Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board Why Twins dont eat their cranks like triples

I know I refer to this guy too much but we recently had a discussion about how much the centrifugal force of the triumph twin crank helps the big ends and that while triples often poke out their conrods in a wave of disgust, twins almost never do, even with quite low oil pressure. I think Kevin explains both tendencies quite well...

https://www.cycleworld.com/story/bl...con-rods-and-cranks-properly-lubricated/

My first old pre-unit in 1976 would indicate next to no oil pressure when hot (on its little mushroom tale tell button, at low revs. It would even hardly show when running at speed. Never a problem with the crank though. So I think the centre-fed explanation is true.

To some extent the BSA and pre-70 Triumph C-range twins have this weakness, but tend not to be as vulnerable, not sure why.
The Trident/R3 is quite a machine, I have only been involved with one, my mate's T150 in the mid 80's. It was constant trouble and expense. He was glad to pass it on. Unless you've got deep pockets, I wouldn't touch one with a bargepole.

A twin can go round the world, it's quite something when a triple does a long trip across America, or any other long journey.
I understand the buzz of that full-throated roar of the triple, it is fantastic and irresistable. I just don't think they are bikes to run for many tens of miles.

Oh dear, what have said?

Too much.....obviously!!!!

Horses for courses, only my opinion.
I think they are lovely machines, I'm just glad other folk pay for their upkeep.
A bit like other exotica, like MVs.
How many tens of thousands of miles have you put on your T150 I wonder?
Just from what I've seen, they're not as durable as the twins.

...a twin is not durable actually so I can feel for a trident owner if that is right.

John Young seems to have cracked the secret of making Trident engines last. I've had a couple of T160s but not for long enough for either to give me any problems. The oil is fed to the big ends via the two central main bearings, which means than any excess wear on them causes oil pressure to drop and what doesn't go in isn't going to come out!. Similar design in principle to the bush timing side mains of A65s, earlier T100/T90 etc. End feed seems a much better way of doing things but there must be ways of overcoming the inherent problem with Trident cranks. Better main bearing materials? Increased capacity oil pumps? Improvements to oilways?

Back in the 80s I had a 1974 trident that I put over 30,000 miles on before a drunk woman rear-ended me and ended the bike and almost ended me and my wife. That was one very reliable and enjoyable bike - the only problem I had was burning a hole in the center piston once. The throw-out bearing did rattle for years, but I never fixed it and it never completely gave up. Although the bike was mostly reliable, fast and enjoyable to ride, I still prefer the twins.

I now have, and have also had, both twins and triples. On one T160 I rode from the UK to the north of Spain and back, another threw a rod on the motorway, the day I rode it home from its previous owner. So the article and this topic is of some interest to me. However, I am having trouble with the meaning of this sentence

"With such a system, oil from the mains must be forced radially inward into the main journals against “centrifugal force” to get it to the crankpins."

The main bearings lie on the axis of rotation and the crankpins are outboard of this, So surely the oil must need to be pumped radially outward, which means that centrifugal effects will assist its flow. Or have I misunderstood something ?

The axis of rotation is the centre line through the crankshaft.
Anything outboard of that centre line experiences “centrifugal” force, which pushes stuff (like oil) away from the centre line.
So for the oil to find its way into the crank it must (irrespective of the small channels in the main bearing shells and small holes in the main bearing journals) it must be forced against that “centrifugal” force.

That is why the triple engine needs much higher oil pressure than the twins, just to get the oil into the crank in the first place.

With the B-range and later C-range twins the oil flows without such hindrance via their centre feed. They also have a large reservoir of oil in their sludge tubes, so any brief interruption in pump supply won’t cause a problem.

By contrast, the triples just have drillings to connect the mains to the big ends. As such they are critically dependent upon oil supply and pressure. All will be well with that proviso.

I can only imagine that BSA twins and earlier C-range Triumphs aren’t so sensitive because they’re only delivering oil to the 2 bigends, have a sludgetube reservoir with centrifugal benefit and don’t have to cope with heavily loaded centre plain bearings.

I had no intention to offend any triple owners, just not my personal cup of tea, and seeing my pal’s suffering 36 years ago reinforced this to me.
I don’t think 30K miles on an engine is really that amazing, but perhaps for a triple it is…

The factory triples as used at Daytona, was the factory doing anything different to the bottom ends?

