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propshaft induced rollover.


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Because I have been stuck at home indoors for these past couple of weeks I have been bored witless ,and some may have noticed that I have far too much time to spend on these forums. So just to bore you all a little more I would like to discuss the effects of propshaft torque on vehicle instability when climbing offcamber (drivers side down) slopes, out of holes etc. And whether or not the reduction at the wheel hubs that portal axles provide, have the effect of halving the rollover force?

For those of you that aren't familiar with what I am on about.

The vehicles engine rotates in a clockwise direction when viewed from the front. When driving in the forward gears so do the propshafts. According to Isaac Archimedes Divinci, every action has a equal and opposite reaction, which means that the vehicles body/ chassis tends to roll on its suspension in an anti clockwise direction and can cause the truck to roll over at an angle considerably lower than the trucks centre of gravity suggests that it would.

So once again,the question I pose here for discussion is ''does the extra reduction (around 2:1) at the wheel hubs provided by portals reduce the torque roll effect by half, or does it have no effect at all"?

Bill.

PS, I have also posed this question in a thread over on Pirate to get as wide a variety of knowledgable opinions as possible.

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with all other factors the same (speed, angles, acceleration, transmitted power etc) then - with portals - the shaft will be turning twice as fast and hence carrying half the torque. So half the turnover effect.

My tuppence worth.

TS

Incidentally - for the same reason my father was taught (in WWII) not to use the transmission brake on a jeep while the vehicle is moving in case it flips it over. Anyone know if there is any truth in this one?

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with all other factors the same (speed, angles, acceleration, transmitted power etc) then - with portals - the shaft will be turning twice as fast and hence carrying half the torque. So half the turnover effect.

My tuppence worth.

TS

Incidentally - for the same reason my father was taught (in WWII) not to use the transmission brake on a jeep while the vehicle is moving in case it flips it over. Anyone know if there is any truth in this one?

Would a non portal differential of the same ratio give the same effect ? or does the fact that the additional reduction is generated at 90 degrees to the driveshaft give portals an advantage here?

With transmission mounted handbrakes and open differentials there is a chance for the wheel or wheels(4x4 mode) on one side only locking up and causing the vehicle to slew sideways and flipping that may have been your fathers instructors concern.

Bill.

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It will reduce the roll torque for a given set of circumstances compared to a higher geared vehicle. Of course, the max torque output of the transmission is the same, so it can force as much roll as before if the wheels are encountering more resistance. In this circumstance, the higher C of G may increase the likelihood of a roll.

Si

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characteristics of damping more relevant? specifically insufficienct low speed rebound and compression damping, torque multiplication happening after the props.

COG change could be an interesting with portals due to increased unsprung mass, increased/decreased track width too?

why are you stuck at home anyway ? :unsure:

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It will reduce the roll torque for a given set of circumstances compared to a higher geared vehicle. Of course, the max torque output of the transmission is the same, so it can force as much roll as before if the wheels are encountering more resistance. In this circumstance, the higher C of G may increase the likelihood of a roll.

Si

Agreed.

I'm not sure its correct to assume a portalled vehicle has a higher C of G though - as Dolly says - they are generally wider (improving roll over angle, but not C of G of course), heavier (low down weight), and tend to allow the vehicle to not be so 'jacked up' on its suspension which could be a plus (vehicle dependent of course). there was an article somewhere about a portalled vs non-portalled vehicle and the rollover angles. The portalled version did better if I recall correctly.

Also, I guess for a given reaction torque you'll see less wheel lift/unload on a wider tracked vehicle, which would also point generally to the portalled vehicle being more stable?

Al.

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why are you stuck at home anyway ? :unsure:

Looking after SWMBO while she recovers from illness.

Al, I have reservations about that example from Russia I think it was, of two almost identical Defenders, one on conventional axles, the other on Volvo's, claiming the Volvo axled one had a higher tipover angle than the other. Stock Volvo axles are only 2'' wider than defender ones and the Volvo rig was easily 4'' higher. It doesn't add up.

Bill.

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Well, it might not be big differences, but I bet the combined effects of more weight lower down, not much increase in height and a small width increase could justify the results?

I guess it depends entirely on the two being compared. But my instinct would certainly not be to always pick the 'regular' axled truck as more stable.

Al.

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I’m trying to sort out the excessive amount of body roll at the moment on my truck, I’m not sure portals will help to improve the situation by having lower overall gear ratio's, the same amount of energy is still transfered from the engine to the axles. The torque applied through the prop is reduced, but the propshaft and gearbox has a higher rotation speed. I think the answer to help reduce engine, gearbox and prop induced body roll is getting the suspension setup correctly. Portal axles do increase the center of gravity of the overall vehicle if the track width is not changed, but they do not increase the amount of body roll.

