Twingle, Twumper, Close Firing V, Big Bang

[The Prophet of Doom]

I've mentioned this in a couple of other posts, but it prompted little discussion.  Either I'm the only one who's interested or it deserves its own thread.

Check out this post from the board archives...
http://ridersofvision.net/rovforum/index.php?topic=7779.msg71403#msg71403

I was going to do this later but since I was doing the shims anyway and had messed up my cam timing no time like the present so I've done the small amount of work required but have some wiring to do yet before I can fire it up.

Apparently it's a common mod for hill climbers and flat tracers as it improved torque immensely, though at some expense to the top end where I never go anyway

People do side by sides as well.  They also make side by sides fire 90 degrees out to give it a V twin sound and power delivery but that means messing with the crank and custom cams.

I've seen nothing negative from people who have actually done it.  A few armchair wankers saying it will do in your big ends, clutch, rip tyres apart, cause teenage pregnancies etc.  (Actually that last one might be true :-) )

All I've done is invert the cam so the front and rear fire in the same crankshaft rotation rather than opposite ones so firing will be 70 / 650 degrees rather than 290 / 430.   The bike is wasted spark with sensors for each cylinders and individual carburetors already so all that can remain without change.

As far as I can see it creates NO balance issues as balancing is typically only for the reciprocating mass of the piston, and this does not change. It must be this way on the XZ because cog on the crank and the bancer both have 45 teeth -  the balancer goes round with the  crank, not with the cams.   So yes vibrations (from explosions) will double, but they will be half as often, so the Root Mean Squared (RMS) vibration should be identical.

You can see from the picture the sound would be quite different.  At 1200 idle it should sound almost like it's doing 600RPM (10Hz) -  That's the frequency they use to give you the shits in horror films. :-)  apparently long term exposure to 18hz can cause hallucinations so I'd best be careful   At open road speeds it should be quite relaxing.

It's only a few hours to undo it, so assuming it hasn't done in the big end, no harm in trying.
The nice thing is that RMS power should be the same, though delivered differently so it has a different effect on road holding – more controlled rear wheel breakaway apparently - not that I care much about this as I'm not usually riding anywhere near the limits, but interesting that no-one I found says it makes things worse.  After a couple of incidents I tend to over compensate but if this gives advance warning of breakaway it might change my riding habits a bit :-).

Adhesion is not really the primary reason for doing it.  I'm much more interested in the torque change – it should feel gruntier, and the sound change - it should sound gruntier, and the mod factor – I like having mods just for the sake of them.  Of all the possible benefits, only that last one is guaranteed till I've actually got the machine going again.

Any thoughts on this?

[Rikugun]

I've talked with POD about some of this so I'll keep this (relatively!) short.  ;) Actually, the 2007 forum discussion I was unaware of until just now. Thanks for the link, I'm definitely going back to study that more...  :)

I like how (relatively) easily achievable and reversible this seems to be. I'm glad POD has the time and inclination to investigate as I'm really anxious to hear a report.  I'm particularly interested in torque delivery differences but hadn't considered engine note much. But since you mentioned that I'm curious to know how that's changed too.

Any thoughts re: less/more strain on the starter motor and clutch? Is the close interval firing purportedly detrimental to rod journal bearings?

So how far off is the maiden voyage?

[Rikugun]

So reading the '07 post it seems treedragon rode one and thought it a torque monster. He also claimed it felt stronger up top although some have theorized lower top end performance. This was all seat of the pants dyno so that could be a thing....  :) 

Superfly claims to have altered his but then went back to stock. Treedragon expressed a desire to alter his but I'm guessing ended up not? Any further info on why to either?

[pinholenz]

This is fascinating stuff. I haven't got my head around the theory and what has actually got to happen to make the mods but would love to see the documentation and results. Should it just be a matter of rotating the cam sprocket so that both cylinders are on an intake cycle at the same time?

I used to ride a Panther 600 sloper single back in the UK. The prospect of a twumper from a v twin really  fires up my imagination.

