/06 by ChrisGamlin]

Hellfire, although it may be the case now, I think the two piece prop "fashion" pre-dates the single donor cars like the Indy etc, so Im not sure its that. A lot of BECs such as mine use a prop with a transit centre bearing for example.

Chris

Sound about right Chris.

I wonder what the terminal length/speed/diameter equation is?

Carl - go for a centre bearing!

Pat...

If you do a search on the yahoo BEC list there was a veeeeeeeeery long string about this some time back.

Yep, thats the one I remember along with Pete Osborne's single piece prop horror story. Its a reals shame it's such a bu**er to search and nobody got round to putting it into a searchable database cos there's been some great info buried in there over the last 5 years or so!

Also, just thought, if you've got a live axle then you'll need a sliding joint. This might add additional weight and/or change the resonant frequency making it even less viable

The prop I need will be only 1200mm long(4ft in old money). I will nip to the prop people in Leeds this week and go with what they say and I assume they will say centre bearing. Plus I thought centre bearing would be safer. it would be better to mod when I get around to fitting a reverse box.

Thanks Carl

What reverse box???

Pat...

Here's a web site that does the calculations for you.

Assuming the bike has a 12k red line, 1:1 top gear and a 1.6:1 primary reduction ratio (rough figures), an average BEC prop will rotate at ~7500rpm at top speed

With a 60"/ 5ft long prop (roughly a normal BEC tunnel length), a 3" prop will fail at roughly 5500rpm regardless of the wall thickness. A 4" prop will fail at around 7300rpm so you'd need to use 5" to be safe, failing at just over 9000rpm - but that wouldnt fit down the tunnel on most cars.

If you bring it down to 4ft, a 2" prop which is what most use in a 2 piece setup would fail at ~5500rpm, and a 3" prop fails at about 8200rpm so still fairly close to its resonant frequency at full chat.

All these were using the "1053" material property which was the middle of the two as far as results were concerned. I don't know what material is most commonly used but you can bet that the one that gives higher speeds is more expensive!

Thanks Chris.

Looks like a centre bearing is definately required then.

Pat...