Technical toying with the idea of changing the castor setting

[dumbledore]

Have been trying for ever to sort out the vibration at speeds above 60 to 75 and never managed to get rid of it. I have tried different wheels alround, tyres, rebalancing etc. no really satisfactory results.
Now I just thought I should try to alter the casrtor setting on my car. The car is manual steering and the top of the strut is set for it. I am tempted to set the top of the strut in the power steering position. This should change the caster angle. I am not sure how but I think it should reduce the angle. I am hoping that the reduce castor may reduce the vibration. What do you think? Is there a chance that this might work? It looks like it is easy to do so is wotth a try,

 

[blu73]

i'd be looking at condition of suspension bushes, swing arm bushes and bushes in general first

 

[dumbledore]

Got a small success here over the weekend. Tried to see if I can do the mode but relalise that I cannot do it unless I remove the strut so given up for the time being. Done the brake pad instead as they look well down on one side. Then as the wheel that I recently brought forward from the rear to match a new tire I just bought in the front the other side was up I checked to see how the condition was. Started the engine and let the tire spin in first gear. And yes it did not look good. The wheel looked bucked. I decided to replace it with a spare wheel that is in better shape. Today on the motorway speeds the vibration are considerable down. The car is almost OK. I still believe that excessive castor angle will amplify the effects of a bad wheel. So reducing it might help. It will from what I read make the steering lighter too, but will affect straight line stability and worsen cornering. So will be reluctant to do it as yet anyway.
I will check the bushing next time I am at it.

 

[fingers99]

It'll increase the castor and make the steering pretty heavy. Don't think it'll do anything else.

 

[dumbledore]

It'll increase the castor and make the steering pretty heavy. Don't think it'll do anything else.

I am not 100% sure but looking at when you change the setting from manual steering to power steering it looks like the castor angle should go down which should make the steering lighter?. You would expect powersteering to handle more castorr (heavy steering) so not sure what is what. It will be nice if someone can confirm before I do it and find it the hard way.

 

[fingers99]

Depends how you measure the castor.

If you think bikes, power steering is like olde school Ducati bevel 900ss, non-power is like something out of a modern Moto GP bike. (Of course, it doesn't relate to how bikes steer, but for illustrative purposes only!)

 

[dumbledore]

Depends how you measure the castor.

Well the way I am seeing it is that if you draw an imaginary steering axis to the ground the point of contact of this axis falls in front of the tyre contact patch (all cars are setup like this). The angle between the imaginary axis and the vertical axis from the patch is the castor. The larger the angle the more the castor effect (harder steering). The top of the strut can now be moved slightly towards the car (in the power steering position) which according to the above will reduce the castor angle. It will be nice to know why Fiat decided to have two castor settings anyway. Most of front wheel cars don't bother with this. Lowering the car (which my car is) also probably changes castor setting, but not sure how that works.

 

[fingers99]

This from a quick scan seemed appropriate.

Dunno about other makes, but Centos use a different castor angle for power steering models.

From memory, the power steering position puts the top of the strut closer to the windscreen, which increases castor angle and makes steering harder.

 

[dumbledore]

From memory, the power steering position puts the top of the strut closer to the windscreen, which increases castor angle and makes steering harder.

Well yes, this is what I wanted to think, but the strut is parallel to the vertical axis, so if you push slightly to the right, it will make the angle smaller, unless ofcourse the strut is parallel with the imaginary steering axis in which case it will make the castor angle larger. Can't see any reasons why power steering should reduce the castor angle though.

 

[fingers99]

The strut isn't vertical seen from the side!

To over simplify more than a little -- left and right, camber, back and forward, castor.

Castor also afect camber. There's a school of thought that says more castor is "a good thing". Nobles and later Puntos are effectively undriveable without power steering because they have so much castor.

So, more castor because it's "a good thing" but more castor = power steering.

