Firing order and traction

[antti]

Hi All,

This is a question I have heard a bit about but have never read a decent believable technical explaination. It relates to changes in firing order has on traction and engine characteristics, especially concerning motorcycle engines.

The basic belief is that a 1000cc V-twin powered bike can effectively find more traction off a corner than a, say, a 1000cc in-line 4, purely because of the wider spacing of firing pulses. Also, back in days of 500cc GP motorcycles (pre-2002), the last decade was dominated by so called "big-bang" engines. Here the firing order of the 500cc V-4 was changed so the cylinders would fire more closely than the usual, evenly spread firing orders. Again, it was supposed to improve the traction of the bike.

The explainations I've heard is that having longer periods between firing allows rear traction to be regainged if the tyre was pushed into slipping during the pulse (basically a static vs kinetic friction thing). Personally I would expect larger widely spaced pulses to more readily drive a tyre into slippage than smaller even pulses.

Some also claim the close firing order makes egines more "torquey" or more suited to running at lower rpms, but whether that is due to the additional flywheel weight, I don't know.

I was wondering if anybody had some insight into the situation because it does seem to be effective, eg, in the "big bang" 500 cc GP bikes?

 

[rd400guy]

AFAIK, Ducati's NOT using a conventional firing order. The forward two pistons are on the same stroke and fire simultaneously, and the vertical pistons fire with each other as well.

It's my understanding that they intentionally kept it this way, not only to keep the familiar feel of Ducati twins for factory riders, but also because it improved rear wheel traction, which is what I believe antti wanted to know in the original post.

I agree that the cases must be seeing a LOT more stress than if it were "conventional" firing order. Kudos to Ducati's engineers on that one.

As for flywheels, every motorcycle racer I've ever talked to wanted the lightest flywheel possible to spin up the motor quicker.

So far, Chumley's answer seemed like it makes the most sense. Does anyone else agree?

Oh, and by the way, the link was:

http://www.ducati.com

 

[antti]

Yeah, the explainations I have seen relate to the rate that the pulses are applied and how the tyre copes with regaining traction in between pulses. But I've not seen a good techinical expalination. The flywheel weight stuff is all straight forward, but many believe (me included) there is more to it than simply flywheel weight.

RD400, I think mburgess is correct in that Ducati did test both it's "twin pulse" and "four pulse" versions of the gp bike ealry on, but all of the bikes that have been raced were latter. I have video of both bikes in action and sound difference in unmistakable. Ducati's offical word was the twin pulse didn't bring enough (any?) significant adavantage so obviously the NVH issues would favour the four pulse.

Mburgess, yes, a least one rider switched back to "screamer" (conventional) type 500cc bikes in the late 1990s after the early part of the decade was dominated by "big bang" engines. Specifically it was Mick Doohan who did it in the 1997 season. And he claims to have done it for psycological advantage reasons - because it was harder to ride. Others followed suit (like his team mates alex criville and tady okada), but couldn't cope with the engine so returend to a big back firing order. And Doohan's strangle hold over the 97 season inidcates complete superiority over the rest of the field (intrestingly, Doohan was also the first rider use the big bang engine in 1992). Right to the end the majority (I believe) rode the big bang engines.

 

[GregLocock]

Um, the dominant stresses in high speed motors are inertial, not due to combustion, so the choice of firing order makes little difference to that.

Cheers

Greg Locock

 

[Azmio]

Guys,

I wonder if Ducati has ever tried firing the two cylinder not from the same bank but from 2 opposite banks. This way the movement can 'cancel' out each other thus minimizing the rotational imbalance and extra vibration. The engine will still get longer interval between combustion thus the theory of the tire to regain the traction can be applied. Perhaps someone out there has tried it before and I am interested to know what was the outcome.

Anyway, I have ridden a sportbike with twin engine, V4 and I4. The vibration is so much different at low and high RPM. I have to agree that the firing order can contribute to the increase in stress to the crankshaft torsional vibration, crankcase, bearings, engine mount, bike chassis and critical fasteners. I also know that the twins require bolt retightening in every few thousand miles.

 

[mburgess]

RD400guy ducati wre originally running the twin-pulse firing order (front cylinders then the back cylinders together) but as mention above have since changed. Now from what I understand similar to the firing order on Honda RVF V4's
Regards,
MB

 

[antti]

The motoGP championship kicks off this weekend and one of the intersting things to come out of the summer testing is the new "big bang" in-line 4 yamaha. The word is that they worked on 4 different firing order configurations (basically on the same in-line 4) but the final product is very rough sounding indeed. Sounds closer to a single than inline 4, but I don't know exactly what intervals they are using.

 

[Metalguy]

76GMC1500,

Gyro forces have nothing to do with motorcycle "countersteering". It's all in the lean. You can use countersteering a very low speeds where any gyro effects would be only a few grams.

 

[GregLocock]

Could we start a separate thread, in the motorcycling forum, for countersteering? It is fascinating, but perhaps a tad o/t for the engines forum.



