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Post Info TOPIC: Understeer & Oversteer
Collyn Rivers


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Understeer & Oversteer
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Many RV-related forums have posts that indicate many members misunderstand understeer and oversteer (often confusing one for the other). Understeer is desirable for road vehicles. Oversteer generally is not. It (particularly) must be avoided whilst towing.

Understeer

A vehicle that understeers will, if cornered too fast, adopt a slightly wider radius, thereby (automatically) reducing the cornering force. The effect is minor and the driver is normally unaware that this is happening. It is a desirable and stable condition. Understeer also ensures that vehicles will (unless steered) be unaffected by a cambered road, and barely affected by wind gusts. All motor vehicles (excepting race cars and some rally cars) have inbuilt understeer. Some rally cars have this adjustable.

The degree of understeer is obtained by ensuring the front tyre's slip angles are (under all designed conditions of loading) slightly greater than its rear tyres. It must not be too much - or the vehicle is then unable to make an emergency swerve at speed. There will, however, be an understeer margin so that oversteer is avoided (unless grossly overladen at its rear) - or its rear tyre pressure too low (and/or front too high).  

Oversteer

This is (as the name suggests) is the opposite of understeer. In essence, if cornering too fast the vehicle will actually tighten the radius of that turn. Doing so tightens that radius yet further. Unless controlled by the driver (steering so as to take up a wider radius turn) it will continue that cycle - and spin. Many (most) early sports cars had minor oversteer. This was safer back then as the cross-ply tyres used back then had a far wider range of predictable behaviour.

An oversteering vehicle (or one with a reduced margin of understeer will (misleadingly) feel 'more responsive'. That may seriously mislead caravanners.

That understeer/oversteer be understood, or at least accepted, is vital for safe caravan towing. An oversteering car in the hands of an experienced driver can usually be controlled. An oversteering caravan rig requires considerable expertise and experience - and often cannot.

I will expand on this in a week or so time - but happy to respond sooner to any queries.

 

Collyn

 

 

 

 



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Radar


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Hi Collyn

I am interested in reading your post but would like it written a little bigger please. Ralph.



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PeterD


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Hold down the Control key on your computer, tap the + key a few times and the text will expand.

If you are using Firefox as your browser (and if not download it and try it) tap the F9 control key and the text you are viewing will toggle to a better reader view.



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Radar


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PeterD wrote:

Hold down the Control key on your computer, tap the + key a few times and the text will expand.

If you are using Firefox as your browser (and if not download it and try it) tap the F9 control key and the text you are viewing will toggle to a better reader view.

 Hi PeterD

I am reading it on a Samsung tablet and find it a bit harder to just expand.

Will look for another Avenue to follow his longer post which has some good read in it. 



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Mike Harding


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Radar: it is not the original poster who determines the text size with HTML but rather the device/software upon which you are reading it.

Personally I consider HTML to be the work of the devil but I seem to be outnumbered by his demons....



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Aus-Kiwi


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Under steer frightens the driver . Oversteer frightens the passengers ! Under steer is when front wheels loose grip . Often more so on front wheel drive cars . Rear wheel drive can be balanced using power to promote a little oversteer . Not that you would drive to these limits .Especially towing . To reduce either ? Softening sway bar gives grip . But theres equal and opposite reactions when it comes to suspension tuning.,

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Roy E


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Colin,
Looking forward to your continued article.
For us caravanners, hooking a caravan onto any car changes the balance and dynamics of what was (hopefully) a perfectly behaved road car.
I believe that some time ago, the Americans (Bless them), determined that use of WDH system to fully restore front/rear axle balance can seriously raise oversteer problems and that only around 50% of rear sag/front height increase should be corrected using WDH, so to decrease chances of oversteer and possible subsequent jack-knifing. Any comment on this or do we have to wait a week?
Cheers,
Roy.

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Collyn Rivers


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Roy

 

 I will respond to your post later today. 

Re font size. How is this now?

(I have also changed the font to digital-friendly Georgia.)

Collyn

 



-- Edited by Collyn Rivers on Wednesday 5th of December 2018 02:29:48 PM

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montie


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Hi Collyn,

Good to see you up and about again!

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Collyn Rivers


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Date: 09:41 PM Dec 4, 2018
 
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Re: "Under steer frightens the driver. Oversteer frightens the passengers ! Under steer is when front wheels loose grip".
 
 
The first two comments are about right!
That understeer (or oversteer) is when wheels (front or rear) lose grip, however, is only the ultimate situation and with radial ply tyres, sudden and violent.
 
 
 
That of interest (and under our control) is the considerable range between tyres that are being steered or subjected to side forces (as from a yawing caravan). 
 
 
In essence, the tyre's footprint on the ground has both a frictional and molecular bond to any firm surface that it roll over. When steered (or as with rear tyres) that force is caused, via the wheel's rims, to distort that footprint in a more or less diagonal manner. As a result the steered wheel points in one direction - but the vehicle adopts a radius that is always less. This angle is (misleadingly) called the 'slip angle'. 
 
 
Understeer is thus the condition where the front slip angle exceeds the rear.
 
 
The magnitude of a slip is related to tyre wall stiffness, tyre pressure - and imposed load. Increasing tyre pressure decreases slip angle.
 
 
That vital to understand (or at least accept) is a side force (such as caravan yaw) imposed on the tow vehicle's rear tyre will generate a slip angle. The tow vehicle is then literally steered by its rear tyres. Read that a few times and it may be clear why it essential to increase tow vehicle rear tyre pressures when towing.
 
