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Solid rear axle steering


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Hello All,

Last week I was fitting an electronic sensor to one of my back wheels and I noticed that I could move the whole rear axle slightly if I grasped the brake backplate and twisted it. Only a few degrees either way, but the axle was definitely moving.
I checked the U bolts that attached the spring to the axle on that side and discovered that one was not fully tight. Not exactly loose, but I could do the nuts up a couple of turns.

“Good” I thought. “It’s nice to find a positive answer to something.”

I have to mention that I had never had any noticeable problems driving the car over 500 miles with the axle like this, (although I had heard the occasional clonk)  but it was obviously wrong.


The next day I drove to the Worcester car show and was baffled that the car would lurch (Slightly) to the left when I lifted off the accelerator and the same to the right when I accelerated. It was slight but it was noticeable, and the car seemed not a stable as it had been.

On arriving home I checked the U bolts on the other side and, again, one of the U bolts was loose. This time not even finger tight. I tightened it up as it should have been, did a test-drive and all seemed perfect. In fact the car seemed more stable than it had ever been. (Even though I thought it was fine before; maybe it wasn’t.)

My conclusion was that when the axle was loose both sides all was well. Under lifting off and acceleration the axle must have moved back and forth evenly both sides. When I tightened up the one side it left the other to move back and forth a bit, and so give a “Rear wheel steering” effect.

It made me wonder how accurate the wheelbase was each side on our cars. After years of use I would guess that the back axle was no longer exactly square on to the body, and would cause (even slight) drift of the car one way or the other.

Any opinions, or have I got it totally wrong?

Charlie.

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21 minutes ago, Charlie D said:

Hello All,

Last week I was fitting an electronic sensor to one of my back wheels and I noticed that I could move the whole rear axle slightly if I grasped the brake backplate and twisted it. Only a few degrees either way, but the axle was definitely moving.
I checked the U bolts that attached the spring to the axle on that side and discovered that one was not fully tight. Not exactly loose, but I could do the nuts up a couple of turns.

“Good” I thought. “It’s nice to find a positive answer to something.”

I have to mention that I had never had any noticeable problems driving the car over 500 miles with the axle like this, (although I had heard the occasional clonk)  but it was obviously wrong.


The next day I drove to the Worcester car show and was baffled that the car would lurch (Slightly) to the left when I lifted off the accelerator and the same to the right when I accelerated. It was slight but it was noticeable, and the car seemed not a stable as it had been.

On arriving home I checked the U bolts on the other side and, again, one of the U bolts was loose. This time not even finger tight. I tightened it up as it should have been, did a test-drive and all seemed perfect. In fact the car seemed more stable than it had ever been. (Even though I thought it was fine before; maybe it wasn’t.)

My conclusion was that when the axle was loose both sides all was well. Under lifting off and acceleration the axle must have moved back and forth evenly both sides. When I tightened up the one side it left the other to move back and forth a bit, and so give a “Rear wheel steering” effect.

It made me wonder how accurate the wheelbase was each side on our cars. After years of use I would guess that the back axle was no longer exactly square on to the body, and would cause (even slight) drift of the car one way or the other.

Any opinions, or have I got it totally wrong?

Charlie.

Sounds right. What you had was rear axle steer. On backing off your axle was probably further forward on the left side and Vise versa under load. It was most likely evident on hard cornering too but you may not have consciously noticed it. 

I don’t know whether TR race cars are permitted to use 3 or 4 link suspension at the back with coil overs,  or standard leaf spring, ( M/Mick would know) but you can actually set forward facing link bar angles high or low etc to induce oversteer or understeer in cornering as a tweak on the track also, and your experience appears to prove it works !

Kevin 

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I would be inclined to look at the mounting plates under the axle where the locating pin on the spring sits as if its been loose on the U bolts for a while it will probably have ovalled the holes. New U bolts are in order too as they do stretch. then measure the wheel base up.

Stuart.

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14 hours ago, boxofbits said:

Sounds right. What you had was rear axle steer. On backing off your axle was probably further forward on the left side and Vise versa under load. It was most likely evident on hard cornering too but you may not have consciously noticed it. 

