Your thoughts on roll oversteer please

[Colin Reed]

Hi John,

Rollsteer is not confined to the rear axle only .
the front axel does the same only the roll puts the axel into under steer
I will try and explain if you look at the front of the car and imagine a horizontal line going from front to back starting from were the gooseneck  bolts to the spring , if the radius  ball bolted to the cross member was mounted on that line, as the chassis rolls the axel would stay at a right angle to that line.
But the Ball is mounted about 4 inches below so that it moves in an arc so that if the chassis rolls to the right the ball moves to the Left and because the radius arms are attached they pull the axel back on the right hand side and forward on the left  putting the axel into under steer.
This is why the front suspension is tightened  to reduce roll
The only thing I would do differently is stiffen the leafs of the spring and use the dampers as dampers
but I under stand why the do it because its much quicker
I would also do the same on the rear

Colin
Tinopai NZ

[Alan]

I’m reminded of the first race I had at Donington. I was suffering horrendous wheel patter. Neville Barr, then with the Forrest Special told me after he could have overtaken at Redgate Corner but he didn’t dare come past. I eventually fixed it by carefully balancing the wheels and fitting dampers as Stuart suggests. There are actually two issues. Patter and Shimmy. Patter is up and down motion, and I think another fix for this would be to increase the torsional rigidity of the chassis, or at least move the spring frequency away from the chassis frequency. One day I’ll do some sums and work it out. Shimmy is oscillation about the steering axis and it’s as good a reason as any not to increase your track by spacing the wheels out. Again wheel balance is a big factor, but you also get it on acceleration and under braking ( at least you do in cars more powerful than a box saloon) a cause for that if variation in the torsional stiffness of the tyres. Not much you can do about that, but again anything that moves the natural frequencies involved will help.

[Tony Press]

Wheel patter, which I know as axle tramp, is not confined to Austin Sevens. Large and heavy cars are very prone to it. A friend always drove Rover 14s and 16s which suffered from this. I had a Daimler Light Straight 8 which was also prone to it and W O Bentley while working for Rolls Royce testing the new 3-1/2 litre Bentleys had an accident caused by it. On these cars it happens at around 70mph while traveling in a straight line. At a certain frequency the front axle starts to oscillate with the front wheels jumping in the air alternately. It's very scary.
The cause is believed to be lack of rigidity in the front section of the chassis. On these cars there's a big section with no cross bracing because the space is taken by engine and gearbox.
It is aggravated if the shock absorbers are weak and Daimler adopted cross-braced chassis where the bracing extends forward alongside the engine. All these cars were fitted with harmonic front bumpers consisting of a flat spring steel bar across the car with lead weights at the ends. They were tuned to the frequency at which this happened. Akin to the dampers fitted to high-tension cables on pylons.

Ah ! So we are discussing 'axle tramp' - now I understand - thank you Austin.

Patter was a term I hadn't heard in relation to front axles.

Cheers, Tony.

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Hi John,

Rollsteer is not confined to the rear axle only .
the front axel does the same only the roll puts the axel into under steer
I will try and explain if you look at the front of the car and imagine a horizontal line going from front to back starting from were the gooseneck  bolts to the spring , if the radius  ball bolted to the cross member was mounted on that line, as the chassis rolls the axel would stay at a right angle to that line.
But the Ball is mounted about 4 inches below so that it moves in an arc so that if the chassis rolls to the right the ball moves to the Left and because the radius arms are attached they pull the axel back on the right hand side and forward on the left  putting the axel into under steer.
This is why the front suspension is tightened  to reduce roll
The only thing I would do differently is stiffen the leafs of the spring and use the dampers as dampers
but I under stand why the do it because its much quicker
I would also do the same on the rear

Colin
Tinopai NZ

I assume that these front suspensions still use the transverse spring - converted from the hammock shackles to one fixed. 

This must introduce some steering effect when the spring assumes an 'S' shape under roll ?

Or am I confused again.

Cheers, Tony.

[DavidL]

"Shimmy" - don't you get rid of that by triangulating one of the shackles? (...or I've seen some with rubber wedging the shackles to the axle).  I locked the one at the steering end.  (One has to move to allow spring length change...and I didn't like the idea of the wedging and where the forces went).   

I've seen cars with no shackles and the spring resting on the axle -  extra bit of damping?

