Author Topic: decompressor linking  (Read 415 times)

Online RDfella

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decompressor linking
« on: 24.04. 2021 18:37 »
About ready to test the project's starter. I note on another thread that a member alluded to the fact that when vincents are fitted with an 'electric leg' one can ease the initial burden on the starter by using the decompressors.
Now I have a decompressor for each cylinder, with the levers on opposite ends of the handlebar (because there ain't room to fit both on one side).
As I've only got two hands and arms clearly I can't operate the starter button and work both decompressors simultaneously, and so I'm looking to pick brains - how best to link them? Should I use a cable splitter (I have one for my chokes, and it's a real pain to assemble / disassemble) or does anyone make a double lever? I'd thought of perhaps using a double air/mag lever, but guess that's not really appropriate.
I appreciate linking decompressors probably isn't the greatest idea, but on balance seems the best option for easing the considerable initial torque going through the starter mechanism.
Of course, if it transpires that the starter design is a failure, then the problem evaporates. Will know next time my son is around so he can apply the decompressors whilst I hit the start button. BTW, wasted over an hour yesterday trying to work out which terminals on the honda vtr starter relay worked the solenoid. Didn't help that the maker's workshop manual shows how to test it - but clearly illustrates the wrong terminals.
 
Clarification - by 'cable splitter' I meant a junction box (tube) where one cable goes in and two out the other end, as sometimes used on throttle cables.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online Greybeard

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Re: decompressor linking
« Reply #1 on: 24.04. 2021 20:10 »
For the decompressor, I'm thinking about a cable from one lever going to a tee bar that pulls on two cables. Is that what you mean by a split cable?

Offline chaterlea25

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Re: decompressor linking
« Reply #2 on: 24.04. 2021 20:15 »
Hi RD
Would it be possible to use a front brake twin cable mounting as used on bikes with twin drums? I think some RE's had them as well as European bikes

Another idea would be to series link the two decompressor actuating levers ? It would need a long travel  lever and also on how equal the pressures are on both levers on the cylinder heads are ???

John
1961 Super Rocket
1963 RGS (ongoing)

Offline sean

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Re: decompressor linking
« Reply #3 on: 24.04. 2021 22:40 »
had decompressors on a harley motor I built they had a poppet you just pushed in on each cylinder they were drilled and tapped into the heads available from HD dealers

Offline WozzA

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Re: decompressor linking
« Reply #4 on: 25.04. 2021 00:45 »
had decompressors on a harley motor I built they had a poppet you just pushed in on each cylinder they were drilled and tapped into the heads available from HD dealers
Ditto.. easy to fit & operate...  I think the cable type are old school...
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'57 Golden Flash Swingarm

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Re: decompressor linking
« Reply #5 on: 27.04. 2021 21:00 »
Well, finally got to try the starter. It spins the engine easily decompressed. Spins it fine with one cyl decompressed but struggles really badly with both compressed. With one decompressed, motor spins @ 304rpm. Reduction ratio of starter gearbox is 1:24. Would've been nearer 1: 30 but that's a long story, starting with the rear support bolt .....
During the exercise I noticed more deflection of the starter assembly than I'd have expected but, with 3bhp trying to spin an engine, the torque must be quite high.
So today I tried to start the engine, given my son was around to give me a hand. Not good news. We had to abandon the exercise as the starter gearbox was trying to rip itself from the engine. And I thought making it out of 6mm alloy plate would be sufficient! I now need to get my pal to weld in some 10mm sections to beef it up. Been making the fresh parts this afternoon, but it'll be another couple of weeks delay before I'm ready to try again.
In the meantime, I have to admit to being an electrical ignoramus. Yes, I can wire single and three phase, I have wired houses, boats etc to good standard. I was good at physics at school - just don't ask me to understand Ohm's law! V=IR just gives me nonsense figures. So, for those that do understand - and thanks to those members who have tried to help me in this endeavour previously - I want to softly pre-engage the starter. I intend using a separate switch to do that, after which I'll hit the start button. Using the bike's original 6v battery achieves a gentle engagement quite nicely. So what resistance do I need to drop 12v to 5v or 6v? The battery is 12v x 200A CCA and the starter resistance is .7 Ohms.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online mikeb

