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Tired torsen? 4 channel wheel speed logging

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Tired torsen? 4 channel wheel speed logging

 
Old 04-15-2019, 05:51 PM
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Default Tired torsen? 4 channel wheel speed logging

I had my first sprint (for both myself and the car) yesterday and it was my first time launching the car. The car did well, but one issue stuck out in particular, it sounded like it was doing some severe wheel hop off the line. This was evidenced by me having one of the slowest 64ft times of everyone there, 2.75s was my best!

A friend filmed the start and you can see there is what appears to be a torsional mode of the diff with the right rear (and presumably the left rear in antiphase, since the engine speed seemed smooth) visibly speeding up and slowing down.


Pertinent car info
FM2 with GTX2860 ~280whp here
Supermiata big grip kit 700/400 and race alignment
15x8 with 205 Federal RS-R (yes I know but the tyres on my 9Js weren't legal)
6 speed w/3.6 torsen type 2
BBFW low NVH mounts and diff fillers

I have datalogging with wheel speed sensors at each corner and this is what it showed (note, may not be same run as the video, but it did it every time)



Frustratingly, the data doesn't look to be logged at a high enough rate to determine whether it was in phase or anti-phase (it says 25Hz, but that's just the polling frequency, it only updates at 12.5Hz) but this isn't what I would have expected. Has anyone taken similar data that could show to compare?

The odd behaviour doesn't end there, the first turn (visible on right of above plot where MAP goes positive again) is a 3rd gear left hander and it looks like the diff is having the same torsional mode but at times goes open. It seems to me that having a torsional vibration of that kind of amplitude is just going to break the rear tyres loose all the time like an impact gun.

I've only ever driven this car on the Supermiata Big Grip Kit and I've never been fully comfortable with how it's driven. I've changed the rake, the alignment, the ARB settings, swept front to rear damper clicks, tried 225/9, 205/8, 195/7 road tyre (wet), removed and checked (measured stiction) at every bush, changed all ball joints and never been happy with how the car transitions from off power to on-power mid corner and has zero traction on exit. Every time I rake tune, I end up with the rear so much lower than the front I tell myself that's ridiculous, can't be right and raise it back up. I even tried rear ARB off and it was only a tad pushy. Whereas I tried this set of dampers (but stock ARBs) on an otherwise stock NB with supermiata race alignment and the same 205/8 Federal RS-R and it was by far the nicest handling Miata I've driven. So I'm hoping the diff is the cause of all of this!

Anyway, here's a video of the sprint. I managed to win the class (A5 - 2WD >2301cc road cars) despite just running the car I had, rather than building one to the rules. The driver of the GT3 RS in second place didn't seem best pleased with a rookie in a 25 year old Mazda beating him!

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Old 04-16-2019, 06:40 PM
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In an attempt to understand what is actually going on, I've exported the data to Excel and had a play around with it. Having the lines on top of each other let me see a few things



1. When I've almost released the clutch, the revs reach their lowest and I'm starting to add TPS, (31.2s on plot) the rear left and right wheels (red and light blue) lines start to oscillate in anti-phase
2. This oscillation gets worse and worse and by 32.3s on plot the oscillation in phase. I'm not even sure what could be causing an oscillation this big at this kind of frequency. It doesn't appear to be clutch because calculating the speed of the gearbox output/diff input shaft from the rpm channel gives a value that appears to be close to a time averaged value of the left and right wheel speeds. I have to back out to <25% TPS for over half a second to bring it back under control. Could this be a severe case of traditional wheel hop and the entire drivetrain/PPF is bouncing about? The mode is ~9Hz. This is with BBFW low NVH engine mounts and diff fillers in the rear. Gearstick vibration is visible in the onboard, but I've not yet done any calculations as to whether that kind of vibration could cause this kind of speed fluctuation.
3. When I shift into second, as soon as I tip in, the same behaviour resumes
4. The GPS speed lags the front wheel speeds by 0.2-0.3s which seems high!

It turns out the wheelspeed data was logged at a true 25Hz, but I can turn it up the logging frequency 500Hz. There probably aren't enough teeth to get usable data at this frequency and speed but I'll get some measured, though I'm away for a while so can't do it soon.

I'm struggling to take conclusions out of this data though.
1. Is my torsen broken? If yes, maybe another torsen would be enough for me for the moment, if not, I'd better get eating ramen and saving for an OS Giken.
2. Maybe I need stiffer diff and engine mounts, but the oscillation may be exacerbated by the torsen being broken?
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Old 04-16-2019, 09:05 PM
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Wheelhop has nearly nothing to do with the diff and nearly everything to do with bushing compliance, suspension flex, tire stiffness, and driving skill.
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Originally Posted by codrus View Post
Basically I've come over to the camp of "If something is a reliability problem on the track, just ask Andrew and do what he says".
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Old 04-17-2019, 08:32 AM
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The reason why you like the stock powered car so much is because you can drive the throttle like a lightswitch. Go from 0% to 100% throttle coming out of a turn and nothing weird happens. Part of it is just realizing that you're not going to be able to put the pedal straight to the mat coming out of lower speed corners in autocross environments.

Wheel hop can be helped with better suspension bushings, better diff bushings, better engine mounts (the good kind, not MazdaComp or stock).

Your torsen could also be tired but if you're looking for outright performance then maybe consider just going straight to the OS Giken. One of the downsides of the torsen is that if one wheel gets really loose, it can't recover from it and suddenly "lock up", it'll just let it spin, so if one wheel has much less weight and you throw all 300WHP at it at once, well, game over. Throttle control is key.
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Old 04-17-2019, 11:17 AM
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OS Giken will be far easier to drive than Torsen with 300whp. As Marcus pointed out, the Torsen has a more abrupt nature when one wheel begins to slip. The OSG masks that. Customers often ask us, "what will the Giken feel like?". Our answer is that when a diff is working correctly, you don't feel anything. The car just accelerates and turns according to throttle inputs and is more stable overall.

Torsens do wear out. Only two sources in the US I know of that service them. Both are Spec Miata shops in the midwest. Search spec miata group on facebook for more info on that.
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Old 04-17-2019, 06:09 PM
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Originally Posted by emilio700 View Post
As Marcus pointed out,
flier129 change your car back plz, we must look too similar now lol.
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