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  #16  
Old 08-18-2004, 05:35 AM
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JDW-SVX JDW-SVX is offline
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Well - I can't figure this one out!

If the noise disappears when the DIFF LOCK fuse is inserted, then it would seem that the noise is being generated by the rotation of the planetary gears - as these seem to be the only gears inside the transmission whose movement is affected by that fuse.

Is the thumping noise repetitive, or is it a single thump when you tap the brakes?

The only time that I experience a thump in my drivetrain or transmission is when the car is coasting with no throttle applied, and the throttle is quickly reapplied. This, in effect, takes up all of the "slack" in the drivetrain at once, and causes a soft thump.

It may be worth fitting the standard set of wheels and tyres to your red SVX (if possible) to see if this has an influence on this problem!

......................did any of that make sense?





Jason.
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  #17  
Old 08-18-2004, 06:02 AM
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thumping sans Diff Lock

Jason thanks for the quick reply, I was intending on doing the re test with my original good set of std rims, but in the interim I was going to slip the 18s on my green car across to the red and vice versa onto the green to see if the effect is duplicated when the staggered set are fitted to the green car and whether the effect is apparent on the red car when 18s [same size fr to rear are fitted]..some variables to cover..
The sensation is like you describe only when brakes are applied 60kph and over, its like some one has tapped/run into the car in the rear, if you apply the brakes on and off [just tapping the pedal no deceleration as such] it thumps away on each application..it only thumps the once when the brakes are applied..yet wth DIFF LOCK fuse in place braking is sweet, no thumping sensation at all....but how does this sensation of AWD cutting in and out [if thats whats really going on..] how does that relate to wheels tyres...

I did the static 'standing height' front to back tonite on the staggered wheels the backs do stand 0.25inches [~ 6mm] higher than the fronts ...allowable tread wear variance on tyres is around 8mm..I'm aware as per threads elsewhere here that AWDs are fussy with air pressures and tyre tread wear but 1.02% variance in size [0.25"/24.5" x %] ...are we really that sure that a manufacturer would design a system with that closer tolerance...what about bumps in the road, turning corners, turning corners in the wet !!! the varainces just keep coming...ANYWAY !!

More ideas on why the AWD system would cause the thumping sensation ..assuming DIFF LOCK test has identified this as the culprit.

David
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  #18  
Old 08-18-2004, 06:28 AM
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OK - this is going to sound very weird, but here goes......!

If the thumping occurs when the brakes are applied, but not applied hard enough to cause any deceleration, then it is possible that the clutch which controls the planetary gearset is engaging, and locking the diff when it shouldn't be.

It is possible that the slight difference in the rolling diameter of your front and rear wheels could be fooling the TCU into thinking that wheelspin is occuring, even when it is not, and causing the planetary gearset clutch to engage briefly, to vary the torque split, or to lock the diff completely.

As before, since the thumping does not occur when the diff is constantly locked, I believe that the thumping is due to the clutch engaging at the wrong time.

.................I am probably completely wrong about all of this..........



Jason.
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  #19  
Old 08-18-2004, 07:44 AM
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Re: thumping sans Diff Lock

Quote:
Originally posted by grnSVX96

I did the static 'standing height' front to back tonite on the staggered wheels the backs do stand 0.25inches [~ 6mm] higher than the fronts ...allowable tread wear variance on tyres is around 8mm..I'm aware as per threads elsewhere here that AWDs are fussy with air pressures and tyre tread wear but 1.02% variance in size [0.25"/24.5" x %]

More ideas on why the AWD system would cause the thumping sensation ..assuming DIFF LOCK test has identified this as the culprit.

David
It's the difference in your tires.....
1/4" difference in height on a tire that is about 24" diameter compared to a 24 1/4" diameter is converts to a rolling difference of 75 3/8" compared to 76 3/16 or 13/16" difference...Subaru specs less than a 1/4" "Rolling" difference, not standing height difference, so your tires are what destroyed your planetaries....

