Self centering
 

My car does not self-center at all. The mushroom inserts are with their hole forward, Toyo R888 tyres inflated to 1.5 bar (tried 1.8, too, no effect), 1 degree toe in, -2 degrees camber. The wishbones are built to book dimensions, there is a visible caster angle in the kingpin axis. It does seem to self-center in reverse, though. Any pointers, ideas?

Bit of toe out for IVA sorted out self centre issue

*sigh* Thanks, I'll try that. But there's a reason why self-centering is a required for IVA, it's a safety feature.

I'm considering several ways to increase the caster angle. One is slotting out the holes on one of the front suspension brackets and fitting some eccentric washers, much like this: http://www.jegs.com/i/Specialty-Prod...23765/10002/-1

Another idea is shortening the four bushes on the top wishbones by some 2.5 mm (bush tube and crush tube) and packing with washers to increase the caster angle by about 1 degree.

That's what i was thinking of doing some time when i finish my project as it seems every car with Sierra uprights has the same issue.

If you have a dig about on here someone did an explanation of all this. The mushrooms dont add caster they give you trail. Its this trail that helps with the self centering, a bodge is a better word for it. There is not enough caster in the front suspension set up and no way to alter it.Thats why I am making my upper bones with adjusters on the inner joints. When they are on the car I will take some measurements of them in the corrected adjusted position then perhaps someone can do an ammended top wishbone drawing so they are built to include the caster without the need for adjusters. Caster is an important must have for straight line stability not just self centering. I am no expert on trig but i bet its only a few mm added to the front top wishbone tubes to sort it.

================... KPI ......... caster

Lotus Elise........................ 12° ......... 3.8° caster
Corvette C5, C6................. 8.8° ...... 6.5° caster
Pinto/MustangII/Wilwood... 11°
Ferrari 512BB............... 9 or 13°
ATS AFX Tall..................... 8°
Triumph Spitfire................ 7° ..........4.7° caster
Lotus Seven..................... 9° .......... 5° caster
Mazda Miata................... 11.3° ........ 5° caster
Ferrari F355................... 13.16°
McLaren F1...................... 9° ........... 6° caster ...... 16.25mm scrub

Bob

I would suggest that you lower the tyre pressures to 1.1 bar as a starting point,

set the toe to 0deg and try it. if that fails then 0.5deg toe out will get it self centering but it will feel twitchy at high speed.

One is as good as the other:



Fixing the Sierra's trail inceases positive caster distance, too. It's the distance between the kingpin axis' projection intersection with ground and the centre of the contact patch, and it is precisely the lever that does self-centering.

I thought so. But now I have 1.5 bar in the rear tyres, zero camber, and the marks left on the asphalt when spinning wheels are just narrow stripes along the edges.

Not quite the same thing because the trail wont become effictive untill you move where as the caster will be effiective at all times. Hence the lack of self centering when stationary. That castor like on a shopping trolley could be kicked arround 360 degrees and it will only straighten if you move forward. Also the inside wheel on turn in on the proper set up is forced downwards whereas the trail set up remains static. That in turn makes the steering feel vague, thats the reason all decent suspesion systems have caster built in. The mushrooms help but its only a patch up fix for an inherent problem, they do infact make the set up act as scrub which most would regard as undesirable.

Bob

I have no problem with lack of self-centering when stationary. I would say it's a feature of most cars. Turned wheels stay turned as long as the car is not moving.

Now, the kingpin inclination angle, which is cast into the upright geometry, should provide some self-centering effect, as turning the wheels results in slightly raising the car, but, apparently, with the Roadster's weight, this effect is not enough.

You're missing the bit that Sierra's upright has negative trail, and offset mushrooms set it to zero, making the wheel axle intersect the kingping axis.

I have not gone for the sierra set up so I cant comment on it but dig about with the search and its was made clear that the offset mushrooms were used to give the suspension trail. That might or might not be correct so moving on to the basics and there is not enough caster in the system to give it self centering end of. i have gone for the mazda rx8 uprights and will be making fully adjustable top wishbones to address the problem. Caster is what is needed. If the mushrooms set the trail to zero that makes the whole thing worse!! Zero or close to zero caster and zero trail "no thanks"

Bob

You don't merely need caster angle. What you need is the benefit that caster angle gives you. This "benefit" is actually called "mechanical trail" and this along with the "pneumatic trail" gives the "total trail".
Caster is one way to obtain mechanical trail. Another way is moving the kinpin forward of the wheel center.