Centrifugal force = 4* W * R * Pi^2 * n^2 / 3600 / g
W is weight in pounds
R is radius to rotation centre
Pi is 3.1415926...
n is revolutions per minute (RPM)
g is gravity 32.174 ft/sec^2

SAE 40 oil density is 0.0315 lb/in^3
Triple centre main journal is 1.917" diameter (0.8575" radius)
The oil hole is about 0.16" diameter so the oil in the hole to the centre of the crank is about 0.0005431 lb (0.08^2 * Pi * 0.8575 * 0.0315)
At 1000 RPM the centripetal force, assuming all the oil is spinning at the radius (actually the force would be an integral of the forces along the hole), is ...
0.01323 lb
At 7000 RPM the force is 0.64815 lb.
The pressure at the surface of the oil hole is 32.236 PSI (0.64815 / 0.08^2 / Pi)
I think the pump can handle the back pressure. If mercury was used instead of oil it might be a problem.
The oil on the other side going to the journal is pulling the oil the other way and since the throw is larger than the main bearing radius the effect should be nullified.

I've that centripetal/centrifugal force nonsense so many times regarding a65 cranks i just laugh.
99.99% of engines are fed via bearings, there are plenty of things on the triples that may be suspect
but the oil feed is fine as is the pump.

Whoops ! Pressure = Force / Area, not Force *Area.

This gives a pressure of 32 PSI at 7000 rev / min. However, this would be an upper bound estimate and the actual pressure would be less, because as you correctly point out the force is the integral of the forces acting along the radius.

But I take your point about the effect being reversed when the oil flows from the centre of the crank to the main bearing journals. The centrifugal effect here is to aid the flow (which is how a centrifugal pump works) and that is what prompted my original posting.

The vast majority of Automotive engines use the main bearings to feed the crank and so the big ends with oil, I doubt the Trident engine gets its issues from that setup unless there is some factor they missed in the design. Modern main bearing shells are much narrow and on a much bigger diameter that the ones from the 60's and 70's. They are narrow to shorten the engine and the bigger diameter brings the bearing surface area back into the design limits and adds stiffness to the crank.

Hi folks, thanks for the input. Sealong, Dave's numbers felt wrong to me but I've not had the time to check them (wife dragging us around Xmas markets!!!) So thanks!

My original post was not made to point out problems in the T150 design, the oil system is fine (although all the triples were really supplied as a loosely assembled kit for the owner to finalise) it was primarily related to the previous discussions that indicate a twin with low oil pressure will run forever while a triple will get rather expensive in short order.

I've a pressure gauge on my trident just for this reason.

Cheers,

BMF.

Wonderful comment ! I had never thought of it like that but you are correct. I rebuilt a T150 and a T160 back in the late 1970s (both were a mess) and they ran much better afterwards. About a year after I sold the T150, the new owner phoned me up to tell me what a wonderful machine it was.

And thanks for the tip, I think I will fit an oil pressure gauge on my current T160.

Thanks for correcting the dumb error. However, I did say that the centrifugal force calculated had all the oil in the hole at the crank radius which greatly over estimates the force.
The integral of r from 0 to the bearing is r^2 / 2 = 0.36765, so the centripetal force at 7000 RPM is 0.27789 lb and the pressure is 13.821 PSI (this is assuming I punched the numbers into this crappy four function calculator right).
Possibly the difference between twin and triple crank life is that people with triples tend to use them more "sportingly", whereas twin riders tend to plod more. Few twin riders wind the engine out to red line and since triples make their power at higher RPMs, they do.
Also a factor could be owners that bodge a fix instead of buying new parts and fixing it properly. Or wiggling the rod up and down to check clearance.

[/quote]
And thanks for the tip, I think I will fit an oil pressure gauge on my current T160.[/quote]

The best ones come from Greece 👍

+1 to oil pressure gauge from Greece. It is really necessary on an old Trident.
My explanation of this phenomena is completely different than any oil system design problems. John Young trips showed any stories about some specific weakness of Tridents oil system / cranks are "an old woman tales". Some Trident owners from trilesonline.com have 80 - 100 thousand miles on their cranks and still going strong. I believe most of young buyers back in a day used to ride Tridents fast, but wasn't particularly interested in necessary oil changes being used to twins and singles where such neglect wasn't dangerous. Plus lower quality of oils from 50 years ago was a factor as well.

Anyway all Trident engines I've seen dismantled but not destroyed had their big ends and mains wiped to the copper and most new owners from this forum were forced sooner rather than later to rebuild bottom ends of their triples because of low oil pressure. I had to do the same with my 73 Trident, which was not used for the last 30 or more years spending this time in different garages with previous owners not able to set its valve timing properly. I found a dot on a crank pinion 180 degrees off.
However after rebuild my Trident holds the same oil pressure for seven seasons and roughly 20 thousand km and I hope it will not change in a long time. I use fully synthetic 10W60 oil ( because of my particular circumstances - big town, lots of traffic lights and high temperatures during summer ) and feel really safe with this engine. The second problem mentioned - center piston holing got solved by bigger main jet in its center carburetor.
I like a twin, but prefer a triple because of it's lack of vibes, better comfort and being heavier - I used to be blown by fast moving trucks on a twin, which I don't experience on my Trident. Besides I love it's sound, easy starting, 5 speed tranny and a front disc brake. The best British bike from this era in my eyes.

