Does GR86 EPAS help with steering self centering?

[Jianlun]

Opinions purely my own from whatever limited experience i have encountered. Raising it here cos wanted to hear if anyone has any prior experience.

Car: 2022 GR86 JDM-only RC-version (bare bones version with nothing basically and steelies hahah).

Premise: i installed my works boss kit wrongly (blame my stupidity and arrogance for not trying to Google Translate the japanese instructions), ie the spline position/angle of the boss kit is not correct leading to the clock spring which is pushed by the boss kit (and thus steering angle sensor) offset to say 1 o clock position (lets say 30deg too clockwise) when steering is straight.

Observation : VSC error came on (duh) after driving for a few seconds after the install. Cos car got weirded out on why it is sensing a non zero angle of the steering yet its g sensors and wheel speed sensors are not reading the car actually turning. But the kicker is below:

Observation 2: The EPAS seem to consistently want to pull the steering to the left (rem i wrongly offset the clock spring/amgle sensor to the right). Which actually means the EPAS is trying to pull the car about 30deg to the left. Its as if the EPAS wants to center the steering ie make the steering sensor read zero.

My analysis/opinion/dismay: I am puzzled why the EPAS would want to pull the steering to zero the steering angle value. I mean if its a fail safe mode/behavior (rem during this time i have a VSC error), i dun see why anyone will code in a behavior to pull the steering anywhere cos this seems dangerous. So i can only wonder whether the EPAS is coded to ALWAYS center the steering (and angle sensor). Meaning, it continuously helps to center the steering, effectively assisting the usual mechanical self centering effect due to caster/KPI/SAI. Which means the steering feel is NOT PURE! A little dismayed about this though, that one of todays "most analogue cars" actually has the EPAS helping with steering self centering effect. Ie what i feeling is not just pure mechanical. Btw this pull by the EPAS seems independent of speed (ie i had to exert roughly the same amount of counter torque to the steering to keep the wheels pointed straight regardless whether i am 5mph or 70mph). Which is scary cos the caster effect at 70mph should be so strong that it would have cancelled some of the EPAS force, ie i wouldnt have needed to counter the same amount at high speeds vs low speeds. This means the EPAS actually is coded to have more effect at high speeds to self center the wheel.....

A little background: i came from the subi wrx/sti world where i had all the fee caster mods, rotated top mounts, ALK, offset bushings just to sqeeeeze out more caster to get more steering "feel" and self centering (and of cause the camber gain benefits haha). Before this were fwd hot hatches with violent mechanical LSDs that will rip my wrist off the wheel at any opportunity (and sometimes the wrist themselves). So, i like my steering feels purely mechanical pls....

By the way i corrected the boss kit misalignment and all good now. No cars were harmed in the course of my stupidity (i hope). But again the point of this post is to question if the EPAS is too ummmm.... busybody for its own good in what is supposedly an analogue car.

[Ohio Enthusiast]

Yes, EPAS uses the motor to self-center the steering wheel, on top of caster effects. This is stated in this Jalopnik BMW suspension article (search for "centering" in the article) and I assume applies to other manufacturers as well.

For your specific case, I wonder if the disagreement between the steering wheel angle sensor and the vehicle yaw sensor led to a "panic mode" that is not representative of the EPAS normal operation mode. The fact that the self centering force was the same across speeds makes me suspect this, as EPAS assist should drop with speed, and it makes sense to do the same for self-centering.

Finally, your concern about "pure" steering feel is a little exaggerated - any power steering system decreases the "pureness" of the steering. The only way to get pure feel (i.e. only suspension geometry effects) is fully manual steering. It's better to look at the final result instead of caring exactly how it was achieved. So far the consensus on the Twins' steering is that it is among the best EPAS and compares favorably to good HPAS.

[Jianlun]

Originally Posted by Ohio Enthusiast

Yes, EPAS uses the motor to self-center the steering wheel, on top of caster effects. This is stated in this Jalopnik BMW suspension article (search for "centering" in the article) and I assume applies to other manufacturers as well.

For your specific case, I wonder if the disagreement between the steering wheel angle sensor and the vehicle yaw sensor led to a "panic mode" that is not representative of the EPAS normal operation mode. The fact that the self centering force was the same across speeds makes me suspect this, as EPAS assist should drop with speed, and it makes sense to do the same for self-centering.

Finally, your concern about "pure" steering feel is a little exaggerated - any power steering system decreases the "pureness" of the steering. The only way to get pure feel (i.e. only suspension geometry effects) is fully manual steering. It's better to look at the final result instead of caring exactly how it was achieved. So far the consensus on the Twins' steering is that it is among the best EPAS and compares favorably to good HPAS.

Yup yup. Thanks! Just a surprise to me. Cheers!


And reminds me about the news or article i read some where that the new lotus emira uses HPAS cos they deemed the inertia of the motors (directly coupled to the steering in the case of a EPAS) too high and muted the feel. Hopefully our EPAS code somehow counteracts this inertia for the better. Well at least we do not yet have steer by wire....