Technical toying with the idea of changing the castor setting

[fingers99]

Yep. Mind you, it's only about 20 minutes work on a Mk 1. Jack (with block of wood) under wishbone, just keeping wheel off ground, and you should be able to turn the mount round without removing the strut.

 

[dumbledore]

I tried this over the weekend with both wheels off the ground could not push the strut down more the an inch, which is not enought to rotate the mount. May be I should have been more persuasive. Anti roll bar is not letting it going down anymore I suppose, even with both wheels off the ground.

 

[fingers99]

That's why you put the jack under the wishbone, to put some tension into the spring. If you do it without load on the spring you will kill yourself.

 

[dumbledore]

Still not get it. If I put load into the spring will just push the strut assembly upwards which will make it impossible to turn the top mount.

 

[fingers99]

The opposite, methinks. But have to admit, never actually done it this way.

 

[tinytim]

Changing the castor angle (not castEr, that's sugar) will NOT solve the vibration problem. More likely to be poorly balanced wheels or worn bushes.
On a front wheel drive car you need negative castor on overrun and positive castor when power is applied to the wheels; think about pushing your sofa around the room; the castor wheels will swivel so the the axis of rotation of the wheel is behind is behind the swivel pin; then imagine the wheel has power applied and turns of it's own accord; the castor will swivel so that the wheel is now in front of the swivel pin.
Most FWD cars are set to have zero castor angle for this reason. On the punto the top of suspension unit (actually the top of the shock absorber/damper on a Macpherson strut) can rock fore and aft to allow for these differing requirements. To change from power to manual setting you simply undo the top suspension mount, (the one in the engine compartment) taking the appropriate precautions regarding coil springs and rotate it 180 degrees.
However I see absolutely no point in doing this.

 

[dumbledore]

Fiat put the castor mod there for a reason. We have not fully explained that. In my case the vibration is caused almost sure by buckled wheel. I check the culprit wheel in the front rotating and the side and was not rotating on a perfect line you could see side movement. I believe the castor effect is amplifiying this movement causing vibration. Reducing castor angle may reduce the vibration. Fortunatelly I have replace the wheel with another that is almost OK (but with a soon to be replaced tire) and the vibrations are mostly gone. Also putting a backled wheel to the rear causes less or no vibrations which is an indication that castor does have an effect.

 

[fingers99]

Fiat put the castor mod there for a reason. We have not fully explained that.

Pretty much have, I'd say. We've not fully explained the effect of cator on handling, or even dipped into the relationship between castor and camber, but we're not wtiting a book on susspension, init?

Also putting a backled wheel to the rear causes less or no vibrations which is an indication that castor does have an effect.

More, I think, that the front has more components and more potential for play.

 

[fingers99]

On a front wheel drive car you need negative castor on overrun and positive castor when power is applied to the wheels; think about pushing your sofa around the room; the castor wheels will swivel so the the axis of rotation of the wheel is behind is behind the swivel pin; then imagine the wheel has power applied and turns of it's own accord; the castor will swivel so that the wheel is now in front of the swivel pin.

That's right, study the steering/suspension of your car by looking at your sofa/shopping trolley!

Cars do not have negative castor on front Mac struts.

Most FWD cars are set to have zero castor angle for this reason.

But this is clearly untrue! Just look at the fookin suspension from the side with the wheel and arch cover removed, it's clearly inclined backwards.

On the punto the top of suspension unit (actually the top of the shock absorber/damper on a Macpherson strut) can rock fore and aft to allow for these differing requirements. .

The top mount is a bearing within a peice of very stiff rubber within a steel shroud. The rubber is there purely for NVH -- more sporting set ups bin the rubber entirely.

The castor angle will change depending on whether the strut is in bounce or compression. Hence the type of bearing used, but the strut cannot know whether the car is under power or on over run (how the hell could it)?

Next time can we please talk about how many angels will fit in the head of a needle?

 

[dumbledore]

Pretty much have, I'd say. We've not fully explained the effect of cator on handling, or even dipped into the relationship between castor and camber, but we're not wtiting a book on susspension, init?

Punto is not a performance car so it must be a simple reason why Fiat make the castor change possible. Unfortunatelly you do not know why.

 

[fingers99]

Punto is not a performance car so it must be a simple reason why Fiat make the castor change possible. Unfortunatelly you do not know why.

Because you can! Quick racks, less unwanted camber change, are all good things. You can only have them with power steering. If you're fitting power steering because Joe Public is getting fat, lazy and stupid, you may as well have the advantages.

 

[dumbledore]

Because you can! Quick racks, less unwanted camber change, are all good things. You can only have them with power steering. If you're fitting power steering because Joe Public is getting fat, lazy and stupid, you may as well have the advantages.

What stops me to get all that advantages with my manual steering by moving the top castor setting to the power steering position?

 

[fingers99]

You don't have a fast rack and the steering will (assuming you don't do bench presses, play rugby or eat lots of spinach) be too damn heavy, at least for sustained use.

 

[dumbledore]

You assuming that the castor increases and the steering will them become unbearable. You do not believe that do you!! You already said that the steering is light on manual steering MK1 on another post, how come a small castor angle change make that much of a difference? Also higher castor angle actually increase camber when the wheel is turning is that an unwanted camber change that you mentioned earlier in this post? The fact is that you do not really know why Fiat done it,

 

[fingers99]

You assuming that the castor increases and the steering will them become unbearable.

I'm assuming nothing. Unlike you, I've actually done it on a Sei. The differences in castor -- at least in that application, are very clear indeed. I've also played with the Mk 1 top mounts. On your own admission, you can't even get them off! :bang:

Unbeareable is unquantifiable, but I certainly wouldn't like to drive a PAS Sei with the power steering disconnected for any length of time. A good test might be whether a normal person could deliver pizzas in the middle of a city

You do not believe that do you!! You already said that the steering is light on manual steering MK1 on another post, how come a small castor angle change make that much of a difference?

Working out "how come" is beyond my current knowledge. All I can tell you is that it does.

Also higher castor angle actually increase camber when the wheel is turning is that an unwanted camber change that you mentioned earlier in this post? The fact is that you do not really know why Fiat done it,

To +ive or - camber? And under what circumstances? It's entirely true that FIAT sometimes do utterly crazy things. But in suspension design they're not clueless. While I cannot claim to know why FIAT did it, I think I give a reasonable explanation.

 

[dumbledore]

The thing is the manual steering is no where near as heavy as PAS as there are more turns. The manual steering is not a quick rack. This means that it will be certainly bearable to drive. And if the PAS castor setting gives better handling caracteristics this will be certainly worth a try.

 

[fingers99]

The thing is the manual steering is no where near as heavy as PAS as there are more turns. The manual steering is not a quick rack. This means that it will be certainly bearable to drive. And if the PAS castor setting gives better handling caracteristics this will be certainly worth a try.

I see what you mean. It's worth a try, although I'd not count on the bearability, especially if you fit decent tyres (quality and size). And the tyres are the most obvious improvements to be made to your car.