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Re: decompressor linking
« Reply #6 on: 27.04. 2021 22:15 »
in theory if you want to half the voltage (to 6v) by putting a resistor in line then the resistor would be the same resistance as the motor - 0.7 Ohms.
but its not that simple and I'm a bit skeptical of that 0.7 Ohm.

if power = VxA (you knew that from wiring your house) then 12 x (12/.7) = 204W - but you say that motor is 3bhp (2200W) and I'd have guessed 1000w to 1200w at least.

but motors are inductive and when a motor starts its pretty much a dead short til it gets turning. that's why people make soft starts to prevent popping fuses.

remember that other thread you started about this? https://www.a7a10.net/forum/index.php?topic=16088 - I tried an old started motor and needed a 0.15 Ohm resistor in series to soft start it at half volts. the problem was 0.15 Ohms trying to dissipate 600W glowed red hot in less than half a second (there's a video link on that page). So you'd need to make a monster resistor with lots of heat sinking (or carry a fire extinguisher and have flame retardant leathers).

I still think it would be better to rewire the internals of your starter motor to separate the pairs of brushes and power initially only one pair then both (using a second starter relay)... is that feasible? Or - two 6V batteries in series where you switch over between one and two

(I'm following this project keenly RD as I'm also interested in electric leg options for by plunger b33)
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

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Re: decompressor linking
« Reply #7 on: 28.04. 2021 10:58 »
Thank you mikeb and others, but after much deliberation last night I now realise there's an insoluble problem here and consequently have decided to abandon the electric start.
The starter motor / gearbox work satisfactorily and I could beef up the gearbox casing, but I now realise the problem lies elsewhere. As early conversions by Triumph found out, strengthening a component when weakness is discovered only leads to more serious / expensive failures further down the line and I've no intention of potentially risking breaking crankcases. Yes, I have spare castings, but I really don't need a complete engine strip and machining fresh cases.
Recalling how the starter motor was behaving yesterday I now see what's happening. If the engine fires but fails to burst into life, the crank is 'unhooking' from the ratchet. Then the starter motor has to re-engage which it does with a mighty clunk. This interrupted transmission of power is putting immense strain on the starter gearbox and its mountings. Of course, a sprag clutch would prevent that, but the reason I went to the trouble of designing the ratchet is because I couldn't use a one-way bearing. With a wide primary belt and idler (chosen because I didn't trust a chain to take the power) there is no room for the transfer chain usually adopted by aftermarket starter manufacturers. And a sprag bearing fitted to the crank large enough to take the torque would limit engine revs to 3,000rpm or less. I don't see a way out and so don't intend to waste any more time and effort on this.

Which of course is a huge disappointment. Not so much the time spent designing / manufacturing the device (though challenging it's sort of fun) but the fact the bike will now rarely be used. Starting on rollers means stalling on the road will likely require the services of a recovery vehicle. A pity as the engine goes really well, the frame handles well (as one would expect from A series) and the front brake is excellent. Will now have to keep playing the videos we took of it underway rather than riding it. Was really looking forward to riding it with electric start.

 
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online mikeb

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Re: decompressor linking
« Reply #8 on: 28.04. 2021 11:05 »
Noooooooooooooooooooo!
come on RD - so much good work done - there must be a solution somewhere.
time out yes, but don't give up!!
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline chaterlea25

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Re: decompressor linking
« Reply #9 on: 28.04. 2021 16:40 »
Hi RD,
Sorry to hear that the setup is not going to work *sad2*
What about a fresh thought?
I have seen an electric starter on big bore Vincent's where the starter mounts across the front of the engine
and drives back to the outer end of the crank via a chain
I am thinking of a starter with integral gear reduction that would engage with a shaft with a 2 to 3 times bigger driven gear and a sprocket to transfer the drive to the crank
OK the chain and sprockets would be running all the time bit this does not seem to be an issue on the Gold Star
electric starter system?
I would also think that a duplex primary chain would be capable of taking the power to the gearbox?
The BSA clutch and gearbox I feel would be the limiting factor in your drive train ???