Mechanical trail is not an "angel". You dont need "much" of that, because if pneumatic trail is only a small percentage of the total trail, then when you are at the limit of traction (where pneumatic trail tends to disappear) you will not get a good feel of it.

.
Caster is one way to obtain mechanical trail. Another way is moving the kinpin forward of the wheel center.
[/quote]

Which we wont be doing as we are using ready made uprights so caster it will have to be. Unless of course we fabricate our own uprights! I have checked the mazda uprights the best I can and the king pin line runs through the center of the stub axle so there is no trail built into those particular uprights. All valid stuff this and needs looking at.

Bob

Offset mushrooms is another way to obtain some mechanical trail and you don't need any special uprights to do this.
What i want to tell is that caster is not what you actually need. What you need is trail and there are a number of ways to get trail including caster angle.

You're missing the bit that Sierra's upright has negative trail, and offset mushrooms set it to zero, making the wheel axle intersect the kingping axis.
Quote.

Alga has already pointed out that the mushrooms set the trail to zero<which i was told gave a positive trail> so where do the sierra upright boys go now. More caster.?????? We have what the donor of our choice gave us and the only practical way to rectify the problem is to frig the top wishbone or pivot points. Any other method would involve purpose made uprights which no one is going to do.

Bob

Robo, here's a piccie of the Sierra upright. The negative trail, while exaggerated, is fairly obvious.



The book design has the caster angle of approximately 5.5°, which, while not a lot, is far from zero.

Hm, moving the upright back by 2.5 mm will increase the caster angle to about 6.15°. That's a pretty small difference, I doubt it's worth it.

Actually, making another set of top wishbones with larger offset might be the neatest way to increase the caster. The question is, by how much?

Alga, have you actually measured your car's caster angles in an alignment shop?

Albert help might be at hand here. The extract below was from a drift forum. Where I cant comment on the 2wd sierra stuff I can check out the 4wd stuff because I have two sets of 4wd hubs here. If the below is true that might be the best option. I will check them out for trail today and let you know. Its the negative trail that is wiping out the effect of the caster and causing the problems. Its a shame it was not noted when the cars were being developed because they could have mounted the rack behind the front axle and just reversed the hubs giving positive trail, but thats not where we are.

For drifting Sierras lack caster and lock, so I want to use 4WD front hubs as they give more caster angle than the 2WD hubs. I had them left over from when I broke the XR4x4 I had. I've already removed the CV joint innards.........

[image]

.......and to increase lock I want to get the hubs redrilled to change the location of the track rod end. Like this.......

[image]

.......although I want a tapered hole so I can use the OE track rod ends, instead of the bolt and rose jointed affair I'm the photo.

Using 4WD hubs means I must use the pinch bolt type track control arms (which I've also got) and the larger diameter 4WD dampers too (which I don't have).

Bob

Just to clarify,
When you move the inserts, it doesn't change the castor angle as the top and bottom pivots are fixed, it effectively moves the centre line of the wheel forward and backwards changing the trail, so in the diagram above the steering axis stays fixed the wheel effectively moves for and aft.

From recollection the only thing that would give you any self centering when stationary is king pin inclination as this causes the body to rise and fall when the wheels are turned so the vehicle mass forces it back down due to gravity. (sorry missed someone already said this!)

No, I haven't. Do you think it's the only way to get a reasonably exact measurement?

Here is a picture of the 4x4 hub with a steel ruler through the two pinch bolts, I know this is not 100% accurate but it looks like the drift boys are right. It does appear there is no trail + or - , the strut diameter is 50mm so I dont know how that compares. Might be a cheap quick fix. Edit :have a read of this, the cobra builders have the same problems http://www.cobraclub.com/forum/pilgr...or-angles.html




Bob

Absolutely. You will also be able to find out if you have any difference in caster angle between left and right wheel.

Very interesting, thanks for the pointers! Alas, RWD sierra's struts are 45.5 mm in diameter...

I just realised, it is pretty easy to check to within less than 1 degree with a steel ruler, a square, a reference plane (e.g, wall, big square box, etc.) touching front wheels. Just measure the difference of the distance of centres of balljoints from the reference plane. A measuring error of 1 mm will result in the error of about 16'.