[dingleberry]

I used to ride a Panther 600 sloper single back in the UK. The prospect of a twumper from a v twin really  fires up my imagination.

Mmmmm, you wish you still had that I bet. I just found a 1926 Matchless engine I want to build bike around (after the Vision is finished)

[The Prophet of Doom]

I don't know why Superfly went back to stock.  Unfortunately he's long gone from the forums.

Good thinking about the starter and clutch...

Both power strokes are overlapping so would expect some extra stresses there.  If it slips, then perhaps some heavy duty springs and new plates would be advisable.   

As for starter, the compression strokes will be overlapping also.  Not much to be done about that except perhaps a freshly rebuilt starter, high CCA battery and good cabling.  All of which I've done already

I'm still a fair way off running, but I'm all over excited about this and tempted to crack the 400 open and twingle it as well except at the moment I can't get the stupid thing going.

@ pinhole
Careful study of the picture in my earlier post should help visualise it.  The pic is split into two sections, what happens to each cylinder  now, and what happens when twingled.

In stock form, our bikes fire one cylinder per crank rotation, but 70 degrees apart. so while one is on a power stroke, the other is on an intake stroke.  This leads to a slightly lopsided but largely even bang (290) bang (430)   bang (290) bang (430)

The crank makes the piston go up and down, but on the vision the ONLY thing that determines if a piston going Upwards is on a compression or exhaust stroke or a piston going downwards is on a intake or power stroke is the orientation of the cams.  Fuel delivery and spark happen the same either way.  It's just that the (wasted) spark has no effect on a cylinder not primed with fuel.

By re-orienting the cams on one cylinder, you can make what was the intake stroke the power stroke, what was the exhaust stroke the compression stroke etc.

We do this by popping one of the cylinder valve adjustment cover off, releasing the cam tensioner, and re-positioning the cams so they are where they would normally be exactly one crank rotation later.  That's all there is to it.  It takes less time than doing shims - mostly the process of juggling with radiator, removing valve cover etc is identical.  Realigning the cams against the marks can be a little tricky but fuzzlewump wrote a great tutorial a week or so in the "Shimming Woes" thread about cam alignment.

By firing very close together, the sound will be quite different as the crank will rotate 650 degrees after the rear fires before the front fires again. 

Bang(70)Bang(650)                  Bang(70)Bang (650)

The effect of this is that the bangs will be (almost) twice as loud, (almost) twice as powerful, but (almost) half as often.  It should sound a bit like an old slow single, yet still maintain most or all of the power as that is largely a function of the size of the explosions rather than when they happen.

[Rikugun]

I must be missing something.... As I look at the schematic, it would appear either front or rear could be altered to achieve the pulses 70 apart followed by the long 650 degree pause. I'm going back to the diagram for more investigation....   :)

Good thinking about the starter and clutch...

Both power strokes are overlapping so would expect some extra stresses there.  If it slips, then perhaps some heavy duty springs and new plates would be advisable.   

As for starter, the compression strokes will be overlapping also.  Not much to be done about that except perhaps a freshly rebuilt starter, high CCA battery and good cabling.  All of which I've done already

Actually I was referring to the starter and it's clutch, not the drive clutch so good point about the HD springs and plates if needed.  :)  Maybe heavier springs in the sprag if need be?

[Walt_M.]

If I remember correctly, the close firing order was done to improve traction by spreading the power pulses at the expense of maximum power. I suspect that on the Vision it will make a different exhaust tone and increase vibration. The balancer is set up for the standard timing. 

[The Prophet of Doom]

@ Rikugun
Yes you are right of course.  Either way would have the same effect.  I'll edit my last post.

Unlike the drive clutch, I don't think the starter clutch uses spring force to enhance grip, so much as to position the rollers so they wedge into place.  They are pathetic little springs.  Perhaps roughing up the surfaces for additional metal on metal grip ????

@Walt
The balancer on the Vision as far as I can tell  rotates once for each crank rotation (both gears have 45 teeth).  It can't be set up differently for power and intake strokes.   