I also have both, for sustained higher speed touring IMO the triple is much better. I haven't had anything, nothing [brackets, lights, tank etc] fracture or fail. The vibes just aren't there. The biggest plus is you can increase the capacity to 1000cc and it makes it a very good unit.
I like the twin for shorter trips, it will do longer trips albeit at a lower overall average speed....unless you want to see something break or fall off. I've had the coil bracket fracture, the head steady fracture, The heady steady frame bracket fracture, the rear guard fracture, the front guard fracture, the rear guard frame bracket fracture, the tank fracture, the headlight ear fracture, my numberplate fracture, and I've probably forgotten some.........fixed or repaired all of them.
Plus sustained higher speeds on the twin can be hard work. Any increase in capacity over 750 affects reliability unless you have a Nourish setup.
By higher speed, I'm talking 130km/h for several hours, approx 1,000km in a day.
I think the weight of the triple helps, especially in cross winds and for second rate road surfaces.
Both my bikes have done more than 100,000km.....both have had several rebuilds to get them there.....mainly due to oil leaks or engine issues. No twin or triple will ever do more than say 30,000km before they need new valves or guides or clutch work or pistons and rings etc etc .....even if they are baby'ed.
BTW, I've never had an oil issue with either bike....except for it escaping or trying to escape .

Granted, I've never had a rod escape on anything but a triple. However, this triple had seen regular service over 11 years since a rebuild and was showing low oil pressure when it blew up. It was my only means of transport at the time. I can't compare it to the twins as I've never had to run one of them after it was wise to rebuild.
Currently my Trident shows 60psi at 4-5000rpm once warm, and I rarely ride it at under 5000. My Bonneville shows 70psi all day. The thing with triples is that they need FLOW more than pressure for healthy rod bearings.

"Word got around quickly: Rolling bearings can survive on a lot less oil than can bronze bushings. The faster a shaft turns in a bushing, the hotter the oil in that bushing becomes, and the more it “turns to water” (loses viscosity with rising temperature). It may survive if that hot oil is constantly replaced with cool, but if not, it may seize."
BSA never got the memo.

I agree, and it was my understanding that much of the reason Pick and Peckett developed the +50% pump (now +30/50) was to enable thinner oil to be circulated faster. Phil Pick favored 10w40 oil, at least for racing. Peckett, according to a friend who knows him and Pick, also opens up the oilways slightly to encourage flow.
http://www.triple-cycles.co.uk/products/oil_pump/index.htm
My Trident shows much the same pressure as yours by the way. Tales of 80psi at idle with a steaming hot engine can be dismissed, though we've all heard them.

I wonder how the crankcase bearings line up as the temperatures change? Modern computer simulations can predict these things very accurately, while Hele and company had to rely on traditional methods. I can imagine how the center mains pedestals would possibly push the bearings upwards as they heat up, while the outer cases may stay central. I don't know, just a theory.

SR

Regarding vibration, talking to the proprietor/mechanic of a shop in California (Mototown) who's a british expat that grew up with these bikes about my tridents vibration, she says that its a dirty little secret of the tridents that they they have vibration, but that its a higher frequency (from the rocking couple). Some folks feel it more and some feel it less.

I had my clutch balanced (it was out quite a bit) but surprisingly, the vibes (Which only come on strong around 4.5 k RPM) were completely unchanged. The bike came to me with 10'000 km on the clock, totally stock, engine never opened, but with a cracked oil tank mount and a cracked chain guard mount.

Im guessing crank /rods/pistons out of balance but with oil pressure at 80 PSI at anything over 3k RPM its going to be a long time before I open the bottom end to find out...

All of them vibrate, and to my butt, about the same level whatever you do to them. I changed to heavier pistons with little perceived difference, and changed the crank later. Still no big difference that I can detect. As I see it, every crank throw is doing it's own thing and balancing can't remove the rocking couple anyway. Who's to say to which side it rocks the most? I've only owned two triples but have ridden quite a few, and to me there is little between them. The big thing with a triple is that any vibrations there may be are forgotten soon as you get on your way a bit. On many twins that's simply not so.

Looking forward to be contradicted!

80psi from 3000, at what temperature?

SR