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Firing order and traction

Firing order and traction

Firing order and traction

(OP)
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?

RE: Firing order and traction

Antii
    This is an interesting question. I also wonder whether the V twin gives more traction to the tire. I am also not sure who is right or who is wrong.
    One thing that i know is that the V twin power pulse is bigger than the 4 cylinder power pulse. This comes from the bigger displacement for each of the twin cylinders. This is the reason why I keep on wondering whether the theory of bigger interval of the twin gives more traction to the tire.
     Anyway, one good reason that I can think of is that the twin cylinder has better low and midrange torque. The torque spread is much better than the 4 cylinders. The twin also has less peaky torque if compared to the 4 cylinders. Better low and midrange plus the less peaky torque will translate to better traction.
     We also have to understand that the current world superbike championship decision makers are dominated by Italians. I heard that the technical group of the comittee made the calculations for 2,3 and 4 cylinders engine. They made it extremely difficult for bikes with more than 2 cylinders to have advantages by restricting the intake flow through the use of restrictors. Before this, the 4 cylinders can have only 750cc compared to 1000cc for the twins.
     That is the reason why WSB is dominated by the Italian Ducati two cylinders bikes. Japanese OEMs were very unhappy over the unfair rules and made a right move by staying away from the championship.
     Perhaps, the only reason that the twins to be winning a lot in the WSB is that they have the unfair advantage over other bikes.

AO
     

RE: Firing order and traction

HP/cu in vs fuel use/HP--that's the difference.  With everything else equal, more cyls. favor the former, less cyls. the latter.  Many cyl. engines can be built to have very high amounts of low speed torque, but they have a huge advantage at high RPM.  This is all very elementary.

RE: Firing order and traction

Metalguy,
    Back in the 90s, F1 engines can have as many as 12 cylinders. Yet the most successful engines have only 10 cylinders and not 10. The 12 cylinders benefit the most at Monza and Hockenheim. The rest of the tracks favor the V8 or V10. They ended up having only 10 cylinders and not more and not less. To win the races, it's not about having the most cylinders to get the most power.

RE: Firing order and traction

(OP)
OK, I think I might not have made my question clear enough. I wasn't asking regardling different numbers of cylinders, just different numbers of firing impulses. Will firing order ALONE, make any difference to an engine's ability to get traction or any other characteristic.

The 500cc GP engine is the best example I can thing of. Take 2 V4 engines otherwise identical - one with many cylinders firing at once, one with the pulses evenly spread. The former is supposed to provide better traction. Why?

RE: Firing order and traction

Azmio,

Probably a HP vs fuel use issue.  *Never* bet against more cylinders if you need HP/cu in.  Yes, HP alone doesn't win races.

RE: Firing order and traction

I think in the 90's F1 had fuel use limits, which would completely distort the picture, as both maximum power and good fuel economy were both important. As the requirements for each are often conflicting, therefore, the trick was the best compromise.

Regards
pat

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.

RE: Firing order and traction

Interesting question.  I'm not sure if braking generated traction and  power generated traction would be similar but, intuitively they seem similar.  

Engineers who design braking systems seem to think that ABS when properly applied is more effective than standard brakes in that the ABS systems lock and unlock (pulse) the brakes as many as 20 times per second depending on the system providing shorter stopping distances as opposed to being in a partial to complete skid.

The coefficient of static friction is usually significantly higher than the coefficient of sliding friction for most materials.

So, if leaving a corner with a smooth power application causes the tire(s) to break loose or be right on the edge of it as opposed to lots of power pulses which continuously break the tire(s) loose then allow it to regain traction in a similar way to an ABS setup, that could cause the tire to continuously move back and forth from static to sliding friction.  Since the coefficient of static friction is higher, "in theory" it might just make sense.  In practice, well, the stopwatch tells all!  I'm sure rider/driver experience, other engine torque characteristics, weight and balance, and track  design/conditions are just as significant, if not more.

Chumley     

RE: Firing order and traction

Ok, I figrued out why big twins get better traction than the 4 cylinder engines.  The engines with 2 large cylinders require more of energy be stored in a flywheel than a similarly displaced 4 cylinder in order to compress that large volume of air.  A large flywheel means the rotating assembly has a lot of inertia and cannot accelerate fast enough to break traction.  It also means that if you lose traction, the rpms wont build as quickly so you'll have time to react.

RE: Firing order and traction

(OP)
Thanks for your reply.  I figured the flywheel weight would be a big part of it, but I wonder if that is all.  People do make 4 cylinders with differing firing orders, hence, maybe require different flywheels.  I think if it was purely a flywheel weight issue, why not just use a heavier flywheel on a normal 4 cylinder.

I wonder also if people working on abs find significantly different performance from different system cycle rates.