I don’t know whether TR race cars are permitted to use 3 or 4 link suspension at the back with coil overs,  or standard leaf spring, ( M/Mick would know) but you can actually set forward facing link bar angles high or low etc to induce oversteer or understeer in cornering as a tweak on the track also, and your experience appears to prove it works !

Kevin 

I have never understood how inclining the rear springs up or down affects the steering. It obviously does, it is mentioned in a lot of the period road tests of the TR2, but a spring is just a spring, and an axle just basically a solid bar, and how these items can alter steering input escapes me.

Ralph.

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Kevin,

Thanks for the reply and confirming what I thought.

Stuart,

Thanks for the advice. I will fit new U-bolts (Cheap easy job) and if the problem comes back I’ll look at the plates under the axle. I noted recently that Bob (Lebro) mentioned that he welded new plates onto his axle. Hope I don’t need to though.

Ralph,

It’s not that the axle is going up and down, it’s the fact that it is twisting.
Imagine it is fixed solidly to one spring, but not fixed at all to the other.
The non fixed wheel could then go forwards or backwards relative to the fixed wheel.
The car would then “Self steer” in the direction that the wheel was pointing.


Charlie.

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The take up of loosened or worn components can give a  pronounced "torque steering effect" upon cars.

A good example of this is a Dolomite or TR7 which has short angled radius arms affixed to the top of the back (live) axle. The rods are a different length to the parallel rear pointing trailing arms which carry the axle and so soft or disintegrating bushes in these Radius arms combined with similar disintegrating bushes in the trailing arms ,carry a disproportionate affect upon the torque transmitted through the rear Trailing arms, there being no location by leaf springs on these models having a rear coil spring suspension. 

The shortening or lengthening affect of either end of the live axle gives an acceleration torque steer to the drivers side and under braking a brake steer to the passenger side as the axle is allowed to twist around the diff so shortening the wheelbase first at one side of the car and then the other.

As for springs...they are just not springs when they are leaf springs, they also carry a proportion of the axle location being a fixed length with the axle afixed broadly in the centre of them, but react to the twisting motion passed into them by torque (either acceleration or braking) of the axle tube so adopting an S type shape (geometrically) and allowing the wheel base to shorten or lengthen on different sides of the car. This causing different torque steer affects.

When racing these oddball affects are sort to be reduced or removed  entirely by mechanical means, spherical bearings in place of rubber bushes or at the least hardened rubber bushes with end washers (on leaf springs) just allowing the spring to rotate on them but preventing across spring movement and angular application of the spring movement. Other methods of locating the axles end are of course radius rods affixed as far across the axle as can be made and to limit across car movement a Panhard rod attached to chassis one end and the diff the other, or an A frame (my choice on the TR 7V8) which controls not only across car axle movement but through a spherical bearing mounted on the diff underneath allows full articulation and swing throughout the axles movement range. Whether any or all of these are able to be employed in a race series is up to the series organisers which should be complied with.

Mick Richards

Edited by Motorsport Mickey
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Charlie - is your car a TR4 with the dished rear springs and the axle sitting on top of alloy blocks?  This has always struck me as a most unsatisfactory way of mounting the rear axle as it doesn't provide the really positive method of locating the rear axle which adopted on TR2/3/3A and many TR4s, where the axle is clamped directly to the springs on both sides (i.e. no alloy blocks).

Ian Cornish

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21 hours ago, ianc said:

...is your car a TR4 with the dished rear springs and the axle sitting on top of alloy blocks?...

Ian,

The car is a 3A, although I do have a 4 back axle.

I think my problem was simply because of the loose U bolts.

 

Interestingly I was looking at the parts book and wondered why the 4 had those ally blocks you mentioned, seeing as I didn’t.

 

Charlie.

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On 9/16/2020 at 6:54 AM, Ralph Whitaker said:

I have never understood how inclining the rear springs up or down affects the steering. It obviously does, it is mentioned in a lot of the period road tests of the TR2, but a spring is just a spring, and an axle just basically a solid bar, and how these items can alter steering input escapes me.

Ralph.

