8 replies to this topic

[mariner]

The question is "why do test reports on EV's so often criticize the pitch sensitivity?

 

Here isa quote from long term test of the Audi SQ8 E  Tron, which weighs 2.650 kg unladen and has a 2.9 mtre wheelbase. 

 

Yu might expect it ride well given it has a massive 106 KWH battery slung low down but it apparently  has  a problem.

 

"My problem with this car's gait, as with many heavy EVs, is its tendency to pitch uncomfortably end to end on a particular kind of lumpy A- and B-road common in the UK. Many EVs do it; this one more than most, with less excuse because of cost. Weirdly, no one in OEM-ville even acknowledges this condition, let alone fixes it. But at times I found it excruciating"

 

Any thoughts why these things have poor pitch ride ?

Edited by mariner, 23 July 2024 - 13:04.

[Bloggsworth]

Perhaps because many of them are indeed overensitive to pitch - perhaps they need a modified version of htydrolastic suspension to damp the instability.

Edited by Bloggsworth, 23 July 2024 - 16:06.

[Greg Locock]

German engineers aren't especially good at ride for third world roads. It certainly doesn't seem to have been a priority for BMW in the last 2 decades (more or less when they introduced run flats) they decided they could make more money by making their cars into Timezone clones.

 

"2.650 kg unladen and has a 2.9 mtre wheelbase."

 

OK, that is slightly shorter and much heavier than my Everest.

"as with many heavy EVs, is its tendency to pitch uncomfortably end to end"

 

Well that's a useful observation, but without numbers it doesn't help me much. I should point out that a large part of development engineering is converting subjective observations into relevant and repeatable measurements. That's more or less a done deal for primary ride, but secondary ride (impact harshness et al) is still a work in progress. Pitch would be primary ride, hence calculable. Rather excitingly you don't need a full vehicle model for primary ride, an elegant spreadsheet is sufficient for at least a first pass solution.

Edited by Greg Locock, 24 July 2024 - 00:05.

[gruntguru]

I can't see why pitch sensitivity would be a characteristic of EVs. Common sense suggests a high polar moment (y axis) and/or short wheelbase would cause it. High CG wouldn't help - generating greater pitch moment under braking and acceleration.

 

None of these 3 would seem to be necessary characteristics of an EV. The heavy bit is typically located within the wheelbase and low.

 

More likely the problem is down to chassis tuning decisions.

Edited by gruntguru, 24 July 2024 - 04:14.

[mariner]

Greg, I think you have just explained the Audi  ride problem 

 

Rather excitingly you don't need a full vehicle model for primary ride, an elegant spreadsheet is sufficient for at least a first pass solution.

 

 

What German auto engineer would use  something so simple as a spreadsheet  to do design work!

Edited by mariner, 24 July 2024 - 11:00.

[Greg Locock]

A friend of mine at Toyota had a full kinematic model of a double wishbone suspension in Excel, complete with animations. Bored engineers do strange things

 

Thinking about the pitch thing it could be related to the ratio of pitch moment inertia to vehicle mass*wheelbase^2, where I suspect an EV has a relatively low index. But that is a wild stab in the dark.

[mariner]

Greg, I THINK you ar saying that an EV with so much battery weight close to its CG has a very low moment of inertia in pitch so is confirming Olley's k^2 ratio analysis?

[gruntguru]

So a low value of Greg's "Index" would yield a high natural frequency in pitch, which should make it easier to control pitch oscillation - no?

 

OTOH a low value will see pitch angle tracking the road profile more accurately and responsively (which might be uncomfortable)

[Greg Locock]

Bastow has a good discussion of it. As that index drops the car is more susceptible to pitching, which Olley and everyone else agrees is the worst side view ride problem (head toss and being generally thrown around on rough roads is probably a bigger issue, but let's stick with side view). Primary natural frequency of a seated human is 5 Hz, which includes head nodding. So yes, you want to keep the pitch frequency low (and it is) but I suppose the thing that makes it more complicated is that you don't have a pure bounce mode and a pure pitch mode, you typically  get two bouncy-pitchy modes with the main differences being the frequency and the location of the centre of rotation. Typically one of the modes is more rear axle deflection, and the other is more front axle deflection.

 

So in ADAMS you get this as part of the static vehicle characteristic (not what it sounds like) report

 

  Bounce node loc. wrt H-pt.    mm            2451.32                       

    Bounce natural frequency      Hz               1.24                       

    Pitch node loc. wrt H-pt.     mm           -1039.18                       

    Pitch natural frequency       Hz               1.50                       

    Ride frequency ratio                           1.16  

 

So by convention it calls the lower frequency mode bounce, and the node (centre of rotation) is 2451 mm behind the H point of the driver, the H point  being the thigh to hip joint. The further the node is from the h point, the more the mode resembles pure bounce. As you can see neither frequency is anywhere near 5 Hz, so pitchiness should be ok (as it turned out that is true). So, if you managed to get the node located on the shock absorber, it would see no motion, all the damping for that mode would have to be on the other axle. So the pitch mode is certainly going to be more responsive to rear shock tune in this case.