 
Roy
That a WDH introduces unexpected issues was originally noted by Richard Klein, Donald Johnston and Henry Szostak in 1978. The two main ones are SAE 780012 'Effects of Trailer Hookup Practices on Passenger Car Handling and Braking, and in 1981, 'Development of Maximum Allowable Hitch Load Boundaries for Trailer Towing'.
 
 
Richard's work strongly influenced the SAE J2807 Recommendations re FALR (Front Axle Load Restation). Cequent (the world's largest WDH maker) now recommends that a WDH only correct 50% FALR.   
 
 
Collyn
 
 
 
 



-- Edited by Collyn Rivers on Wednesday 5th of December 2018 02:24:38 PM



-- Edited by Collyn Rivers on Wednesday 5th of December 2018 02:25:37 PM

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Collyn Rivers


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Thank you Monty.

I sort of migrated . . ..

 

Collyn



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rockylizard


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Gday...

I am not sure how you created that post Collyn but, on my pooter, it was unreadable as it did not 'wrap' to allow all the text to be viewed.

I have taken the liberty to copy your post/text and insert it here -

 

Re: "Under steer frightens the driver. Oversteer frightens the passengers ! Under steer is when front wheels loose grip". 

The first two comments are about right!

That understeer (or oversteer) is when wheels (front or rear) lose grip, however, is only the ultimate situation and with radial ply tyres, sudden and violent.

That of interest (and under our control) is the considerable range between tyres that are being steered or subjected to side forces (as from a yawing caravan).

In essence, the tyre's footprint on the ground has both a frictional and molecular bond to any firm surface that it roll over. When steered (or as with rear tyres) that force is caused, via the wheel's rims, to distort that footprint in a more or less diagonal manner. As a result the steered wheel points in one direction - but the vehicle adopts a radius that is always less. This angle is (misleadingly) called the 'slip angle'.

Understeer is thus the condition where the front slip angle exceeds the rear.

The magnitude of a slip is related to tyre wall stiffness, tyre pressure - and imposed load. Increasing tyre pressure decreases slip angle.

That vital to understand (or at least accept) is a side force (such as caravan yaw) imposed on the tow vehicle's rear tyre will generate a slip angle. The tow vehicle is then literally steered by its rear tyres. Read that a few times and it may be clear why it essential to increase tow vehicle rear tyre pressures when towing.

 

Roy

That a WDH introduces unexpected issues was originally noted by Richard Klein, Donald Johnston and Henry Szostak in 1978. The two main ones are SAE 780012 'Effects of Trailer Hookup Practices on Passenger Car Handling and Braking, and in 1981, 'Development of Maximum Allowable Hitch Load Boundaries for Trailer Towing'.

Richard's work strongly influenced the SAE J2807 Recommendations re FALR (Front Axle Load Restoration). Cequent (the world's largest WDH maker) now recommends that a WDH only correct 50% FALR.  

Collyn

Cheers - and welcome - John 

 

 

 

 



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Aus-Kiwi


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Yes the better the tyres, handling vehicle . The more severe when you over do it . Drive a 70s car then jump into late model of similar brand . The latter models do turn and go a heap faster . Thing is young drivers dont know seat of the pants driving . The grip limits are much higher !

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Radar


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Collyn Rivers wrote:

Roy

 

 I will respond to your post later today. 

Re font size. How is this now?

(I have also changed the font to digital-friendly Georgia.)

Collyn

 



-- Edited by Collyn Rivers on Wednesday 5th of December 2018 02:29:48 PM

 That is good. Thank you



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Collyn Rivers


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Thank you John

I have no idea what went wrong but it was odd at my end too.

 Collyn



-- Edited by Collyn Rivers on Wednesday 5th of December 2018 06:15:15 PM

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Collyn Rivers


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Aus-Kiwi wrote:

Yes the better the tyres, handling vehicle . The more severe when you over do it . Drive a 70s car then jump into late model of similar brand . The latter models do turn and go a heap faster . Thing is young drivers dont know seat of the pants driving . The grip limits are much higher !

 

The limits are now indeed much higher, and when overdone there's a lot of momentum there when it finally 'lets go'.  

As I noted before much of the misunderstandings re vehicle handling is through people assuming a pneumatic tyre behaves like a solid rubber tyre. It does not. The best way see them is as spheroid-shaped balloons that are free to roll -and steered by pressing the sides together and then twisting. As will be seen the section on the ground distorts diagonally - but it only slides if turned very strongly.

Collyn    



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rockylizard


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Gday...

Like this -

Tyre distortion 01.jpg

http://what-when-how.com/automobile/vehicle-steady-state-directional-stability-automobile/

Cheers - John



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T1 Terry


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Hi Collyn, a serious topic that effects a rigid vehicle like a motorhome virtually the opposite to the way it affects a vehicle towing a caravan and about half way in between is the effect on a fifth wheeler where to tow hitch is over the rear axle rather than behind it as in a caravan on a towbar hitch.
An old motor racing explanation of the two steering problem, under steer is where the front corner hits the fence, over steer is where the back corner hits the fence. When the front, rear of side of the vehicle hits the fence it is known as lost control :lol:

T1 Terry

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iana


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This is interesting, because previously I did not feel a semi passing me, now I feel a slight "suck in" at the rear of the van when a semi starts to pass, the tow vehicle also seems more sensitive on the road. The only change being a new set of light truck tires on the ute.

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vbc1_75


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The other parts of the understeer/oversteer equation - Horsepower is how fast you hit the wall, and Torque is how far the wall moves when you hit it.

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Collyn Rivers


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What is the tow ball mass - and what tyres pressures are you now using?

(Am away from my computer until Monday).

Collyn 



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