The moving of either of the ends of the leaf spring changes the application of the suspension movement ie if the spring is perfectly level when mounted and the axle is mounted centrally with a symmetrically built spring, by dropping or lifting one end of the other the suspension movement will either be neutral or have bump understeer (which most manufactures seek to have, it's safer). Hence some models of the TR4 with a commission number of 22000 onwards (from memory), have the alloy spacer between axle and spring which solves the problem of how to change the spring orientation and reduce bump oversteer from the ride characteristics.

Having a roadgoing TR4 with these alloy spacers I've found the ride and characteristics on the road to be very acceptable, yet when racing with a different TR4 I used the negative rated flat springs and enjoyed excellent handling but obviously under different application and circumstances. The fact that both systems can work well makes me wonder how aggressively were these roadgoing TR4s with the flat springs being driven that necessitated the alloy block addition !

Mick Richards

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9 hours ago, Motorsport Mickey said:

The moving of either of the ends of the leaf spring changes the application of the suspension movement ie if the spring is perfectly level when mounted and the axle is mounted centrally with a symmetrically built spring, by dropping or lifting one end of the other the suspension movement will either be neutral or have bump understeer (which most manufactures seek to have, it's safer). Hence some models of the TR4 with a commission number of 22000 onwards (from memory), have the alloy spacer between axle and spring which solves the problem of how to change the spring orientation and reduce bump oversteer from the ride characteristics.

Having a roadgoing TR4 with these alloy spacers I've found the ride and characteristics on the road to be very acceptable, yet when racing with a different TR4 I used the negative rated flat springs and enjoyed excellent handling but obviously under different application and circumstances. The fact that both systems can work well makes me wonder how aggressively were these roadgoing TR4s with the flat springs being driven that necessitated the alloy block addition !

Mick Richards

So if the car leans into a corner, the weight comes off the inside spring and goes on the outside spring, and the geometry alters causing rear wheel steer which promotes understeer at the front. Have I grasped it correctly?, and altering the way the spring is mounted this effect can be lessened or increased, is that right?

Sorry, but I have been struggling to understand this for a while, but think I now have a glimmer of understanding.

Ralph.

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Half way there there Ralph, having a car that bump oversteers can cause grievous accidents. Hitting a bump especially with one wheel first, causing the cars rear end to "step out" means opposite lock has to be applied which takes the car neatly into oncoming traffic on left hand bends or into immovable objects, road lights, bus shelters, trees etc on right hand bends.

Manufacturers like understeer and most will develop vehicles so that however ham handed you are with the throttle the overlying response is understeer, too fast into the corner,...understeer which will scrub off the excess speed until running wide on the corner the regained tyre friction grips and eventually turns you into the corner...safe. The compromise between the front suspension movements understeer and oversteer and the rear suspension whether understeer or oversteer gives the overall handling package of the car.   

I've found the best way to get an idea of suspension behaviour and how to make it work for you is to read a book by Fred Puhn called "How to make your car handle" ISBN 978-0-91265-646-5 

It's got references to American muscle cars and Japanese saloons in it but the methods used remain true in their scientific explanations and it's what I used to" make my car handle"

Mick Richards

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As Mick has said previously any rear axle steer is going to be an undesirable effect for road or most circuit racing.

Personally, we used it sparingly or perhaps not at all on the oval Hot Rod circuits as we were on a clockwise track. If the left rear link bar was set say two/three degrees upwards at the front and the right horizontal  ( 3rd link was off top of diff) body roll would flatten the left link and push the left side of the axle backwards, encouraging the car to turn in as it were. It did work, but the Avon tyre specified for use was not that good and that was partly the reason for using it, but I can’t think of any other particular reason. It’s really all about employing basic geometry to the best advantage.

Kevin

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Perhaps a really silly question, but I will ask it anyway,  If the rear spring had the fixed end at the rear on the chassis and the swinging connection at the front what would happen?

I know rear springs since the very beginning have never been mounted like that, perhaps for a very special reason, I have seen some of the 4wd offroaders have the fixed point at the rear of the front spring, presumably it was to stop the axle moving about a bit and upsetting the steering perhaps?

I suppose it depends how much camber there is in the spring, to stop it steering the axle.

Just an interesting thought, please ignore if silly.

John

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