Ackerman?

[capt jack]

I've noticed in this very cold weather that on full lock and moving slowly, the front wheels of the S90 will appear to "skip". It's most noticeable on wet / icy / loose road surfaces.

I Googled this and it seems as though it's a feature of RWD cars particularly, especially those with larger wheels, really good turning circles and older and part-worn tyres.

All of those things are true of my S90 - big wheels, a turning circle to match a London taxi, and tyres that are 4 or 5 years old, and about half-worn.

Apparently it's all to do with something called Ackerman.

It does look as though it's just a quirk of the engineering of big RWD cars, and nothing to worry about. But I'd be interested to know if anyone else has had similar experiences, and especially if ignoring it actually isn't the right option!

The car has within the past few hundred miles had two MOTs at two different garages - failing at one on the handbrake, and, after new handbrake shoes, passing at the other with no advisories. I mention this because I'd have thought that were there any major issues with the front suspension at least one MOT tester if not both would have spotted it.

The car otherwise drives perfectly. The steering wheel is perfectly central, the car does not wander on the road at all, and the brakes pull up in a straight line. The tyre pressures are all correct.

Thoughts and advice as ever are most welcome.

Cheers

Jack

First posted in For Sale section - and now moved to the right place!

[360beast]

I'd look for worn bushes on the front end, my 940 did it really bad when I got it. I polybushed the front end and it almost eliminated it, but it does do it still occasionally but it is worse in the cold.

Rubber will go soft and not look worn out so if they only did a visual inspection and didn't pry on anything they won't spot a soft bush.

I had a nearly new Rolls Royce come in that was scrubbing tyres out, pulling to the left and wouldn't wheel align properly, the car was a year old with 3k miles on the clock. Rolls Royce said they couldn't find anything wrong with it, I found a lower wishbone bush was soft from the factory, after changing it I aligned it fine and it drove spot on.

[griston64]

I've noticed my 940 doing it a bit more lately. I replaced all my front bushes ( superflex spaceship and gen Volvo rubber ) and suspension so it can't be worn bushes

[Forrest]

My experience across four RWD Volvos is that this is normal but can be worse if the steering geometry is a bit out and/or the car is fitted with larger, wider wheels/tyres.

Ackerman steering essentially means that the inside wheel on a turn will be steered to a greater angle than the outer one thus allowing the inside wheel to steer a tighter radius. This is rarely implemented perfectly and camber and toe-in also play a part.

By the time your car has done a couple of hundred thousand miles over Britain’s third-world road network and had the wheel alignment adjusted a few times by the trainee at a national tyre chain it’s anyone’s guess what direction your wheels will point at any given time.

[GingerMagic]

I'm following this thread with interest, as my car would only skip on full lock and reversing, but now it's does it on say 3/4 lock and going in either direction....

I was planning to get the front bushes done anyway ( sadly a job a bit beyond my confidence level ) so am I better off getting the whole lot done or just the usual suspects?

Not sure if I can afford to polybush them all to be honest, but I know they last longer so probably cheaper in the long run.

Kelv

[360beast]

I've noticed my 940 doing it a bit more lately. I replaced all my front bushes ( superflex spaceship and gen Volvo rubber ) and suspension so it can't be worn bushes

That'll be the cold weather Mark, mine barely does it in summer, I'd even be tempted to say it doesn't do it at all in the warm weather but now it is very cold and the polybushes are solid at the best of times

[griston64]

That'll be the cold weather Mark, mine barely does it in summer, I'd even be tempted to say it doesn't do it at all in the warm weather but now it is very cold and the polybushes are solid at the best of times

I've not noticed in summer and it has been pretty cold lately

[griston64]

I'm following this thread with interest, as my car would only skip on full lock and reversing, but now it's does it on say 3/4 lock and going in either direction....

I was planning to get the front bushes done anyway ( sadly a job a bit beyond my confidence level ) so am I better off getting the whole lot done or just the usual suspects?

Not sure if I can afford to polybush them all to be honest, but I know they last longer so probably cheaper in the long run.