My thoughts on it....not that they're worth much
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  #20  
Old 08-18-2004, 07:50 AM
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Unhappy Thumping etc

I just finished reading lots of threads under 'Diff Lock" search, quite a few from Harvey, re this subject..as well as quite a few from folk in England [same box...some in similar predicaments to mine..with box repaired and few drama...etc, ...the more I read though the more your last reply could be right..I misunderstood the Difff Lock to be like as described for the US models [isolating AWD making FWD...but the fuse in ours does what it says its labelled to do..it locks the diff holding the car in AWD [thats now my new understdg]...I did notice with the fuse in binding in slow turns, but definitely no thumping under brakes...I was hoping that we may have been able to isolate/identify this whilst he car was driven in the air ...but the rolling diameters of tyres driving on thin air ain't gonna do nothin'...bugger...but then if the thumping sensation can be seen in the air then the wheel/tyre thing is knocked out...I will show the AT guy tomorrow the problem and see what he diagnoses it as ....he's seems to know a great deal about our boxes...so I'll get his 2 bobbs worth..

..I did read thru Harveys and some of other threads about Duty "C" may cause some TCU mixed messages as well ..??? Will keep digging !!

David
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  #21  
Old 08-18-2004, 08:02 AM
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Question Huck Subarus 0.25inch

Hey Huck

Thanks I think...where did you get the 0.25" figure from Subaru, and excuse me for my rusty brain the diameter converted to circumference was what old formula ....like 2 pie r squared..I know thats not the one ??

FYI your thoughts/diagnosis in this case worth around $2.5K AUD, plus blowing hard earned cash on wrong wheels [hmm about the same price as the AT rebuild] !!...

David
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  #22  
Old 08-18-2004, 08:10 AM
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Hey David,
The BEST way to get the rolling diameter, is to raise the car and use a flat measuring tape (a seamstress tape works great), and measure the actual circumfrance or each tire around the center.

But the formula is "Diameter X Pie = Circumfrance"......the squared gets you the area, which you don't need

Hope this helps clear it up

I've been were you are....that's why I've got a 5-speed in mine now, wasn't going to replace my auto box again!
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  #23  
Old 08-18-2004, 08:36 AM
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Question But wait there's more.

re wheel diameters etc ..thanks for the formula the one I quoted was the old radius, so 2 of them = circumference, yes I skipped the squared bit...thanks for that...now back to the thumping...

If I do adjust tyre pressures then as I get the pressures/diameters as close to fr=back...then the thumping ..'planetaries etc' engaging should be less..?? it follows

...however tonite I have already got psi @ 40 fronts and 35 psi rears ..[tyre diameters measured as above] still thumpin'..I know futher back in the thread that some of the cars are upto 8psi apart [same tyres f to b]...so thinking whilst writing if my tyres are taller/longer on the back then they will need lesser again than OEM specs for the rears so as to compensate for shorter rolling diam on front because of the weight of the motor..?? OEM ~ 10% more air in fronts....low profile vs lower air pressure, thats not usually recommended ???

David
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  #24  
Old 08-18-2004, 08:40 AM
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Quote:
Originally posted by huck369
Hey David,
The BEST way to get the rolling diameter, is to raise the car and use a flat measuring tape (a seamstress tape works great), and measure the actual circumfrance or each tire around the center.

But the formula is "Diameter X Pie = Circumfrance"......the squared gets you the area, which you don't need

Hope this helps clear it up

I've been were you are....that's why I've got a 5-speed in mine now, wasn't going to replace my auto box again!
I'm wondering if it might be more accurate to mark a front and rear tire at say "6 pm", mark the ground at the same location, and then push the car forward until each tire has made several revolutions and the marks are now back in the start location. This way, the effect of the weight of the car is automatically taken into account when measuring the rolling circumference. I'd push it so that each tire does at least 5 revolutions. You'll need a long tape measurer.

