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Self centering....

Eggy - 26/9/08 at 07:31 PM

Ok, it's driving me mad!

I've read loads of posts, looked on the net, and asked at the Speed Trials. All with diferent answers - what's correct?

Is there a definate fix for this (raising the rack, using valve springs, over inflating tyres etc).

What's been done before that works??

Photo's would really help too!!!

Cheers,

Kev.

Paul TigerB6 - 26/9/08 at 07:37 PM

trial and error seems to be a good starting point - see what works on your car!!

Start with a bit of toe out and see how that goes

blakep82 - 26/9/08 at 07:37 PM

then a bit more castor

chrisg - 26/9/08 at 07:45 PM

blakep82 - 26/9/08 at 07:46 PM

mmm worms on toast

jacko - 26/9/08 at 07:56 PM

What car you building ie MK / locost /MNR / it may help us help you

paulmw - 26/9/08 at 08:38 PM

I got really pi$$ed off with this when building my Indy. Most answers to the problem were either sarcarstic or wrong.

In the end I added some valve springs to the rack and on the SVA the steering would magically spring back.

....... step away from the soap box................

simonk - 26/9/08 at 09:02 PM

Some sensible tips would also be appreciated here also. Castor is the obvious one but that's set by the front end geometry for most of us. I can rotate the mushroom to drop the upright back a bit to get a little more trail and was planning some toe-out and 40lbs of tyre pressure. I have already juggled wishbone washers to move the bottom bone fully forward and the top one fully back and on my exploratory drives down the lane it's still not great.

Any other practical tips would be very welcome.

Eggy - 26/9/08 at 09:25 PM

quote:
Originally posted by paulmw
I got really pi$$ed off with this when building my Indy. Most answers to the problem were either sarcarstic or wrong.

In the end I added some valve springs to the rack and on the SVA the steering would magically spring back.

....... step away from the soap box................

I've heard about this but can't get my head around it, anyone got some pictures?

NeilP - 26/9/08 at 09:45 PM

Some trailing hub helped mine a fair bit after shuffling the wishbones as far as washers would allow - From my experience, toe in/out and tyre pressure seemed to have a very limited effect...

whitestu - 26/9/08 at 09:48 PM

Valve springs in the rack were the only things that made a difference on my Indy as well.

I think the proper fix is to make new top wishbones with more castor.

Stu

mark chandler - 26/9/08 at 11:14 PM

Caster is the key, look at any motorbike, the forks always lean backwards, the principle is exactly the same.

So twiddle the mushroom, pack the bottom bones forward with washers and the top bones backwards or better still make some new top ones, they are easy to make, you just need a bit of board and some nails to make a jig.

Regards Mark

zetec7 - 27/9/08 at 04:09 AM

Or, maybe it's possible to cut the upper a-arm mounts off and move them rearward, ideally 1" or so. We designed our a-arm mounts to be adjustable for caster angle - we can add as much as another 1" rearward displacement of the upper 'bones.

And, yes, caster is the ONLY right way to add self-centering. Toe-out = twitchy steering, toe-in = rapid tyre wear & rapid onset of understeer in corners.

As for valve springs in the steering rack...well, I suppose it works, but it strikes me as fundamentally wrong for some reason...probably because self-centering is suppose to occur automatically as a result of good suspension design and adjustment. I suppose you could use bungy cords on the steering column...

David Jenkins - 27/9/08 at 07:00 AM

quote:
Originally posted by paulmw
I got really pi$$ed off with this when building my Indy. Most answers to the problem were either sarcarstic or wrong.

The fundamental problem is that nearly all the kit manufacturers are selling chassis & front suspension with a design flaw built in - for road use anyway.

The original design with minimal castor is fine for the race track, where self-centering isn't a major concern and the lightness of the steering is a bonus. For the road a lot more castor needs to be built into the design, and no amount of fiddling about can rectify its omission. People worry about it for the SVA, when in reality it's essential for comfortable and safe road driving.

The fix is really easy for the kit manufacturers - make a minor adjustment to the top wishbone jig, so that the top pivot is about 22 - 24mm behind the lower pivot - and a lot of people will be a lot happier.

Until builders put a significant amount of pressure on the chassis makers, the issue will continue to come up here, and the silly answers will continue, as everyone here who's built a commercial kit knows that the fundamental error is built-in and can't be fixed by half-arsed and potentially dangerous bodges.