Cheers

Greg Locock

 

[patprimmer]

Sorry Greg

I started that as a side effect of rotating mass in the engine, and it went off onto overall gyroscopic effect, which of course belongs in the motor cycle forum

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ben91932]

This effet was well explained by Kevin Cameron in an article in Cycle World. The 'big bang' motors work because of tire deflection. On each combustion event, the tire carcas flexes then springs back. If they come too close together, the tire breaks loose. The trade-off was that the gp bikes configured to fire all at once were very hard on the cases. This is also why Harleys have long dominated dirt track racing. Yamaha tried a 3 cylinder 2 stroke TT bike; it had twice the HP of the Harleys, but could get no traction at all.
Ben

 

[MikeHalloran]

Back in the days when I was young, shortly after invention of the wheel, serious dirt bikes had two cylinders and were made in England.

There was, commercially available, a replacement camshaft kit to convert your even- firing Triumph twin into a 'twingle', where both cylinders fired at once. The least incredible reason given for doing so was along the lines that the firing impulse would break traction, but the flywheels would power the bike along smoothly, so more time spent driven by flywheel inertia gave better traction overall. On reflection, I'd guess the argument may be true, depending on the specific dirt.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[crysta1c1ear]

I guess there is a difference here between accelerating in dirt, where average traction might be important, and cornering on a track or road where averages are maybe less important.

The idea that the whole setup can work a bit like ABS, where you 'loose it' on the firing plus, and then the tyre starts to grip again is interesting. Seem a bit far fetched until I read some physics explaining it, but I don't rule it out.

ABS irritates me, as the only time mine operates in parking on grass outisde my house and I'd rather the car slid a few inches that vibrated my foot like that. Maybe that is what gives me butterflies in the stomach at the thought of an effect resembling ABS while leaned over on a bend on a bike.

I looked last night for an article I once read about torquey engines which contains an urban legend about a large single cylinder engine in the Isle of Man TT races - legend had it you could hear the motor fire once every lamppost, which the author mathematically showed was some way from being truth.

So far, the logic here reminds me of the argument that stopped clocks are always right twice a day.

I guess what we are really debating is
1) frequency and
2) amplitude variation
in the torque at the wheels.

I think there are muddy and sandy surfaces where the wheels are useless once they have started to slip. Like telling a woman stuck in a field with the wheels spinning, that revving it harder won't help ... maybe amplitude variation of the torque would help on unpredictable surfaces as you could go right to the point where the wheels started to spin, and then recover from the wheel spin ... but doing that at engine rates? That's another question.

I'm more comfortable with the idea accelerating in dirt than cornering on roads!

 

[Milanoguy]

Hi Antti

To get back to your original question about “Big Bang Motors” I hope the following section is of some interest.

It is from the Nov/Dec 1992 issue of the U.S. motorcycle magazine Road Racing World. The article was entitled “Shaking the Foundations of Our Universe” by Peter Clifford, and was a discussion of why Honda had gone to the “Big Bang Motor.” The most interesting quote is in the third paragraph where it states “That is why it only really makes a difference on the slower corners where the revs are lower…..”



“The important thing is to prevent the tire from spinning,” explains Willing. “As soon as it goes from static traction to kinetic traction that generates heat in the contact patch. When that happens things start to get very hot very quickly, to the point where you even see smoke coming off the tires. The rubber is being overworked and grip is lost.

“If you allow the rubber to mold around the particles and then impart a short, sharp push, then it will push the machine forward,” continues Willing. “At the same time the rubber starts to deform and come out from around the particles of the road. If you give it another push too soon then it will spin. If you allow time for the rubber to reform around the particles it will push forward again at the next power impulse without spinning.

‘That is why it only really makes a difference on the slower corners where the revs are lower and the rubber is having a chance to mold around the particles. After that the pulses come too quickly. If you consider that the tire contact patch is about 120mm long, it is quite simple to work out that at low rpm you have two firing pulses per contact patch with the even-firing-interval engine and just over one with the new engines.”

The gist of this as I understand it; is that selection of firing order has everything to traction and nothing else.

Bye for now

 

[evelrod]

crystalclear1---"...you could hear the motor fire once every lampost..."
Well as luck would have it, I ride a 1948 Norton that was once owned by Bill Young (1950's National Champion from Western Australia) and can attest to the "big bang" of this "thumper" but, not quite "every lampost" as you have found. Makes a good story...
Everytime I ride the canyons around here I marvel at the courage and skill it took to ride this bike in the trials of the 40's and 50's.

As to the original question of closly spaced fireing impulses v. "big bang"---I tried Google for this but with limited success---If I remember correctly, in the late 40's several of the Indy teams tried a "big bang" (a term not used in relation to automotive racing in that era) version of the venerable Offy. It was very fast (high speed traction was improved?) but was 'death' on axels and drive train. My guess here is that the big firing impulses were just too much for the 2 speed gearboxes of the time.
Is there anyone here that can verify/refute my memories?

Pat, there is a long thread in the motorcycle forum pertaining to "counter steering" and gyroscopic stability as I recall. It's a small forum so the thread should not be difficult to find.

Rod