At the weekend my trucks suspension setup was not working well on side slopes, I would not put it down to the valving of the air shocks, I think its down to the setup of the progressive rate air spring and the way shock is positioned on the axle relitive to there position on the chassis. I’m going to move the lower shock mounts on the rear closer to the hubs on the rear axle and fit a discovery anti roll bar to the front I’m just hoping that will work!

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Please humor me if I am wrong.

The unsprung weight can make the truck become a hand full to control at speed as the momentum must affect the COG compared with a smaller weaker axle eg keeping the axles on the floor and not bouncing.

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Sorry jules - a bit lost on what you're saying?

Momentum and CoG are unrelated.

The CoG is basically fixed if you ignore movement of components like suspension / axles etc and changeable loads such as fuel / people / tools etc.

You are right that for stability at speed the unsprung weight being low can be a bonus, as I'm sure you know (so portals = bad :( ), but whats the bit about momentum? You mean axle momentum during suspension flex, or entire vehicle momentum?

Al.

:)

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depends on the suspension Jules, agreed the unsprung/sprung ratio is wierd and by any conventional wisdom its totally unworkable but In reality with a decent suspension setup a portal car is just as fast as anything else - its even possible they may be faster as big tyres soak up knocks and the worry of things snapping isnt there. It might sound like pony but 90% of the top two classes are on portals and on the fast fire breaks a landrover would be dead meat chasing them

I'd like to reach that stage (having been left for dust by other protos with mega buck suspension)... hey ho... this year we have something up our sleeves <_<

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Bill, not thought of this before but now I have it occurs to me that a secondary question is what impact the offset of the diff in the axle has? Thinking of the torque reaction there may be good reason for LR (and I guess other marques) placing the diffs to the right of centre. This creates a long lever to resist the torque and hence generates much less lifting force on the vehicle. Flipping the axles, (Saley?) may then prove detrimental to roll resistance under power.

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Trev - I'm afraid not. A torque is a torque, no matter where along the axle it is applied. The lower the gearing, the less this torque is (for a given power transfer).

Sure? If you consider the interface between prop shaft and axle at one end the torque is being resisted by engine/gearbox mounts while the axle is stopped from spinning round by the ground on one side and spring/axle weight at the other end. On a rover it is the long side of the axle that will be countering the torque (forxe x distance) and that longer distance means less force is required where the wheel makes contact with the ground than if we reversed this and used a short tube. Now every force having to be balance means this lower wheel/ground force will require less upforce of the axle (with less rise in cog,...).

Of course I could be all wrong or it could be a well understood (but not by me) principle of transmission design.

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Sorry jules - a bit lost on what you're saying?

Momentum and CoG are unrelated.

The CoG is basically fixed if you ignore movement of components like suspension / axles etc and changeable loads such as fuel / people / tools etc.

You are right that for stability at speed the unsprung weight being low can be a bonus, as I'm sure you know (so portals = bad :( ), but whats the bit about momentum? You mean axle momentum during suspension flex, or entire vehicle momentum?

Al.

:)

Sorry due to my dislxyer I struggle to write things on email or forum the way I actualy mean them or intend them to be understood.

(which is the main reason people get the ars with me)

Ok not to worry I can't be arsed to re-write it. :)

Axle coming up and body unable to push it back down.

When we were resurching the Axles for the Tomcat the Nissam and Toyota were scrapped as there far to heavy for racing and the weakness of the Rover makes up for the loss of speed due to not being able to put the power down on a convertional drive train.

EG some people on here run the likes of Fox but in racing thats a entry level shock like say a Pro comp is to challanging.

The last comp truck (very light weight RRC) with run of the mill twin Bilstines on each coner will become completly inactive and uncontrolable due to over heating within two miles of racing I can't comprahend how the Portal trucks could deal with that kind of punishment looking at the minimal shock set ups they run from the limited photos of them I have seen.

Or is the punishment not speed related just the mental tarain the big trucks contend with

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every force having to be balance means this lower wheel/ground force will require less upforce of the axle

Ah, that's where the story falls down. The torque is applied to the axle at the diff, no question. One long tube means the force at that wheel due to the torque will be less, yes, but it's not the force that is balanced, it's the torque. There's no requirement to have the same force at each wheel and it's the effect of more vertical force at one wheel than the other which causes dragsters to spin one wheel off the line.

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