[BillyT]

Two stroke Vs Four stroke traction theory.

This is an open letter placing down my curiosity as to the realty of my thoughts of two vs four stroke traction. It is looking at the theory vs reality.

Given that a two stroke crank rotates (under power) with every pulse of the spark plug means that every rotation of the crank is under power and therefore no free wheeling in affect. The pulses are somewhat smooth as there is no coasting just continuous power being sent to the back wheel as long as you have the clutch out and the gas on.

A four stroke on the other hand has the crank make one revolution without a power pulse and one with. Basically the crank is coasting with no power pulse for one revolution and one with. This mean that when you have the gas on and clutch out on a four stroke it theoretically pulses and coast as the crank turns.

This in theory should give less traction and/or smoothness as one has lost a smooth constant grip with the ground not thru the coasting but when the power comes back on and tries to turn the back wheel.

It is more of a on off affect to power in the world of trials and smooth constant traction.

The two stroke is always trying to power or pulse the wheel as the crank is constantly under power.

I wonder how all of this relates in reality to traction as the theory says it has less traction due to its pulsing.

I welcome any input to this thought as all indications are that the bike gets great traction

Maybe I have answered my own question as the on and off again is the secret to why it gives more traction.

Two stoke= One bit of gas equals one power crank turn pulse.

Four stroke = A bit of gas gives one power crank turn and one coast crank turn. Hmmmm...........

BillyT

This post has been edited by BillyT: 04 December 2004 - 06:45 AM

[Woody]

Well, I haven't anything scientific to add to this but from my trials and enduro experience I have the following observations.

A 2 stroke 200 KTM enduro will find grip and is a lot less prone to spinning up the back wheel than a 4 stroke 426 Yam. The KTM delivers its power in such a way that there is no instant reaction to the throttle low down in the rev range, the power is introduced gently. The 426 Yam on the other hand (which I have) delivers instant torque and power at the touch of the throttle and will spin up very easily if you aren't careful with the throttle - even if pulling a higher gear. They just react straight away. Out of the two, the KTM is easier to ride and finds grip more naturally.

In trials, from participating in classic events which throw 2 and 4 strokes together, it is noticeable that the bigger Brit thumpers (which I am sure only fire once every 5 yards....) will find grip - in some conditions - more naturally than the 2 strokes. I am sure this is down to the way the power is delivered as the Brit thumpers deliver the power in a completely different manner from the 426 Yam. It is slower and less aggressive and so has a tendency to spin up less easily. The 2 strokes deliver the power a bit quicker from lower revs, so there is more chance of breaking traction. Unless it is a bog standard Pre65 2 stroke where you will wait about a week for any reaction to the throttle... they grip quite well....

I think whether it is 2 or 4 stroke the ability to grip is down to the way the engine delivers the power and how also well the rear suspension is working as that too can make a difference.

Of course, as is sod's law, just to contrdict the suspension set-up theory I have regularly seen a rider on a rigid Bantam - so there is no rear suspension at all - drive up muddy banks like they are tarmac with no trouble at all, when other machines are spinning and struggling. Must be due to the fact that it hasn't enough power to spin the wheel on ice wearing a slick....

[g4321]

Quote

The 426 Yam on the other hand (which I have) delivers instant torque and power at the touch of the throttle and will spin up very easily if you aren't careful with the throttle - even if pulling a higher gear. They just react straight away. Out of the two, the KTM is easier to ride and finds grip more naturally.

Having owned a 400 Yam you are correct BUT remember how much more power the Yamaha has than the KTM - try comparing like for like - a 250 2st - say a CR Honda - this has a lot less traction than a 400 Yam - watch the start of a MX race - the 4strokes power away first. In addition the Yam can be made more user friendly by addition of a flywheel weight. The 200 KTM is a very soft engine by comparisson.

[feetupfun]

BillyT
You are spot on about the benefits of the pulsing effect on traction in marginal conditions.
Modern (2-stroke) trials bike manufacturers design their engines to provide a strong pulsing effect at low RPM to maximise traction in slippery conditions.
Being a regular rider of 1970s trials bikes since they were new, one of the things that is very obvious to me when riding a modern trials 2 stroke is the strong pulsing effect at low RPM compared with almost electric motor smoothness of the 1970s 2 stroke trials bikes.
The physics is quite simple of why the pulsing helps. A tyre slipping on a muddy rock has less grip than a tyre that has the tread stationary relative to the rock ie static friction is greater than sliding friction. When drive is on the verge of being lost due to low traction, the part of the engine cycle when no drive is occurring (between when the exhaust port opens and when the next power stroke starts) allows the tyre to regain the benefits of static friction in time for the next power pulse. A four stroke engine simply has a stronger version of this effect because it has a longer period between power strokes for the same RPM.
The effect is only a benefit when the tyre is on the verge of slipping. Once a tyre is spinning on a slippery rock, there is no benefit from the pulsing effect.
The same effect is being used in MotoGP bike engine development. These bikes also operate at the margins of traction during acceleration and are being developed to maximise static tyre friction by optimising the pulsing effect.
Another example is ABS braking systems on cars. They are are designed to pulse the braking to maximise tyre friction during emergency braking.