And that's my 2p's worth for the morning!

[Edited on 27/9/08 by David Jenkins]

flak monkey - 27/9/08 at 07:24 AM

My GTS self centres brilliantly. Not sure what the offset is on the top wishbones, dont think I ever measured it.

Front geometry is set to the same as the caterham set up. Lots of camber and very little toe. Sticks brilliantly into the corners and get full tread contact on the tyres running 17psi per corner.

David

britishtrident - 27/9/08 at 07:59 AM

Rotating the Mushrooms will have no effect on caster

Mole - 27/9/08 at 08:03 AM

Anyone would think you'd mentioned that before.

britishtrident - 27/9/08 at 08:03 AM

Increasing the king pin inclination will help ----- this can be done by rotating the mushrooms.

whitestu - 27/9/08 at 09:10 AM

I got next to no difference in self centring however I adjusted mushrooms and toe.
I agree the bones should be made differently, and will be trying this next.

I used springs for SVA which worked [they also limit lock which helps to prevent brake pipes fouling as well], but I took them off straight after.

They are a bodge and I'm surprised MK recommend it [which they did to me when I phoned up for help].

I'm sure someone could make a few quid if they started selling modified top bones. as most people who buy kits do so cos they don't want to start making chassis bits.

Stu

t.j. - 27/9/08 at 06:32 PM

quote:
Originally posted by britishtrident
Rotating the Mushrooms will have no effect on caster

you don't have to scream...

My caster is adjustable by moving the mushrooms/shims in the brackets in the lower and upper wishbones.

So pictures could be helpfull, to understand each other.

I want to know which mushroom is pointing at? KPI adjustable?

[Edited on 27/9/08 by t.j.]

mad-butcher - 27/9/08 at 07:43 PM

Just a couple of observations, not that I know what the F**K I'm talking about.
1. By rotating the mushrooms would this not make the problem worse. my nut is a the front meaning that the upright is fully adjusted to rear, if I rotate the mushroom it will push the upright forward, surely the opposite effect to what we are looking for.
2. if by moving the adjustment towards the the rear of car. by moving brackets rearwards or by remaking the top wishbone, is this not going to put too much strain on the ball joints.. my bottom one alresdy looks at a frightning angle.
3. the use of springs to induce self centering seems a bit weird to me, my transit, my 7.5 ton works vehicle and the wifes pajero all self centre from very little movement of the steering wheel, so what benefit do springs serve, or are we saying that for the sva they only test for self centering on full lock.. (a position of the steering that I think most people would rarely use) when are you ever on full lock in normal motorway driving.

Tony

[Edited on 27/9/08 by mad-butcher]

simonk - 27/9/08 at 10:21 PM

Agree, the ball joint on my mushrooms is currently directly adjacent to the wheel - if yours if fully forward then whatever you do will reduce trail. I will try moving the mushroom round to push the upright back as far as possible.

To clear up the discussion and agree with Triton adjusting the mushroom has no effect on caster or king pin inclination. All you can do with the mushroom is add or reduce trail (how far the contact patch is behind the point where a line drawn through the upper and lower pivot hits the ground) by moving the axle position in relation to the line described above. Increasing caster angle adds trail in addition to adding to the jacking effect of adding lock.

I won't be getting new top bones made by next SVA date so will be adding max trail (with the mushrooms) and will be upping the tyre pressures. Will let you know what happens.

Thanks
Simon

MikeRJ - 28/9/08 at 11:27 AM

quote:
Originally posted by mad-butcher
3. the use of springs to induce self centering seems a bit weird to me, my transit, my 7.5 ton works vehicle and the wifes pajero all self centre from very little movement of the steering wheel, so what benefit do springs serve,

It's a total bodge to get the car through the SVA.

t.j. - 28/9/08 at 07:22 PM

quote:
Originally posted by simonk
Agree, the ball joint on my mushrooms is currently directly adjacent to the wheel - if yours if fully forward then whatever you do will reduce trail. I will try moving the mushroom round to push the upright back as far as possible.

To clear up the discussion and agree with Triton adjusting the mushroom has no effect on caster or king pin inclination. All you can do with the mushroom is add or reduce trail (how far the contact patch is behind the point where a line drawn through the upper and lower pivot hits the ground) by moving the axle position in relation to the line described above. Increasing caster angle adds trail in addition to adding to the jacking effect of adding lock.