Kelv

poly the spaceships as they will be knackered. Easy as pie job. I replaced control arm and strut rods with new Volvo rubber ones but after 124k they actually looked not too bad.

[capt jack]

Thanks all.

The front wishbones and bushes on my car are fairly new, as are the ARB bushes and the drop links. And this skipping is only apparent when it's really cold and the road surface is either wet or icy, or it's a painted surface - in a car park for example, or a gravel drive.

I read that Merc RWD cars do this all the time, but if you've ever watched an older E-series turn you'll appreciate just how complex is their steering geometry. And I've web stories that RWD Lexus cars will do it too.

I've had RWD Volvos before - 240s, 740s and 940s, but a long time ago, and I don't recall this being apparent with any of those cars

Interesting that the wheel size might have a bearing. The wheels on this S90 are 16" with 205/55 tyres, and I'd be pretty sure that on the older cars the wheels were neither this big diameter, nor so wide, so maybe that's a factor. When I think back to the 240s in particular the tyres were very skinny by comparison.

Well hopefully the mercury will stay above the red line for a bit, so we'll see if some warmer conditions affect things at all.

Interesting comments - thanks to everyone who's posted.

Cheers

Jack

[classicswede]

240's can do it when bushes are worn and/or alignment is out.

7/940 are most prone with worn TCA bushes being a common issue making it worse

The other thing that can happen is teh judder is only on turning one way and that can be where the rack has not be set centred so turns more one way than the other

[J liddy]

940 tic auto 1995

Hi my car has been skip effects on turning recently noticed it Saturday but it been doing it few times recently didn't think that much about it till I read this post all the front end bushes have been renewed just over 12 months ago

[Laird Scooby]

Once worn bushes etc have been eliminated as the possible cause, get the wheel alignment checked and corrected, ensuring the rack is centralised before adjusting anything - this may necessitate removing and refitting the steering wheel so it is replaced in the straight ahead position when the rack is central.

The alignment is slightly out on mine at present but mainly putting the rack off-centre as it tracks true when the steering wheel is held so the wheels aim straight. With that in mind, it doesn't skip but is inconvenient because of the self-centreing on the rack.

I'm not sure but seem to recall i read somewhere the S90 had rear wheel alignment adjustment, if so that needs to be checked as well as the front and there should be a different alignment figure for 16" wheels from the 15" figure. This takes into account the Ackerman Principle and larger tyres. :thumbs_up:

[Rversteeg]

Skidding....

The geometry indeed follows the Aclerman principle as in attached drawing. But this is an ideal situation. The dotted lines need to intersect in the middle of the rear axle, not only when viewed from above, but also when viewed from the side. This is only possible on eg a forklift truck without suspension, but not on a normal car.
More important the connecting rod between the steering arms on a Volvo is not a single piece rod, but a steering rack with tie rods at both ends, with the steering rack is fixed to the vehicle.
Another factor is the front wheel struts leaning backwards, so when cornering the wheels will also fall a bit sideways.
This all causes the actual geometry on any car not to follow the Ackermann principle exactly. The deviation gets noticable on larger steering angles. The wheels no longer follow the perfect line, so one of them has to skid sideways. Which wheel will skid will depend on the vehicle going forward or backward and which wheel has the least grip. For the same reason you will probably find the turning circle to be smaller when going backwards.
Skidding at larger steering angles is therefore quite normal and unavoidable.

[capt jack]

Thanks Rversteeg for that explanation - a lot easier to follow than the Wikipedia page!

I've been out in the car this evening - ambient temperature well into double figures, unlike the past week where it's been sub-zero day and night.

Turing out of my drive, and then pulling in and out of a parking space at the local chip shop, and not a skip to be heard.

Very interesting thread - thanks to all who've posted comments!

Cheers

Jack

[stephend]

Mine's also skipping badly on full lock in reverse. I don't know (yet) if it's cold-related, but it was certainly brass monkeys out when it was doing it.

All the visible bushes on mine look dreadful - but not a murmur at MoT, so I guess they must still be free of excessive play, by some miracle. And it's not like my tester doesn't check, I got minor advisories for play in bushes on both of my other cars.