I mentioned some time ago about my friend who made an electric start conversion on his vintage ohv Ariel
The Ariel cylinder was worn out on max oversize when he got the project
I searched my stash of old pistons and found that a long rod B33 had the same compression height , only a thick walled small end bush was needed, the bore is now 85mm opposed to 81.8mm originally so the engine is now 550cc..
He fitted a ring gear around the clutch drum and the starter behind the gearbox, the starter is one without a "nose bearing" intended for a Honda car.  The first starter motor he fitted did not have a gear reduction and so struggled a bit needing the use of the valve lifter , but the new gear reduction starter easily copes without using it, although he does use it when starting from cold.

Here is a link to another Vincent system, Could you mount the starter under the BSA gearbox?
https://www.accessnorton.com/NortonCommando/hegeler-electric-start-vincent-twin.20881/

I will try to find details of the front mounted starter later

John

1961 Super Rocket
1963 RGS (ongoing)

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Re: decompressor linking
« Reply #10 on: 28.04. 2021 17:56 »
Thanks John.
That Vincent set-up is incredible. Trouble I'm facing, as with most aftermarket manufacturers, is having to work around issues that could easily have been avoided had an electric starter been envisaged from the outset. Remember originally this engine was destined for race use, not road. Hence the large belt drive, capable of transmitting 100+bhp. Another reason for choosing that was the ability to make the engine sprocket much larger, in order to reduce torque on the gearbox. Clearly with a vee twin in a frame designed for a parallel twin, things are tight in places, limiting my choice of starter placement. For example, I have space above the gearbox a la Vincent, but alas the intake manifold for my rear cylinder precludes putting anything there. No room at the front either, unless I change the forks to a bobber style! No room between engine and frame, then there's the front exhaust pipe between the downtubes ......
Before this exercise I'd not paid much attention to sprag clutches, though I was familiar with them from auto gearbox repair. What I find is having a sprag and bearing combined in one unit severely limits torque and speed ratings. For example, a CSK40 will transmit 385Nm of torque (I need more like 500) but has a max overrun speed of 3,000rpm. A CSK15 will stand nearly 3x those revs, but only transmits 20Nm. A sprag alone (fitted to suitable shaft and housing) and oil lubricated are often rated 3x those figures and with unlimited speed. Altogether a real pain.
As for alternative ratchets, I had toyed with a face ratchet (larger version of our kickstart ratchets). That would, as Neil discussed some while back, address the issue of having too many degrees between consecutive pawl engagements. Trouble is I can't see any way to engage that other than manually. There just isn't the crankshaft room for fancy face cams to initiate engagement.

 
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline Swarfcut

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Re: decompressor linking
« Reply #11 on: 28.04. 2021 18:24 »
 Great shame, RD.  Lot of effort gone into that project, but after a good sit and a think the solution may come. Workshop electric kettle makes a handy high resistance component......start the bike, makes the tea.

 Swarfy.

Offline chaterlea25

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Re: decompressor linking
« Reply #12 on: 28.04. 2021 23:16 »
Hi RD
I know this would require an awful lot of work but could a Harley gearbox be fitted (these have the starter mounted on top?
Or , Could a "Grosset type" drive  be built into a replacement gearbox sandwich plate?

John
1961 Super Rocket
1963 RGS (ongoing)

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Re: decompressor linking
« Reply #13 on: 29.04. 2021 12:21 »
Thanks for all the suggestions.  Update is I may be onto something.
Yesterday, subsequent to my explaining to him that the starter was a no-go and my annoyance that a sprag clutch would solve that but were unsuitable, my son searched the internet and came up with an American firm making sprag clutches that had no rev limit and 3x the torque rating of those usually available. So last night I looked at their site. Unfortunately, their produce is a stand-alone sprag, requiring shaft, housing, bearing and lubrication (same as those in bikes made with electric start).
But what caught my attention was their explanation regarding rev limits. Apparently it's because centrifugal forces throw the locking elements out and the clutch will no longer lock into drive. That's not an issue for me - I only need drive at slow speeds and it's the shaft, not the elements, that'll be over-revving anyway. So, as fifteen council workmen standing around a hole in the road would say - we're looking into it. After all, the starter works fine, it's the ratchet that's causing the grief. I chose that instead of a sprag bearing purely because of the stated rev limits. I presumed the reason was that they'd seize up or whatever, but it seems my assumption may have been wrong.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

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Re: decompressor linking
« Reply #14 on: 30.04. 2021 12:48 »
RDfella goes down for the third and final time.... but what's this? He's bobbed up coughing and spluttering and ready to swim against the tide again. Hooray!