RE: Firing order and traction

Everything in automotive design is a compromise.  A lighter flywheel gives better throttle response because there is less mass to accelerate, and shifts are shorter because it takes less time for the engine to decelerate from high rpms.  Heavier flywheels improve rideability by reducing the rate at which the engine can accelerate during wheel spin, and it effectivly gives the bike a little more low end torque to get it moving.  One of the major differences between a motocross bike and a trail bike (dirtbikes) is the flywheel.  Motocross bikes are fast reving, high winding engines while woods bikes need to be nimble and torquey.  They only difference internally is the flywheel, compression and camshafts remain the same.

RE: Firing order and traction

I thought flywheels also had a pronounced gyroscopic effect on motor cycles

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.

RE: Firing order and traction

Could be, I ride dirtbikes and they are tall so it is very easy to upset the balance and initiate a turn.  They wheels have a very pronounced gyroscopic effect, and you can use this to initiate a turn.  In order to make a left-hand turn, you push the left handle bar (turn them to the right) and the gyroscopics will cause the bike to lean to the left.  It takes so much less energy than trying to make the bike lean by yourself.  I don't notice any gyroscopic effects due to the engine.  The rotating parts don't weigh very much and are of very small diameter so their gyroscopic forces aren't very pronounced.

RE: Firing order and traction

Guys,
   Are you guys sure that the race bikes competing in the WSB championship have flywheels? I may be wrong but I dont recall flywheel being used in sportbikes. Same with F1 race cars, I guess.
   However, I do know that touring bikes like the ones from BMW have flywheel. The touring bikes are different from the sportbikes in which the sportbikes rev rapidly up to 15,000rpm. Flywheel will really slow it down but the touring bikes will benefit in term of better ridability.
   There is also not much packaging space for flywheel to fit in. Otherwise the riders wont be able to lean the bikes very close to the tarmac.

AO

RE: Firing order and traction

You're right, motorcycles don't actually have a flywheel, they use the alternator as a flywheel.  Formula 1 uses a multiple disk clutch very similar to what is found on a motorcycle and probably uses the basket as a flywheel because there is a lot of weight there.

RE: Firing order and traction

Antti,
To have a different firing order in a 4cyl you would need a different counterweighted crank and possibly a counter weight shaft to control NVH. The combination of these may have a flywheel effect but a flywheel at the end of the crank will not dampen out inherent unbalances in the crank. Also remember that a lighter crank/flywheel can decelerate at a greater rate as well as accelerate (ie spin up faster) and this itself can have a major effect in the stability of a motorcycle, esepecially under braking. A greater flywheel effect will not make an engine more "torquey" but maintain a greater energy for a given rpm, this may feel like torque in some instances like when releasing the clutch. It will also carry you into a corner more under a closed throttle as there is more crank inertia to dissapate. Also from what I have read GP engines prior to the big bang firing order had a very savage power delivery (ie a big step in the torque curve) and the big bang firing order an effect of softening the throttle response making the bikes easier (relaitvely) to ride. In the last years of 500's many riders switched back to the even firing order engines as electronics & engine design were able to deliver a more manageble power delivery.

76GMC1500,
The major effect on stability of a motorcycle is the chassis geometry and also the geometry of the tyres. Mainly the rake and trail of the front fork. The inertia of crank and wheels is minimal by comparison.

Regards,
MB

RE: Firing order and traction

(OP)
OK, thanks for your thoughts folks.

Antti.

RE: Firing order and traction

I remember a teacher telling us about motorcycle hill climbs. Some people were cutting and welding the camshaft so the big four cylinders had #'s 1&4 firing at the same time, then 180 degrees later #'s 3&4 would fire followed by 1&4 540 degrees later. The idea was to let the tire hook up. Now it seems like the big Harleys dominate the unlimited classes. Great question, Mike

RE: Firing order and traction

I'm a little late in responding, but are you referring to Ducati's "Desmosidici" GP bike?  www.ducati.com  Check out the MotoGP part under Racing links.  It details the Desmosedici, AKA "Twin Pulse" GP bike.

I think this is exactly what antti had in mind with the question.  Ducati designed a 989cc, 90 degree V4, but the pistons in the forward bank travel side-by-side and fire simultaneously, as do the pistons in the vertical bank.  Claimed horsepower is 220+ @ 16,500 RPM.  

In one of the articles they claim that having two smaller pistons fire simultaneously gives the "feel" of one big piston.  Or in this case, a V4 that rides like a big twin.  BUT, there is a blurb about testing a "conventional" firing order where each piston goes through its cycle independently, however there were concerns about the rear tire losing traction.

RE: Firing order and traction

Prob with the theory was ducati found in testing that a more conventional V4 firing order was a faster setup. Didn't destroy the cases as much & NVH was far superior.
Cheers,
MB

RE: Firing order and traction

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:  www.ducati.com

RE: Firing order and traction

(OP)
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.

RE: Firing order and traction

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

RE: Firing order and traction

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.



RE: Firing order and traction

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

RE: Firing order and traction

(OP)
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.

RE: Firing order and traction

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.  

RE: Firing order and traction

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

RE: Firing order and traction

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.

RE: Firing order and traction

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

RE: Firing order and traction

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

RE: Firing order and traction

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!

RE: Firing order and traction

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

RE: Firing order and traction

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

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