I won't be getting new top bones made by next SVA date so will be adding max trail (with the mushrooms) and will be upping the tyre pressures. Will let you know what happens.

Thanks
Simon

I don't know what, but you can increase your caster by moving the upper wishbone to the rear and the lower wishbone to the front.
wishbones seen from above

[Edited on 28/9/08 by t.j.]

David Jenkins - 28/9/08 at 09:17 PM

An alternative would be for the chassis builder to fit extra-wide top pivot points - the builder could use washers or spacers to put the wishbones precisely where he/she wants them.

So, as I said earlier, the kit makers could remake their top wishbone jig, or the pivot point jig, or the shape of the pivot points, to permanently solve the self-centering problem. None of these are hard or big jobs.

So why don't they?

mad-butcher - 29/9/08 at 08:15 AM

OK I accept the point of moving the top wishbone mounts rearwards, But by how much? and would the extra strain on the ball joints be worth it
Tony

mangogrooveworkshop - 16/10/08 at 07:02 PM

Rescued attachment Wisbone.jpg

IDONTBELEIVEIT - 16/10/08 at 07:12 PM

CASTOR not CASTER(sugar,holiday resort)or is that caister

Antnicuk - 1/1/09 at 11:46 PM

I am replacing my lower wishbone bushes with rod ends to make the castor adjustable.

I know to adjust the castor i can slide the whole wishbone forwards or backwards, BUT, If i adjust each rod end in and out, this will twist the wishbone which will also change the caster, is this an ok method to adjust the castor or does this have another effect?

NS Dev - 2/1/09 at 10:58 AM

the car, if correctly designed, should self centre without bodges like springs in the rack, which, to even consider using, you must be barking mad.

If it doesn't self centre, either make the upper or lower wishbones adjustable fore and aft, or remake the upper or lower wishbones.

NS Dev - 2/1/09 at 10:59 AM

PS as has been said many times before, adjustable mushroom inserts on sierra uprights make no difference at all as you haven't moved the pivot point at all, the ball joints are still in the same place!

mistergrumpy - 2/1/09 at 01:48 PM

If it helps I've just returned from my SVA in the last half hour and I was struggling with self centreing. I inflated the tyres to 30psi and toed them outwards slightly by eye and it worked.
Remember the car only has to attempt to self centre so the wheel only has to come back a little and not like a standard car.

By the way Wayno, it is caster and not castor see here
You'll notice in little blue writing towards the top too where it redirects you from "castor"

chrisg - 2/1/09 at 05:42 PM

It's caster OR castor - they're interchagable.

You can't just keep adding castor angle to help self centreing - it causes other problems, not least of which is that excessive camber causes the wheel to "flop" from side to side as the wheel is steered.

The so called "adjustable" mushrom is nothing of the sort. The hole is offset forward to increase mechanical trail and so help self centreing. It should not be used to adjust camber, as has been suggested in the past.

Trail and castor are related but not the same thing.

Castor can be increased by moving the top balljoint rearward (or the bottom balljoint forward)

Mechanical trail can only be adjusted by moving the attachment points ON THE UPRIGHT.

Therefore using an offset mushroom increases the mechanical trail, not castor, by moving the upper attachment forward to move the centreline of the upright attachments in front of the axle centreline.

Hopefully these sketches might help.

It's facinating stuff isn't it?

Cheers

Chris

MustangSix - 5/1/09 at 02:42 PM

One of the things that seem to be misunderstood is that a caster increase is only effective if the car is rolling. While at rest, self centering is also a function of the King Pin Inclination angle.

Envision how the KPI is at an angle to the axle stub. As the spindle rotates, the KPI causes the stub to move in a downward arc, not parallel to the roadway, actually forcing the car to rise slightly. The weight of the car pushing against this arc tends to pull the spindle back to center.

This effect is noticeable at both rest and while moving. This is also the reason that you can get a self centering effect even on a car with negative caster, which was common on many cars in the 50's and 60's.

If you increase negative camber, you can get a more pronounced arc from the stub axle, increasing the self centering at rest, but only to a limited degree.

[Edited on 5/1/09 by MustangSix]