75 3.0 - 29mm front anti-roll bar

[Colin Edwards]

Back in 1976 I bought "How To make Your Car Handle" from The Technical Book Shop in Swanston Street, Melbourne.  Still have the book but Technical Books is sadly long gone!  A good book for its day and most of the content still relevant - as long as your not tuning a FWD car!

Anyway, while under the 75 the other day bleeding brakes, I got to thinking about "Roll Stiffness" and how its applied to the TB front end of the 75.  Given how close to the centre line of the wheels the "springs" are determines effective roll stiffness, where do we assume the effective "spring" location is on an Alfa TB equipped car?  Methinks the effective location is somewhat inboard compared to a traditionally coli sprung front end.  This location would tend to reduce the "efficiency" of increasing spring rate.  Thoughts?

[Duk]

Front suspension verticle forces are put into the chassis via the lower control arms.
So efectively the 2 chassis rails are pushed upwards where the LCAs attach. This verticle movement is some what resisted by the diagonal box sections that go from down near the engine mounts to A pillars at the base of the windscreen.

It does look like the chassis is subjected to a lot of torsional loading being a torsion bar front suspension and plenty of people have said that the chassis isn't particularly torsionally rigid.
Fatter front antiroll bars will increase the twisting forces being applied to the chassis.

I was thinking some time ago that a horizontal brace that looked basically like ladder laying on its side, attached to the chassis, in front engine, but as close as possible, could help.
Basically the front of the chassis does very little to resist torsional loading. By adding a brace that could help there, the front could become more effective.

[Colin Edwards]

We found the increased roll stiffness and increased spring rate due to thicker torsion and anti-roll bars exacerbated the tired damper issue.  Kissing kerbs to straighten a corner unsettled the car a tad more with the stiffer front anti-roll bar.  The poor old Koni's were now not providing enough rebound damping. 
A closer look at the dampers revealed "adjustable" on the tube - so out they came!  Seems they were set around 50% stiff - similar setting to the yellows on the 159.  I have now reset the adjustment one half turn from full stiff on the rear and one third turn from full stiff on the front.  A HUGE improvement.
Not tested on the track yet, however on the road the car is SO much more settled over bumps.  A LOT less crashing about and general pitching and yawing.  A quiet bit of "test track" close to home suggests car now a lot happier riding kerbs mid-corner.

Cant wait to try this at Broadford shortly.

[Colin Edwards]

Debrief with driver post Broadford confirms the damper adjustment is close to ideal.  Significantly improved control and spring ringing / oscillations reduced.  Recent brake upgrade allowed driver to attack corners a bit more aggressively.  Respectable lap times given self imposed rev limit of 4750.  A tendency for subtle oversteer on corner exit.  Maybe I will need to consider altering the roll axis!?!  Thoughts? 

Driver suggested a very minor increase in front rebound.  Re-set front dampers 1/8 turn stiffer.  Ride on the road appears firmer but still very ok for general use.  A punt up and down my local test track confirms front end stability seems better. 

[Duk]

Are you using the same offset wheels front and rear?
If you are, increasing the rear track width using suitable spacers, to match the front could be an easier initial option than changing the rear roll centre height.

Making the rear bar adjustable so you could slightly soften it could help aswell.

What are your rear spring rates?
Around 175/185lb/in should give a rear natural frequency about 10% higher than the 28.7mm TB in a full weight 3 litre (heavy bumper bars) and that's right about where you want to be.

What bumper bars are you using?
If you're still using the impact type, get rid of them and use the lighter ones (especially at the front). That'll bring the rear pollar moment a bit further forward and may help a touch.

Have you sorted out the bump steer yet? Inconsistent steering behaviour could be misleading the driver.

[Colin Edwards]

Hi Duk,

Yes - same offset wheels front and rear.  To match the front track I'd need to fit 14mm spacers to each rear corner.  A not insignificant amount.  The Simmons wheels are pretty fat in the hub so longer studs would need to be fitted.  I've seen studs shear and that worries me!  Still a very minor reduction in lateral weight transfer due to the wider track may be worth testing.  Anyone know where longer (and tougher) studs can be sourced?

OKP do a nice looking rear bar with 4 settings.  That could also be worth testing.  This particular bar features rose jointed drop links.  Ok for a race car - high maintenance on a road car.

