Optimal toe-in

[JuhaV]

Hi,

TIS says that in these cars the front total toe-in should be 0 degrees and 20 minutes (plus minus 8 minutes). As one degree is 60 minutes, this equals to 1/3 degrees of total toe-in. For a single tyre this then only 1/6 degrees.

If measured over 75 cm distance, this corresponds about 5 millimeters as a total toe in difference. DIY measurement principle shown here :
https://v2tre.wordpress.com/2016/01/...svarustus-diy/

Please note that the final measurement should be naturally performed with car resting on its wheels.

QUESTION :

If the front suspension qeometry has been slightly changed (due to lowering the suspension by 30 mm, wishbone rear bushings changed to symmetric PU bushings), would the "optimal" toe-in still be the same as specified by BMW ?

Or has anyone played along with the toe-in adjustment in the front and found something else to work better ? (80 % road, 20 % track use).

Also any other comments and wisdom on changing the front end toe, caster, camber adjutments in these cars would be interesting.

Thanks !

[c_w]

I've always had mine set 0 / parallel

[JuhaV]

c_w, care to elaborate why you chose to do that ?

Or whether you noticed any change when going parallel instead of the slight toe-in as originally ?

Myself I have basically no idea / experience how significant the change would be but have only heard during track days that the original toe-in adjustments might not provide the optimal performance.

However, it is clear that all the mods made to the car make it unique and therefore there might not be any general optimal value to use.

br, Juha

[Dave P]

Here's some details from ACS regarding set up etc,

[JuhaV]

Thanks Dave !

It seems that ACS recommends the same toe-in than BMW originally :
"Track overall + 0 min 20 deg"

Negative (-) toe value = tires point outwards -> toe-out
Positive (+) toe value = tires point inwards. -> toe-in

https://en.wikipedia.org/wiki/Toe_%28automotive%29

Just rechecked mine and it seems that after changing the steering rack during winter the preliminary "wheels in the air" adjustment resulted in slightly negative toe in the actual driving situation now when everything has been settled in place.

Will go back to zero or just slightly positive, ie. toe-in. Most likely the steering will feel more stable after that.

[mnbrennan]

Here's some details from ACS regarding set up etc,

It would seem there's a rubbish angle here

[JuhaV]

I agree, one of the angles seem to be crap.

[Dave P]

I agree, one of the angles seem to be crap.

Is it just a bad translation ??

[c_w]

I run parallel because of the swapped top mounts to minimise tyre scrub. I don't think there's much in it and don't think it drives any different. It's only when you start having a lot of toe in or toe out that you might start noticing the difference?

[JuhaV]

Some further reading on the subject revealed these basic ideas :

- A small amount of toe-out encourages the initiation of a turn, while toe-in discourages it. The toe setting thus becomes becomes a tradeoff between the straight-line stability afforded by toe-in and the quick steering response promoted by toe-out.

- A higher amount of toe (in or out) may start, obviously, to affect tyre wear in negative manner.

- There is a difference between static and dynamic toe. When pushed down the road, a non-driven wheel (= BMW front wheel) will tend to toe itself out. For this reason, a certain amount of static toe-in (positive toe) is used so that in a dynamic situation the toe still remains slightly positive or neutral.

After this, it makes sense to stick around the original small toe-in adjustment or just slightly towards neutral. The target could be to have the dynamic toe to be close to neutral. BMW originally may have designed the toe-in to be on the safe side to provide definitive stability in all possible situations but the M-style driving style could perhaps allow to slightly narrow down this safety zone towards more agility to turn in :)

Mine was "accidentally" slightly negative (toe-out) after the winter tinkering and the steering is really very responsive but not really any way restless at least up to 150 km/h. But will go back towards neutral or slightly positive to test the difference.

br, JuhaV

PS: All this depends of course also on the specific tyres and tyre pressures one is running ...

( more stuff see for example : http://www.enginebasics.com/Chassis%...ing%20Toe.html )

[ian]

I don't know much about this subject, but logic suggests that the wheel/tyre should be pointing up the road with tread evenly placed on the road. This would give optimum adhesion/grip in a straight line. I can see that toe out might ease turning on one wheel but would therefore "scrub" the opposite tyre ie right hand turn would "scrub" the near side tyre. Also toe out would likely cause additional edge wear in a straight line on both tyres. If this is so, seems to be uneven tyre wear for the benefit of better turn in.

Is this logic sound for us non experts?

[Dave P]

Fahrachswinkel - Driving axle angle

[JuhaV]

After trying a couple of toe settings here is what I found out in my car.

Please note that I have a 30 mm lowered suspension (ACS), symmetric PU bushings in the front wishbone rear ends and PU bushings at the rear subframe (the big ones). The track width is also slightly wider both front and rear due to custom Braid wheels. Steering rack is also faster (and linear) compared to OEM (2.8 turns vs 3.2 turns lock to lock).

During the winter when changing the steering rack I adjusted the toe to be about right. Checking this now with wheels on the ground showed that I was actually running a slightly negative toe that is toe-out about - 0 deg 15 min. This made steering to feel like "too much power assistance". Not really restless but a bit too much for high speeds (+140 km/h).

Changing toe to parallel (0 deg 0 min) felt like "reducing the power assistance" a notch. Still very easy to turn into a corner but sligthly easier to run straigh without any effort.

Changing the toe sligthly positive that is toe-in to about +0 deg 15 mins still "reduced the power assistance" and made the steering to feel more solid especially at higher speeds. Another unexpected effect was that the car feels more stable during hard brakings. Still very agile to change direction and turn in.

So it seems that the slight amount of toe-in (as factory specifies) seems to have its benefits in the high speed directional stability but also during heavy braking.

I settled with this 0 deg 15 mins which is slightly less than the factory 0 deg 20 mins but still within the "error range" plus minus 8 minutes.

The car feels now very good and the quicker steering is a really positive modification. Suits without question also to street use. BMW was perhaps overly careful when selecting the slower and non-linear steering rack for this cars as original equipment.

br, Juha

[c_w]

The 2.8 turn steering rack is a good swap and one of the first things I did to mine. Also removing the play in the rubber steering column joint helps precision too.