Anyhow, I agree with Huck that its likely the difference in front to rear tire diameter that destroyed your center diff. Its more of a hunch though because with the center diff on your car, I can't understand how a different rolling circumference would destroy the center diff gears. Was it the center diff gears that were destroyed?
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  #25  
Old 08-18-2004, 08:40 AM
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I hate to say it, but i think the only fix will be to get the same size wheels and tires on the front and rear...otherwise you'll have another repair bill in your near future.

Air pressure isn't going to take out 13/16" worth of difference
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  #26  
Old 08-18-2004, 10:07 AM
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I agree with that. I would change the wheels. It would not be worth the problems in the future.
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  #27  
Old 08-18-2004, 10:18 AM
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Quote:
Originally posted by mbtoloczko
I'm wondering if it might be more accurate to mark a front and rear tire at say "6 pm", mark the ground at the same location, and then push the car forward until each tire has made several revolutions and the marks are now back in the start location. This way, the effect of the weight of the car is automatically taken into account when measuring the rolling circumference. I'd push it so that each tire does at least 5 revolutions. You'll need a long tape measurer.
I agree with Mychailo that this would be the most accurate way to measure rolling diameter as a tire's shape & rolling circumference is 'deformed' when the weight of the car is applied. Also, the front wheels will be 'deformed' more due to the 60/40 weight distribution of our cars...

-Chike
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  #28  
Old 08-18-2004, 10:21 AM
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g'day grnSVX96

My SVX (also VTD) does a similar thing as yours, the "thumping" noise, that is.

I have always thought, and still do, that the thump, which on mine occurs in specific situations, is the ECU/TCU unlocking the rear diff.

The only diffrence is that mine also does that if I cruise on a highway, and lift of the throttle, but only once (and you cannot reproduce it by try again like it was a mechanical play or something) , you must go steady for a while and then lift off gentlly, then at a certain throttle point ...thump...EXACTLY as you describe it but maybe not so hard as yours.

This should occur when you brake, because the ABS system does not like when the wheels are connected too much.

The ABS system want's to control each wheel independently as much as it could, and not be disturbed by a mechanical connection between them, like a locked diff or anything.

This is very common in others cars to.

If you get a "thump" when this happens, maybe some binding/force is applied to the drivetrain, like diffrent ROLLING diameters or so, and of course if you lock the diff, no release thump would occur.

This "thump" was very clear on the old subies with a mechanical "diff lock" button, or on-demand "4WD" button, if you disengage it whilst driving straight, but with diffrent rolling diameters on the wheels.


I know....I know... my english sux , but anyway, hope you'll understand what I meant ... not what I wrote



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  #29  
Old 08-18-2004, 10:26 AM
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-ooh forgot ...

I have tested with varoius airpressures in the tires to minimize this "thump", and rigth now i'm at 0.2 bars more in the front, and the "thump" is barely noticable now



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  #30  
Old 08-18-2004, 05:18 PM
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On the road again.

Hi Dave good to see you back on the road.

I don't think the thumping that you have is caused by the center diff. More likely slack in the drive line or in the suspension, that winds up,when the brakes are applied.

The fact that the fuse stops it, can be that with the diff locked, the drive line is bound up, to remove the slack.

I am sure it is not in the center diff. The way the diff works is a bit hard to explain, (it still varies the torque without running,) but when the car is travelling in a straight line the diff spins as a solid unit, just like the gears in a normal rear diff. It is only when turning, or wheel spinning that the gears actually revolve.
It would take a lot of wheel spin, front or rear to get the gears to revolve fast enough to destroy them. This is how they broke it in the first place. Lifting the front and towing it on the back wheels.

All AWD Subes must be 'all up lifted'.

The small difference in the rolling dia. of the front/back wheels would not affect it to any great extent, as the diff will accommodate it, as the Limited Slip Clutch is not applied. It is different to the Transfer Clutch system which cannot accommodate too much difference in wheel dia. as the clutch is applied and has to slip to do it.

The fuse is used for running the car on the dyno. It locks the front and rear together to prevent shunting between them, on the rollers.

Don't worry about the AWD, just look for the brake problem.

Harvey.
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