Really then for the sake of spinning up some 50mm mushrooms and going to the 4x4 hub it would appear that all would be ok for caster and self centering as its the negative trail of the 2wd hub that is wiping out the effect of the caster this built into the suspension system. That would explain why you car self centers when reversing. Best to give tt a nudge and see if there is anything else odd about the 4x4 hub.

Bob

The lack of caster has been a known fault right from the original design, and I believe has been a problem on the MK as well yet there is still no revision to the design, update of the book in this respect. Unfortunatly most builders don't find this out until they've built the chassis/car so end up having to 'bodge' around the problem.
A solution if you can get them would appear to be the 4x4 upright,but it would be better for a sticky on here relating to this design problem so future builders have the chance to 'build in' some extra castor.

I have two sets here if someone wants to have a go. What the f@@k do you do with the front cv because it looks like the same system as the front wheel drive cars where the cv holds the whole hub together. I suppose you could cut the cv bit off the shaft and leave enough of a shoulder on there to make it like a big bolt. I think I will muller one and have a look.

Bob

Well I've been reading this thread and at the moment it's totally incomprehensible to me...:o :o
I really need to read up on this I guess. I don't want to hijack this thread ALGA but will the MX5 build suffer the same fate...:confused:
Here's a photo





I don't believe anybody has finished a MX5 build on here yet so it would be very nice to know.
I believe Nathan modified his top wishbone to overcome the clearance issue with the steering rack extensions fouling the bottom wishbone and moved the top threaded adjuster rewards.
He stated this helped his self centering as well as a result.
Obviously you are looking for a cost effective solution to the problem but would a new top wishbone as Robo suggested be the only option??

Johno

You don't need to apologise, Johno, this is clearly on-topic. From your photo it does look like the wheel axis is a bit further to the front than the kingpin axis, and I assume Andy left the caster angle the same as in the Sierra-based wishbones.

Actually, it's easy to test-self centering in the stage you are in, just put it on the wheels, turn the wheel to the lock, and push the chassis a couple of feet and back. My steering wheel stays where it was on the stroke forward and unwinds a bit when pulled back.

Johno, robo posted an amazing link about suspension geometry a while back:
http://www.haynes.co.uk/forums/showthread.php?t=7371

Hi Alga good site,
Still trying to get my head round it all (too much info:eek: :eek: ).
I'm going to remake my top wishbones soon and need to readup on this before I fab them.
But I actually found out what "bump steer was this weeend".:D
I fitted my new wishbones and moved them throughout their travel with the shock on and noticed the wheel moved in and out...LOL

Johno

Dont panic the steering rack has to centralised mm perfect before you can run that check.

Edit ,another usefull site http://www.optimumg.com/OptimumGWebS...me3_legacy.htm

Bob

Alga, how much caster have you calculated for the original setup?
After a bit of work on CAD, i came up with this rough figure.
I assumed a vertical distance of 200.5mm between ball joints (according to this drawing) and a slight divergence between the upper and lower wishbones towards their outer ends (ball joint ends).

According to my calculations, by offseting the wishbone chassis mount by 2.5mm you increase caster by 0.7deg and by making a new wishbone with 10mm more offset you get 2.8deg of caster! For 5mm offset wishbone you will gain 1.4deg

Original setup: 5.73deg
2.5mm offset chassis mount: 6.43deg
new wishbone with 5mm more offset: 7.14deg
new wishbone with 10mm more offset: 8.54deg

Reading a geometry article by Nigel Dean handed out by Track Developments at a Kit Car show he recommends 3 to 3.5 deg for a 7, 5 to 7 degrees for a Cobra
How much are you guys looking for ultimately?

Self-centering is a safety and stability feature.
If you don't get any sufficient self-centering effect with 5.5deg then you have to go for more.
I think that the best solution would be a new top wishbone with 10mm more offset and rose jointed so you will be able to fine-tune the caster angle according to your specific needs.
I will seriously think of making a pair for my car as soon as i finish building it. Maybe earlier than that..

Thats exactly what I intend to do. The book bones needed an ammendment yonks ago to at least put the caster situation in the picture. Its like the rack being 27mm to long, why have the chassis brackets not been altered 13.5mm on each side to put the bump right. Surely just a few bits like this is going to make a huge difference. Improve the breed with a tinker:)

bob

you may as well ditch the sierra upright and go for some light cortina rep front uprights and redo the whole lot in one go

you cant polish a turd and rolling it in glitter is messy :rolleyes:

A first mock-up: