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Author: Subject: How much caster angle?
John Bonnett

posted on 16/9/07 at 04:10 PM Reply With Quote
How much caster angle?

I seem to remember reading that a rear or mid-engine car needs to have more caster angle built in than a front engined one. I have built the front of the Trials car chassis more or less to the Book leaving some adjustment in the form of spacers that can be moved around within the mounting brackets. Sadly even on maximum adjustment, there is not too much self-centring. Having said that, I've only run the car round the field so it is difficult to tell if it is unstable and whether the self-centring is adequate or not. Any change will mean removing the mounting brackets and carrying out a bit of surgery so I would rather do it now than later when the car is finished.

I just wondered if anyone can give some range of caster that will give a good result. I can then check what i have and act accordingly.

Thank you in advance for your help and advice.

John






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nitram38

posted on 16/9/07 at 05:29 PM Reply With Quote
My F1-2 is rear engined and works fine with 7 degrees.
You will need to alter your wishbones if you can't get enough on your uprights






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Dusty

posted on 16/9/07 at 06:26 PM Reply With Quote
KPI and castor angle combine in producing the part of the required camber changes in the front wheels on steering input to compensate for body roll. (Wishbone geometry is in there as well). Castor is thus dependent on what stub axles you are using as the KPI is pretty much built in. The less the KPI the more castor you will need for self centering but there is going to be a trade of in increased camber change for a given steering input. A lighter front end mayl need more castor for self centering but in practical terms only one or two degrees. I would go for 5-7deg for sierra stub axles.

[Edited on 16/9/07 by Dusty]

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John Bonnett

posted on 16/9/07 at 07:13 PM Reply With Quote
Thank you, Nitram and Dusty for your advice. I am actually using Cortina uprights with a transit top joint that will allow camber adjustment.

I cannot move the top wishbone mounting brackets any further aft because of the change in chassis tube angle. What I was planning on doing was to cut from a piece of one inch box section a wedge shape onto which the top mounting brackets will be attached and then the whole welded to the chassis. Each side, obviously. This will have the effect of moving the front mounting out towards the road wheel thus pushing the top joint backwards and therefore increasing the kpi. I can still retain the wide brackets so that further adjustment is possible by playing with the spacers.

I'd be pleased to hear your opinion on this course.

Many thanks guys.

John






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britishtrident

posted on 16/9/07 at 07:46 PM Reply With Quote
Trials are of course have very different requirements from tarmac motorsport.

With so little weight at the front you will need a lot of caster, a lot more than a Cortina based front end was ever designed for.

Caster is in-directly related to front end weight
Hillman Imps used to have something like 11 degrees plus some trail (stub axle was about 0.5" behind the king pin axis), they were of course design for X-Ply tyres which don't have the in built self-aligning effect that radial tyres do.

MGTF uses 6 degrees caster.


KPI and Caster are inter related, if caster is greater than KPI the camber becomes more negative on the outside wheel when the steeering is turned.

The other thing to watch is how much ackerman effect you can get --- the only way to get any ackerman with Cortina front end parts is to mount the steering rack well behind the outer track rod ends.



If building a trials car myself I would have chosen MG Midget or Triumph Herald front end bits.





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Dusty

posted on 17/9/07 at 12:09 AM Reply With Quote
quote:

This will have the effect of moving the front mounting out towards the road wheel thus pushing the top joint backwards and therefore increasing the kpi.


Will increase positive castor but has the secondary effect of changing the alignment of the inboard mounts so they are no longer parallel to the lower wishbone mounts. Results in changing castor on suspension movement.
How much would this matter in practice. I had a small fight with a tyre wall that left me substituting a friends spare wishbone for the badly bent lower one. Drove home 250 miles with about 5 degrees of positive camber, zero castor on that side, a virtualy coil bound spring and all I noticed was slight bump steer! True it was mostly A roads but that was the moment I realised I can't tell stork from butter (or lard or axle grease for that matter).

[Edited on 17/9/07 by Dusty]

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John Bonnett

posted on 17/9/07 at 07:23 AM Reply With Quote
Thank you everyone for your comments and help. Actually, I'm really confused now to be honest. Although it is not a high speed car it does need to be stable on the road and for this there does need to be a reasonable amount of self centring. What I might do, is to take it over to the Haynes circuit and try it out there as it is. I'll obviously have to check that they will have no problems with me running round on a spaceframe!

Will keep you posted. Once again many thanks all

John






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nitram38

posted on 17/9/07 at 08:10 AM Reply With Quote
You kpi is decided by your uprights and unless you can lengthen or shorten them, you are pretty much stuck with what you have.
Imagine a line that goes through the axis of your top and bottom ball joints. This is your kpi angle.
This line should intersect a point on your tyre patch on the road.
If you imagine a line running through the centre of your tyre tread, where the kpi intersects ideally would be in the centre of this line.
If these lines do not meet, then the difference causes scrub on your tyres as you turn the wheel.
Some scrub is good for grip, but too much can affect your steering (too heavy) wear out your tyres or cause the car to understeer if it is severe.
Overal tyre diameter can change the scrub because this can change the point at which kpi meets the centre of your tyre tread.
Castor is like the wheels on a shopping trolly. If you tilt the wheel turning axis slightly backwards at the top, the wheels will centre because that is the easiest way for them to point.
Hope this helps a bit

[Edited on 17/9/2007 by nitram38]






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John Bonnett

posted on 18/9/07 at 12:45 PM Reply With Quote
Thank you nitram, I can follow and understand what you said.

The thing I am concerened about is the lightness of the steering and whether the car will, when on the road, track straight or want to weave about. There is a vestige of self-centring but I wonder if it is enough. I can at this stage, carry out surgery on the front suspension before the chassis is painted but would prefer to do nothing if it really is okay. I'm favouring a trip to Haynes and give it a few laps to really see how it handles. If it does turn out to be okay then I have saved myself work so in any event it will be well worth the £70 to hire the circuit for half a day.

I really appreciate your interest and help. Thank you,

John






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nitram38

posted on 18/9/07 at 01:04 PM Reply With Quote
As said before, 5-7 degrees castor will give you what you need.






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John Bonnett

posted on 18/9/07 at 03:29 PM Reply With Quote
Nitram, yes, the next step is to actually measure what we have.

Looking down from the top, the projected line through the ball joints falls well outside the tyre because I am using Peugeot wheels and not Cortina. I imagine this will produce more scrub and therefore a heavier steering? Goodness knows how light it would be with the correct wheels!

John






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alistairolsen

posted on 1/4/08 at 05:38 PM Reply With Quote
Anyone know the KPI of a cortina upright? And a sierra one?
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Fred W B

posted on 1/4/08 at 06:56 PM Reply With Quote
See also

HERE

Cheers

Fred W B

[Edited on 1/4/08 by Fred W B]





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Ringius

posted on 1/4/08 at 07:56 PM Reply With Quote
Hi John and all!

I might be far off or misunderstanding the projected line though the balljoints mentioned above, but:

As I understand it, the line drawn from bj to bj to ground end up outside of the tyre - wouldn't this induce instability, since the wheels then will tend to toe-out during braking et.c. I believe Graber had some problems with this when he changed wheels and tires on his Bala...

Wouldn't one option to increase caster be to re-fabricate the upper wishbone, moving the upper balljoint backwards somewhat?

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leto

posted on 2/4/08 at 04:16 PM Reply With Quote
quote:
Originally posted by alistairolsen
Anyone know the KPI of a cortina upright?

You don't want to know (4.3 degrees)
quote:
And a sierra one?

Around 10 I've heard (not confirmed).





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