The rear springs are Eibach EW1004002HA.  Couldn't find a published spring rate but a quick measure and calculation puts them at around 178lb.  These springs reportedly lower the rear by about 30mm.  Given the 16" wheels and lowered ride height I figure the rear COG is near stock but the roll centre is say an inch higher.  Reduced rear roll couple.  A subtle reduction in rear roll stiffness is worth a shot.

I'd love the 75 to be 100kg lighter and going to the lighter bumpers is a step in the right direction, however this is primarily a road car so we'll have to compromise there. 

Haven't looked at bump steer at all.  Given the relatively high front roll stiffness, tie rod vertical deflection is relatively low.  Longer tie rods and shimming the steering rack is a low priority for the moment.

We'll see what the slightly stiffer front damper setting does for us on the track.  I'll also investigate longer wheel studs at the rear.  If nothing else, the wider rear track would look cosmetically better!

The OKP adjustable rear bar is tempting.................................!

[Duk]

Perhaps getting different offset outer rims for the Simmons could be done?
Maybe less work and cost than longer studs, because as far as I can tell, you can't remove the studs from the hubs without removing the hubs from their bearing housings.............. And I believe that would equal new bearings.

And don't be put off by rodends in road cars, just do what should be done and get rubber boots for them. Car manufacturers have been using tierod end style antiroll bar links for well over 20 years. Low and behold they have rubber boots on them. 

[Colin Edwards]

A closer look at the 75 workshop manual suggests only 10mm difference between front and rear track - 5mm per side.  I'd prefer to have all wheels identical given I usually rotate tyres.

Appears Alfaholics do titanium studs for the 75 (and lots of other Alfas!) so a selection of relatively thin spacers may be an option.  Removing the hub to get the studs out does look like a fair old job!! 

The OKP adjustable rear bar does look attractive - good advice re the boots, thanks!

First thing though is to determine a way of defining what is actually the root cause of the very mild corner exit oversteer.  Although inherent in most sedans, clearly and reliably defining its cause can be a challenge.  Too little weight transfer or to much?  Excessive camber change causing insufficient rubber on the road - very likely!  A slightly wider rear track may reduce the positive camber just enough.  Decisions  decisions! 

I do miss karts!

[Duk]

The live axle does an excellent job of keeping the rear wheels perpendicular to the road surface despite body roll.
Requirements of mild camber and toe angle changes are to deal with tyre distortion than trying to compensate the wheel getting pushed into positive camber.

Perhaps a slight increase in tyre pressure to help reduce tyre distortion?

What are you using for the main dedion pivot, rubber, polyurethane or spherical bearing?
Same for the Watt's linkage?
Spherical bearings everywhere will provide the best possible location of the rear suspension and also allows it to move thru its travel more freely than bushes do.
Thought being that, potentially, the main bush could be distorting slighly, especially with a tight LSD. The rear wheels will want to go straight ahead, potentially distorting the main bush in a sideways manor. For all intents and purposes, the outside wheel could be developing a toe out and the inside wheel toeing in. If it was happening like this, it would only be small amounts, but that's all it can take to upset the balance when you're near the limits.

Excuse the Solid Works drawing...............  But I hope it helps get it across what could be happening if you have a rubber main bush.

[Colin Edwards]

We found 44 cold < 47 hot the best tyre pressure compromise.  Car definitely sensitive to pressures!

Not sure what the main DeDion bush is - haven't got that far under the car........yet! 

The existing Watts linkage bushes are definitely all polyurethane - probably around 90 duro.  Installing the spherical SZ like Watts crank bearing is definitely on the to do list! 

Replacing the main DeDion bush looks like major work.  If its still the stock rubber item I'll very seriously consider going poly.  If its the original - out it goes!  Centreline Alfa do an SZ like spherical DeDion bearing.  The SZ certainly handles well.  A plain spherical / uniball bearing could need regular lubricant if it doesn't feature some form of polymer interface though!

The "feeling" of the rear end on corner exit is certainly not unlike an over tight LSD. 

[Colin Edwards]

The front DeDion bush was rubber but not any more!  Installed a polyurethane replacement - the existing one was probably the original.  Also fitted 15mm spacers to rear hubs, longer wheel studs and new bearings. 
The resultant 30mm wider rear track seems to have improved corner exit traction a bit.  Car definately more stable at speed.  Full polyurethane rear axle location via the front DeDion bush and Watts Linkage seems very effective for a road car.
Needs a run at Broadford or Winton to properly evaluate.