 

[dumbledore]

As I am not 100% convinced that power steering setting increases castor angles here are some pointers I thought off.
1. Power steering system itself has some castor effect. Try to gentle turn the wheel while stationary and the system forces the wheel in its centre position back.
2. Power steering has less lock to lock turns. Higher castor angle will make quite violent return of the steering to the straight position making it hard to obtain smooth steering control. Also wear in the steering system will increase.
3. Manual steering lock to lock turn is high making it for slow manoeuvring in fast tight corners. Having higher castor makes the steering return quicker to its straight ahead position hence enhancing manoeuvring speed to the expense of higher muscle.
Above point to possible reducing castor angles on power steering system. Not to mention the fact that higher angles will make driving on bad wheels intolerable as I have experienced with my punto.

 

[fingers99]

You're making it up as you go along!

1. Is gibberish. Castor does not equal "self centreing". Self centring is a phenomenon. Castor is a quantifiable attribute.
2. Isn't necessarily true. Turns lock to lock are a result of steering rack (or whatever) ratio and steering arm length. If you put the same rack and steering arms in both cars, then the ratio will be the same. Manufacturers fit quicker racks (sometimes) in power assisted steering systems for the same reason they use greater camber angles -- because they can!

If you look properly at a power steering system (or talk to that nice Mr.Noble) you'll see that on the small FIATs with power steering it's increased. As an exercise, you might try driving a Mk2 Punto with the power steering fuse removed.

 

[dumbledore]

1. Is gibberish. Castor does not equal "self centreing". Self centring is a phenomenon. Castor is a quantifiable attribute.

This sounds gibberish to me. The fact is higher castor angle will cause the steering wheel to return violently to its straight ahead. And yes I have to make it up as I am going along as you not sounding convincing. In fact I may just do the castor swap to check it out the hard way.

 

[fingers99]

No. Self centring is something that happens -- it happens due to various factors. But you can't measure it. I'd expect only rake (that is, negative castor, like on a trolley wheel) to have a marked effect.

By all means try it. I'd suggest you take the time to read a book or two on suspension first, BWTH. I'm still not sure what the experiment is supposed to establish though.

 

[dumbledore]

By all means try it. I'd suggest you take the time to read a book or two on suspension first, BWTH. I'm still not sure what the experiment is supposed to establish though.

The experiment consists in moving the top strut mount to power steering position. Check if the steering gets lighter or heavier. If it get heavier I'll know the castor angle has increased, if lighter the angles have decreased. If lighter then is possible to use this change, to reduce the effects of a bad wheel has on steering wobble at speeds which is why I was contemplating the change.

 

[blu73]

why not replace the wheel I'm confused by this thread

 

[dumbledore]

Have done so already and have now got some sucees. Have 8 alloys and still struggle to find 2 perfect ones. Fiat must make the alloys out of paper as they buckle as you look at them. Reducing castor angle could make the effects of bad wheels less pronoounces making the drive more tolerable.

 

[blu73]

can't say i have ever buckled a fiat wheel tbh and i had lots!

Whereabouts you hail from, i have 4 very good condition GT3 alloys spare, arrow straight and no kerbing..

 

[fingers99]

The experiment consists in moving the top strut mount to power steering position. Check if the steering gets lighter or heavier. If it get heavier I'll know the castor angle has increased, if lighter the angles have decreased. If lighter then is possible to use this change, to reduce the effects of a bad wheel has on steering wobble at speeds which is why I was contemplating the change.

Will be heavier. I've done this on Seis. But there you actually have to change the top mount (PS ones are different, more castor -- top of strut nearer windscreen -- heavier)

 

[dumbledore]

Will be heavier. I've done this on Seis. But there you actually have to change the top mount (PS ones are different, more castor -- top of strut nearer windscreen -- heavier)

Yes, not change the top mount as that stays the same but turn it by 180 degrees. So you confirm that will make it heavier which means more castor. This is not what I need as this will make vibrations worse and steering heavier.