[QBS]

Could stronger sprag clutch springs enhance starter clutch grip?  Mine sometimes slips a little when cranking.

[Rikugun]

Corrections - I misidentified the starter clutch as a "sprag" when it is actually a trapped roller type. I also overemphasized the importance of the springs.

If the springs aren't use shortened or bent they apparently are deemed serviceable. If the rollers aren't being kept in contact with the hub and ramps it may not be the springs fault. Gouging of the flywheel or sheet metal cover may impede roller movement. While a stiffer spring may help compensate for the added resistance, the spring wasn't the culprit. Galling on the rollers and hub surface also may contribute to the clutch not holding under load. Start attempts with a less than fully charged battery may also cause a weak clutch to slip. A load beyond the design parameters may also. This last one may apply with the close firing cam timing.

Apparently the 2007 referenced bikes started OK. No mention was made of any problems anyway.... I guess I was just wondering aloud if both cylinders' compression strokes now overlapping to some degree would cause a weak starter clutch to fail or shorten the life of an otherwise healthy clutch.

[fret not]

I think there will be more strain on the electric starter and battery rather than the starter clutch.  But, maybe we will see, is anyone gets their XZ sorted for long bang.  Remember that the same starter clutch is fitted to the V-Max and other rather large Yamahas.

[Rikugun]

Remember that the same starter clutch is fitted to the V-Max and other rather large Yamahas.

I do remember that but am unsure if they suffer the same failure rate on those other models?

[dingleberry]

Here is an excerpt from "Motorcycle Tuning - Four Stroke" by John Robinson.

"...the V motors, especially those with two crankshafts, make it easy to get pretty well any firing interval at all, and when the intervals were made unequal - one short, one long, the so-called big bang engines- the riders found they got better, more controllable drive out of corners. The penalty was more severe loading in the driveline, with gear and primary drive failure becoming more common."

Don't think the "drive out of corners" concerns us as he was referring to 500cc GP and flat track bikes, but the "more severe loading.....and failure" might be a worry for longevity of bike.

First time I read this book most didn't sink in, but the more I read on forums, the more points I pick out of books that have relevance. All helps my lazy beer swilled brain to learn.  ???

[fret not]

Failure of gears and drive lines would be expected to be more common in racing machines due to the extreme sort of function they are subjected to.  Can you imagine the loading of the surfaces during high RPM racing conditions?  Usually our street machines don't get treatment that harsh, not on purpose anyway, so, I don't think that is a major concern for street bikes.

[Rikugun]

The modified XZ referenced in the 2007 thread linked earlier had purportedly been in the modified state for as long as a decade or more at the time the post was made. If this is true, it speaks to longevity for that particular example.

Moto GP prepped bikes would have far greater stressed engine and driveline components. I can certainly expect there would be failures of any new configuration going through development. Were there V configured twin crank 4 stroke GP bikes or is the author referring to two strokes?

[dingleberry]

Yes the author was referring to two strokes in that case I think. Thanks guys for your replies, that makes sense to me.

[jasonm.]

Remember that the same starter clutch is fitted to the V-Max and other rather large Yamahas.

I do remember that but am unsure if they suffer the same failure rate on those other models?

starter clutch...on the venture and vmax is the same in the early 80's. But the v-4 engine fires more often...thus the clutch actual receives less ABUSE. simply a v-4 starts easier than a v-2

[The Prophet of Doom]

So I've got my twingled engine running...

Certainly it didn't seem particularly harder to start, and it idles pretty much the same.

I was expecting it to sound a whole lot slower - but it doesn't so much.  It does have a bit more of a bark and I think with a more open pipe like my predators it would sound better still.  I have to remind myself this is the 400 which is a fair bit quieter than the 550

Road test coming soon.  I need to synced and set the mixtures but I don't think the setting is that far off.

I know you'll be wanting pictures so here is...
http://youtu.be/sVCuii3Vti0

[fret not]

We'll be waiting to hear if there is a benefit to the change in firing.  You might have to go power sliding in the dirt to find out.