[Bob Culver]

As previously many axle location arrangements incur some degree of steer. But at the front the interection with the steering linkages primarily determines matters.
With a Seven the compromise there swamps any other effect. Normally when a car is rocked the steering wheel should not turn. On a Seven it moves through a huge angle, esp considering the ratio. Stiff suspension, as favoured on pre war cars where such errors were hard to totally avoid, restrict movement and the effects.

[Alan]

David

Well, yes, maybe, but you would do better to treat the cause not the symptoms, if you can. A favourite one in modern cars is poor wheel balance coupled with too much ground level offset in the steering. A 7 with standard wheels shouldn’t suffer too badly but add wider more offset wheels and tyres into the mix and there might be a problem.
Some cars do away with the shackles and let the spring bear against a low friction surface on the top of the axle. Years ago they used graphite blocks, today I guess PTFE would do. IncorporTed with a Panhard Rod, the idea is to decouple the axle from the spring movements. Not sure how effective it is, I’ve not driven such a car.

[Ian Williams]

With the big seven axle even with one shackle immobilised, carefully balanced wheels, stiff front spring and dampers I could not eliminate wheel patter unless I set the car up so that it understeered horrendously. With and Ulster type axle I did not have the same issues and could get superb light responsive cornering once the springs and dampers were sorted as already mentioned.

John, The front firewall bulkhead area of my car contains a series of interconnected closed boxes, these to my mind are light and stiff they tie into the floor which in turn closes the boxes formed by the chassis rails and under tray. I have limited myself by wanting to retain the appearance of something recognisably Austin, so mine not exactly monocoque but utilising an element of that thinking. Which is why the visible part of the fire wall is extremely Nippy inspired, and the overall body shape echoes both Ulster and Speedy, I tried to creatively think of it as what Austin might have done had they continued more in the Ulster vein with the LWB chassis. I will post some more pictures on the build thread rather than clog up this one.

[Ritchie Wilson]

Stuart's book "Would Suit Enthusiast" is listed on Amazon for $278. There is no author photograph on the listing however.

[Tony Press]

Stuart's book "Would Suit Enthusiast" is listed on Amazon for $278. There is no author photograph on the listing however.

Always check abebooks.com first- two copies of Would Suit Enthusiast at US$70.

Cheers, Tony.

[John Bonnett]

With the big seven axle even with one shackle immobilised, carefully balanced wheels, stiff front spring and dampers I could not eliminate wheel patter unless I set the car up so that it understeered horrendously. With and Ulster type axle I did not have the same issues and could get superb light responsive cornering once the springs and dampers were sorted as already mentioned.

John, The front firewall bulkhead area of my car contains a series of interconnected closed boxes, these to my mind are light and stiff they tie into the floor which in turn closes the boxes formed by the chassis rails and under tray. I have limited myself by wanting to retain the appearance of something recognisably Austin, so mine not exactly monocoque but utilising an element of that thinking. Which is why the visible part of the fire wall is extremely Nippy inspired, and the overall body shape echoes both Ulster and Speedy, I tried to creatively think of it as what Austin might have done had they continued more in the Ulster vein with the LWB chassis. I will post some more pictures on the build thread rather than clog up this one.

Some more detail photos would be very helpful Ian; thank you. Just a couple more questions if I may please?

Did you use for 20 gauge mild steel for the channel sections riveted to the chassis for boxing and mounts for the stressed floor?
What thickness aluminium for the bulkhead and did you weld it?

Thank you

John

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Hi John,

Rollsteer is not confined to the rear axle only .
the front axel does the same only the roll puts the axel into under steer
I will try and explain if you look at the front of the car and imagine a horizontal line going from front to back starting from were the gooseneck  bolts to the spring , if the radius  ball bolted to the cross member was mounted on that line, as the chassis rolls the axel would stay at a right angle to that line.
But the Ball is mounted about 4 inches below so that it moves in an arc so that if the chassis rolls to the right the ball moves to the Left and because the radius arms are attached they pull the axel back on the right hand side and forward on the left  putting the axel into under steer.
This is why the front suspension is tightened  to reduce roll
The only thing I would do differently is stiffen the leafs of the spring and use the dampers as dampers
but I under stand why the do it because its much quicker
I would also do the same on the rear

Colin
Tinopai NZ

Yes I can understand that Colin. A bit like a Panhard rod that moves through too big an arc. So I would imagine that it isn't possible to have a car with compliant suspension (comfortable ride) that will behave well on the road.