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Mushroom Adaptors for Sierra Based Kits
short track 123 - 23/5/08 at 11:31 AM

Is there some thing I'm missing ?

A lot of Sierra based kit cars use a
Mushroom Adaptor with an off set hole for the top joint on the front upright.

Why not just make the wishbones to the caster angle you want ?

This would then make the Mushroom Adaptor a lot cheaper/easier to make and keep down the cost of them.

I have seen costs ranging £45 to £71.50
this is for a pair with VAT and postage.

Jason

Miks15 - 23/5/08 at 11:40 AM

rally design do them for £20 i think with a centered hole.

Yep here

[Edited on 23/5/08 by Miks15]

jabs - 23/5/08 at 11:50 AM

I believe it's not to do with the caster but to counteract the anti dive.

MNR to the offset ones for £38 a pair

Hellfire - 23/5/08 at 11:58 AM

Mushroom adapters have no effect on castor whatsoever. Your top wishbones in relation to the bottoms dictate castor angle.

Phil

short track 123 - 23/5/08 at 12:12 PM

Just my point if the hole was not off set they become much cheaper to make.

Did the Sierra have anti dive?

So then no need to have the off set hole?

Miks15 - 23/5/08 at 12:28 PM

is the offset hole for adjusment in camber?

chrisg - 23/5/08 at 12:39 PM

The offset adaptors increase the trail of the set-up and therefore the self centreing.

Ideally they could do with slightly more but it's impossible with the Sierra upright.

Camber should be adjusted via the top link, not the mushrooms and caster can only be altered by moving the wishbone pivots. Trail can only be altered by moving the attachment points on the upright

So it's up to you but don't forget self centring is part of the SVA test.

Did you think they were made that way just to be awkward?

Cheers

Chris



[Edited on 23/5/08 by chrisg]

short track 123 - 23/5/08 at 12:51 PM

So Chris just for my head

nothing to do with camber, caster or anti dive?

Just trail ( like a shopping trolley ) to help self centering if so i think the penny has just dropped

Jason

[Edited on 23/5/08 by short track 123]

mr henderson - 23/5/08 at 01:17 PM

quote:
Originally posted by short track 123
So Chris just for my head

nothing to do with camber, caster or anti dive?

Just trail ( like a shopping trolley ) to help self centering if so i think the penny has just dropped

Jason

[Edited on 23/5/08 by short track 123]


Caster and trail are the same thing, aren't they. In fact I haven't heard the 'trail' expression used before in this context. Caster is the effect which leads to self centering

mikeb - 23/5/08 at 01:29 PM

Interesting discussion.
If my memory serves me right from my days at TRW Automotive.

Castor angle creates mechanical trail (as opposed to pneumatic trail). It is the mechanical trail in mm that affects the self centering moments (Force x Distance).

Moving the top wishbone pivot will ajust the castor angle so will the offset hole in the mushroom. I would assume moving the wishbone pivot to gain the correct castor angle would end up with the whole upright being rotated whereas a fine adjustment due to the offset hole will leave the upright at it's correct orientation.

Well it make sense to me!!!

short track 123 - 23/5/08 at 01:32 PM

I think this is where it all becomes a bit vague for me to.

If i read Chris's post right they are different but i can not find any reference to trail in any of the books i have.

mikeb - 23/5/08 at 01:35 PM

Caster angle is the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations.

The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel casters around so as to trail behind the axis of steering. This makes a car easier to drive and improves its straight line stability (reducing its tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although, in racing, large caster angles are used to improve camber gain in cornering. Caster angles over 10 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.

The steering axis (the dotted line in the diagram above) does not have to pass through the center of the wheel, so the caster can be set independently of the mechanical trail, which is the distance between where the steering axis hits the ground, in side view, and the point directly below the axle. The interaction between caster angle and trail is complex, but roughly speaking they both aid steering, caster tends to add damping, while trail adds 'feel', and returnability. In the extreme case of the shopping trolley (shopping cart in the US) wheel, the system is undamped but stable, as the wheel oscillates around the 'correct' path. The shopping trolley/cart setup has a great deal of trail, but no caster. Complicating this still further is that the lateral forces at the tire do not act at the center of the contact patch, but at a distance behind the nominal contact patch. This distance is called the pneumatic trail and varies with speed, load, steer angle, surface, tire type, tire pressure and time. A good starting point for this is 30 mm behind the nominal contact patch.

mikeb - 23/5/08 at 01:37 PM

Another interesting side effect is the more castor angle the more ccmber change you get in cornering.
This is why you need decent 3d Kinematic analysis when designing suspension as there are a lot of interactions.

Mechanical Trail or just Trail is a fairly common term I believe, what books have you got on the subject?

[Edited on 23/5/08 by mikeb]

mikeb - 23/5/08 at 01:42 PM

just to bore you even move.

More castor also leads to more jacking.
This is where as you turn the steering the front of the car rises up as the steering axes are not vertical due to the castor. The jacking effect also adds to the celf centering as if you lift the car up gravity wants to pull it back down again.

i use to have an rc car where you could turn thesteering and see these effects!

Don't get me on kingpin inclinations and Vauxhalls!

g.gilo - 23/5/08 at 01:50 PM

hi folks
i think you struggle for ball joint articulation with a central taper top hat.
the aluminium ones supplied by luego are offset and real quality.
regards graham.

short track 123 - 23/5/08 at 03:17 PM

I have the following books
Competition Car Suspension By Allan Staniforth, Chassis Engineering By Herb Adams, How To Make Your Car Handle By Fred Puhn.
If any one knows of others i would be interested to hear of them.

A good subject to get to grips with after all if it boils down to spending £70 or £20
it saves a good part of my £250

tks - 23/5/08 at 03:17 PM

quote:
Originally posted by Hellfire
Mushroom adapters have no effect on castor whatsoever. Your top wishbones in relation to the bottoms dictate castor angle.

Phil


This is wrong.

altough the position of the top wishbone and the lower wishbone isn't changed.

Castor is the amount the hub inclinates backwards. 0 degrees means then that the centre of the struthole = 0degrees from the bottom balljoint.

But by turning the mushroom forward and the mushroom has got an offset the centre of the strut hole then is pushed backwards. and this results in castor.

You can easy see it by watching the wheel from above.

so you do need to use a mushroom wich has an offset in the hole! else it has no effect off course.

To determine the amount of adjust ment.
You need to know the distance between the centre in the hole and the hole in the mushroom.

Importanto also to notice that you only have a 90degrees effective range!!
and that by turning the mushroom other geometrie should be compensated!!

if you turn the mushroom CCW then the hub is pushed backwards and you will need to compensate with the threaded ball joint... i think that the adjustment is about 12mm.

i personally have got them at 45degrees after modding the threaded section on my upperarm (i couldn't turn the joint further in)

i also have had it on 90degrees but then my castor looked almost like a mercedes benz!!!

Also the more castor you give the heavier the steering will be (think in like the go k arts you will need to raise the cars front weight...)

so they do effect the castor but only if they have got an offset in the hole.
turning them then creates an offset as big as the offset between the centres.

Tks

tks - 23/5/08 at 03:31 PM

Like this...

hope the size isn't to big... Rescued attachment CastorSierraStut.JPG
Rescued attachment CastorSierraStut.JPG