Following on from problems with handling on my car following gearbox and diff upgrades (see http://bbs.22b.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=23;t=000113 for details), I'm considering fitting adjustable top mounts to allow me to adjust caster.

What does caster do? Is it purely adding (or removing...) camber when turning the wheels away from straight ahead? And what effect on handling? Sharper turn in?

Also, I have an ALK which adds caster I believe (but is also pro-dive). Should I be removing this once I can adjust caster from the top mount?

Lastly, I've read that I shouldn't use adjustable top mounts to adjust camber, as this raises the CoG. If this is the case, why do people make top mounts which are camber adjustable?

Thanks, Richard

I asked this and didnt get much response.. but a good link was posted.. check the thread I started in here :)

Hi Dowser

Interesting questions.. (apologies david, I must have missed your question).

Caster is (as you suggest) mostly a way of creating dynamic camber through the steering. As with all these things it isn't quite that simple, but that's the main benefit.

The great thing about caster instead of static camber is many-fold...

Firstly, when going in a straight line, you can run less static camber so you will have more traction and better stability.

Next is a little more complex, but not hugely. With static camber, when you turn the steering wheel left, the camber does change (if you assume zero caster), so the left wheel stays cambered inwards (-ve)... what is really nice about caster is that the left wheel will camber outwards (towards +ve) when steering left, and the right wheel will camber more -ve, which is exactly what you want.

This means better turn-in bite at the front.

What you lose (there is always a "but" with vehicle dynamics) is a little bit of self aligning torque which can lead to a slightly more unstable front end.. although front end bite is always going to have to trade off front end stability, so I guess that's obvious.

Regarding adjusting camber at the top altering the CofG.. this is not true. But it will alter the roll centre, but probably not enough to worry about.

Saying that, I really wouldn't run that much -ve anyway.. it's just not necessary unless you are about to compete in the LeMans 24 hour or something :D

All the best

Simon

Simon expressed that very well.
I have MRT adjustable top mounts on my hill climb Stiv3, and the Whiteline ALK, and camber bolts (etc), all in the quest to kill understeer.
It has.
I have max caster for the correct resoning above, AND a huge neg camber all round.
My settings are straight from Whiteline tech help line, and are quite radical.

After a hard hill climb on semi slicks, the tyres are all hot and that heat is even across all the tread width which indicates something is working well!

Tyres are Kumho 17 x 205 x 40 @ 35 psi all round. The car has every Whiteline trick on it I should add and some other bits too....

In hindsight, i should have removed the ALK as it puts the wishbone front bushings in bind, too much friction. They are coming out this winter.

Hope this helps.

911

Hi all,

Adding caster has 1 main advantage. It will increase the weight on the inside turning wheel e.g. it pushes the inside wheel down further. This will normally increase front end grip and traction.

Adding caster will add asmall amount of negative camber on turns although this is quite small.

Adding caster will make the steering heavier and increase the self centering action. It may also make the steering feel a bit 'wooden'.

As for using top mounts to alter camber and caster angles, no problems as far as i am concerned. On some cars it might provide problems with driveshaft 'plunge' though and some machining may be required.

In the real world I would expect a Scoob to be quite nice with 6 or 7 degrees caster. It might be a bit of work to get to this number though.

regards,

Job

I think I have 8!
That extra negative when the wheels are in a turn makes the loaded tyres sit flatter to the road as the body rolls and settles.
Can't help but feel such extreme geometry really only shows a real advantage in extreme conditions, Track days, competition etc, not really needed for the road?
Nice to mod a scooby though and find out.........
911

Thanks All,

Great feedback - where's the best place to buy adjustable top mounts? I'm only aware of the MRT items?

Richard

I think I have 8!
That extra negative when the wheels are in a turn makes the loaded tyres sit flatter to the road as the body rolls and settles.
Can't help but feel such extreme geometry really only shows a real advantage in extreme conditions, Track days, competition etc, not really needed for the road?
Nice to mod a scooby though and find out.........
911

8 sounds ok to me for competition car. I guess you have got power steering?Have you measured the camber change on turns at different castor angles? You'll probably find that there is not that much increase. I think the extra front grip and traction is mainly generated by the inside turned wheel has increased load. Worth checking toe on turns with different castor angles. They shouldn't be much different, but you might find a welcome anomalie?

regards,

Job

911 could you drop me an email with where you get your tyres from.. (dont want to polute the thread)

Other than the Whiteline ALK and adjustable top-mounts, are there any other ways of increasing caster?

Other than the Whiteline ALK and adjustable top-mounts, are there any other ways of increasing caster?


On some one-make series racecars we have pulled the chassis on a jig to get the wishbone pick up points and the top of the inner wings to a position that is more suitable. (inner wishbone pick up points up, forward and out. Topmounts in and back).

Sould be able to make a bespoke wishbone for a Subaru that places the bottom ball joint further forward and out, without changing the dynamics to much.

Quite expensive options though. Normally only done on competitive racecars.

regards,

Job

The tyres are Kumho Ecsta V70A, mine on 17's are 'medium' compound. If you run 16's you can have soft as well as med and hard.
The official outlet is based in Shropshire:

Kumho Motorsport
Building 5 Unit 13
Stanmore Ind Est
Bridgnorth
Shropshire
WV15 5HP
Tel: 01746 764434 Fax: 01746 764435

Just 6 miles from me!!!!!!!!!!! That's a first.

My MRT tops came from Teg Motorsport, but they are not MRT dealers anymore, but worth a call. go to www.mrtrally.com.au for a dealer in UK.
The MRT tops give HUGE adjustment in caster and camber independantly, so there is a lot to play about with. Cusco are popular, but I prefer the MRT in Race spec (pillow ball top bearings).

James, I do not think that there is any other way than the tops. the competition route is extreme and ££££££££££££££££££££££££££££££££££££ but I think my set-up is as far as is reasonable. The car looks very knock-kneed at each corner, but does drive well, steering is a bit heavy, slightly wooden, but during a serious hill climb I'm too scared to notice...........!
911

Thanks All, great info :)

911 - did the MRT race top mounts go on in isolation? Wondering about NVH increases....but then I've got powerflex all round already, so doubt it can get much worse....

Richard

NVH? Not worried too much as the poly bushes everywhere, especially the rear diff and subframe kill your senses after 20 minutes!
The strut tops added nothing to the harshness that i can tell.
I do oil them regularly though when checking the oil levels etc.
Been perfect for 2 years, 6000 miles and 24 hill climbs.
911

BTW, I'll be selling my Cusco struts & pillowball topmounts soon. PM me if interested. NVH will be increased in my experience though

am about to try the HKS top mounts (from Graham Goode) - 1 thing I noticed on the MRT site is that they INCREASE the ride height by 10-15mm. Have double checked with HKS and they think theirs reduce height by about 5mm, more desirable I feel! Will soon discover if any NVH issues, mind you, thought the ALK made things a bit "crashier", so wait and see how they compare.....

I have coil-overs on my car so any ride height was adjusted during set-up.
911

911 - what toe are you running at the rear? Am considering going positive to get the rear end to play...but confess to being a little afraid :)

Richard

To be honest, can't remember, BUT, go to www.whiteline.com.au Check-out the geometry they specify for their Extreme hill climb Classic v5. If you dig about the site a bit you will find all the settings I have.
I just took them at face value and the car if Brilliant.

Ralliart in Dudley initially refused to set the car up this way, and I had to sign a disclaimer!

Any probs, come back to me if you wish.
911

I think these are the MRT settings till told otherwise


Front alignment = camber -3:25, toe -2mm, caster +4.2
Rear alignment = camber -2:5, toe -2.5

Mark A