[stevel]

Sorry, can't go along with the pulsing theory. Even at 1500 revs a fourstroke is still firing 12.5 times/SECOND!! I don't think that this would manifest itself in pulsing at the back wheel

[Woody]

g4321, on Dec 4 2004, 09:57 AM, said:

Quote

The 426 Yam on the other hand (which I have) delivers instant torque and power at the touch of the throttle and will spin up very easily if you aren't careful with the throttle - even if pulling a higher gear. They just react straight away. Out of the two, the KTM is easier to ride and finds grip more naturally.

Having owned a 400 Yam you are correct BUT remember how much more power the Yamaha has than the KTM - try comparing like for like - a 250 2st - say a CR Honda - this has a lot less traction than a 400 Yam - watch the start of a MX race - the 4strokes power away first. In addition the Yam can be made more user friendly by addition of a flywheel weight. The 200 KTM is a very soft engine by comparisson.

I understand what you're saying there but the terrain and grip level at the start straight of a motocross track which is a flat out blast, differs a lot from the snotty, slippy going in a typical woods/forest based enduro, where careful throttle control is needed. Modern high revving short-stroke enduro/motox motors need good riders to overcome their willingness to break traction in those conditions. An XR400 and even an XR650 will be more forgiving powerwise in those conditions than a 400/426 Yam or CRF450 Honda, due to the engine characteristics. My mates 300 KTM 2 stroke, which is more on a par with the 426 power output, is still more forgiving and grips easier than my Yam as it is again quite soft lower down the revs.

I just think, from my experience, that there is no definitive 2 stroke is better than 4 stroke, or vice-versa, in terms of the ability to grip, it's more down to how the engine delivers the power. And after the last 2 events I'm definitely on the lookout for a 2 stroke enduro...

[The artist formerly known as Ishy]

Shed full of both 2 strokes and 4.

250 and 125 four bangers can take the edge off the rear tyre/tire 50% faster than the 125 / 2.9 two strokes.

WHY!!!!

[BillyT]

To stevel

A trials bike idles at around 300 rpm and up so were do you get 1500 RPM from?

BillyT

[neonsurge]

With regards to the power delivery characteristics, the torque curve of a 4-stroke (when plotted against rpm) is far steeper than that of a 2-stroke and this is fundamentally the reason why a 4-stroke bike seems to gip better on low friction surfaces. It's not as noticeable on an enduro or motocross bike due to the higher speeds involved but on a trials machine, more torque at lower rpms means that it's theoretically less likely to break traction. This also accounts for why a 4-stroke bike tends to wear it's tyres more quickly - while the rear wheel spends less time spinning than a 2-stroke (which you'd have thought would result in less tyre wear), the comparitively high torque being delivered to the tyre knobs creates a high-pressure "grinding" effect and that's where the real wear occurs.

Torque = F x D where F is the force being applied perpendicular to the axis of the actuator (in this case, the rear sprocket) and D is the distance from that point to the axis itself (the radius of the rear sprocket). My old physics textbooks (from which I'm getting this info ) doesn't tell me how to convert HP to directional weight to perform the calculations in context Anyone help me out?

This post has been edited by neonsurge: 04 December 2004 - 07:52 PM

[BillyT]

Neonsurge you stated "more torque at lower rpms means that it's theoretically less likely to break traction."

This statement has me confused

More torque at lower RPM will have enough power to break the wheel loose very easly.

Proof in the pudding is that a little 125cc will give great traction due to the lack of torque.

Here in Canada during the winter (ice & snow) any owner of a muscle type car with a manual transmission will take off in the highest gear possible to avoid wheel slip and get greater traction (less torque in higher gears at lower RPMs).

Any good rider will take off using a higher gear in mud, again to avoid torque breaking the wheel loose.

Maybe I am not understanding what you are saying

BillyT

This post has been edited by BillyT: 05 December 2004 - 02:47 AM

[jrsunt]

so what we need is traction control linked to a rear wheel speed sensor and engine speed sensor. I'm sure honda could incorporate this into the fuel injection module.

[Dabs Alot]

Quote

The physics is quite simple of why the pulsing helps. A tyre slipping on a muddy rock has less grip than a tyre that has the tread stationary relative to the rock ie static friction is greater than sliding friction. When drive is on the verge of being lost due to low traction, the part of the engine cycle when no drive is occurring (between when the exhaust port opens and when the next power stroke starts) allows the tyre to regain the benefits of static friction in time for the next power pulse. A four stroke engine simply has a stronger version of this effect because it has a longer period between power strokes for the same RPM.

The problem with this statement seems to me that when "drive is on the verge of being lost due to low traction" you have not broken traction yet so you have no need to "regain the benefits of static friction" because you have not lost static friction.

Traction with a non-spinning tire at a certain point in time is equal to a force, coefficient of static friction, and if the bike's power delivery exceeds that force the tire slips and now you are talking about kinetic coefficient of friction which in my experience has always been less than static friction(usually resulting in me dabbing !). Then something has to change for the rear tire to regain grip, the force exerted to the tire must be reduced below the kinetic coefficient of friction(which means power delivery would have to drop below where it was before the tire slipped since the since the kinetic friction is less than the static friction) or the coefficient of kinetic friction needs to be increased, such as transfering weight to the rear wheel.

Ok, now you have just broke traction with the rear wheel, the first thing that happens is the RPM's go up, and now you have a greater speed difference between the tire and the ground to overcome and the inertia of the bike powertrain is working against that.(no wonder I fall down so much!)

So if a bike has a soft power curve it makes sense to me that you could approach that value of static coefficient of friction without exceeding it easier than with a bike with a stronger (steeper) power curve. So if everything else was equal about two bikes except two different power curves, such as a two stroke versus a four stroke power curve, the traction of the two bikes would be equal, and the rider that could most skillfully approach that coefficient of static friction without exceeding it would "find the most traction".

Make sense to anyone else????
Or have I had too much caffeine this morning????

[bigwig]

I cant beleive the tosh you guys have written. The overlap stroke of a four stroke is what produces grip. When engines of similar power and torqe are deployed in any sport, the four stroke will win hands down in the grip department. Hence Speedway ,grasstrack ,motocross, Ducati in world superbike etc. The only thing that four stokes have suffered with is a relatively poor power to weight ratio. Thihgs have realy changed in recent years with fuel injection,engine management system,multi valves,air type valve return,variable valve timing etc. The only downsides being expence and very complicated engineering both mechanical and electronic. Do we need the modern four stroke in our sport? The customers will vote with their wallets.

[BillyT]

bigwig...............and you are ??????????????

And when did you become so knowledegble and why have we not benifited from your wisdom and advice before now?

Please enligthen us more?

We write to help each other and do not jump down each others throats or belitle them by saying like "what a load of tosh you have written"

Please explain overlap of stroke to us trials neophytes here (that means begineers).

It is any open forum and we write what we want free off riducle.

Are you sure your name is not CHris by any chance


BillyT

This post has been edited by BillyT: 07 December 2004 - 05:27 AM

[Social Climber]

I think the overlap stroke in a fourstroke is the 'suck-squeeze' part of the cycle. The inertia of the flywheel will still produce forward motion to the back wheel in this phase but obviously with a lot less torque (T = Force x Leverage, Force = Mass * Acceleration)

As a result, this reduced power for half of the firing cycle will reduce the force momentarily on the back wheel and allow static grip to be regained.

To highlight this fact even more, in WSB, the reason the Ducati's had trounced the inline fours of the other manufacturers for so long is because a 90 degree twin has two pulses in 90 degrees, then 630 degrees (Remember, the firing cycle is TWO revolutions of 360 degrees) to recover grip, therefore the throttle application did not have to be so precise.

This uneven firing was used in the nineties to tame the NSR500s of Doohan Lawson and co, it is the reason why the RCV was so dominant for the first two seasons of MOTOGP. The clincher? Rossi got the Yamaha engineers to produce an uneven firing pattern crank for his M1 this year and the result? One nice shiny new world title.

The way power is measured on a Dyno means that an even firing order will always produce more power (all other things being equal of course), but even at the the most power critical end of motorsport, power isn't everything, and as you all know by now, it can be a curse in trials. So theoretically a four banger SHOULD produce more grip than a two stroke with the same torgue figures (Torque = Force * leverage for ONE revolution)