C4 drive train stress?
03-16-2009, 03:30 AM
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C4 drive train stress?
I'm thinking of changing my rear tires on my 1990 C4 to a larger size (from 225/50x16 up to a 245/45x16). To do this and match the rolling circumference of the standard front tires (205/55x16) I have to go lower profile, hence the 45. I have checked this on many internet tire comparison sites, however there will be a 0.7% error (about 4 turns/per km) instead of the standard 0.1% (half turn /per km) error. Will this cause drive train windup/stress or is this within tolerance for everyday driving?
Cheers Scott
Cheers Scott
03-16-2009, 08:56 AM
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Thanks guys, newboy your error is 0.2% according to the tyre circumference check. http://www.miata.net/garage/tirecalc.html
I can get my error down to 0.6% if I can find a 255/45x16 with standard front tires but I don't know if they make them? I also don't understand how the PDAS system works to allow slippage between front and rear??
I can get my error down to 0.6% if I can find a 255/45x16 with standard front tires but I don't know if they make them? I also don't understand how the PDAS system works to allow slippage between front and rear??
03-16-2009, 09:16 AM
#5
Three Wheelin'
PDAS is an electronic system which activates the electro/hydraulic differential locks according to it's calculations based on sensor input. Therefore there are no mechanical restrictions as such, the decisions regarding differential lock are decided by the programming - which default to a 6% margin allowance. The 964 4WD system has no concept of slippage - a differential is locked or it isn't. The advantage of PDAS is that it can lock or unlock the centre and rear differentials very rapidly - as determined by the sensor inputs.
Dave
Dave
03-16-2009, 09:52 AM
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Thanks, very impressive. So in Layman's terms, does the PDAS allow the wheels to turn independently up to 6% of each other. Anything > 6% individual diffs can/will lock to achieve traction where lost? Also what is transmission windup? It was stated in a 964 article from a December UK magazine, it refers to having odd sized tires front & rear on the C4?
Thanks scott
Thanks scott
03-16-2009, 10:01 AM
#7
Three Wheelin'
I think I'm going to regret getting involved in this question because it's going to take me out of my comfort zone!! I will admit to being no expert and a little knowledge is probably likely to lead me into dangerous territory.
My understanding (and please, if someone knows better, correct me) is that the 4WD system is purely mechanical and consists of a torque transfer unit contained within the centre differential which provides a split of 39% (front) and 61% (rear) which drives via a centre and rear differential. Both of these differentials are free and there is no possibility of transmission wind-up because of that. The PDAS system is purely a traction control system and plays no part in the torque split but merely controls the activation of the differntial locks (as explained before).
Different tyres sizes will not cause transmission wind-up - they will merely activate PDAS alarms if the defined margins are exceeded.
Dave
My understanding (and please, if someone knows better, correct me) is that the 4WD system is purely mechanical and consists of a torque transfer unit contained within the centre differential which provides a split of 39% (front) and 61% (rear) which drives via a centre and rear differential. Both of these differentials are free and there is no possibility of transmission wind-up because of that. The PDAS system is purely a traction control system and plays no part in the torque split but merely controls the activation of the differntial locks (as explained before).
Different tyres sizes will not cause transmission wind-up - they will merely activate PDAS alarms if the defined margins are exceeded.
Dave
Last edited by Computamedic; 03-16-2009 at 10:32 AM.
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03-16-2009, 11:23 PM
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RL Community Team
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03-17-2009, 06:24 AM
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03-17-2009, 04:01 PM
#12
I drove with mismatched tyres for about a year when eventually my centre diff starting shreding it's self to pieces which ended up in a chunk of friction plate ending up in the planet gear which promptly exploded.
That is how I know!!!
That is how I know!!!
03-18-2009, 01:48 PM
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found the answer in the achieves from 7 years ago!
851 is the standard pre-programed level in both the C2s ABS control unit and the C4s PDAS/ABS control unit. Front and rear. You have a tolerance of 6%. However one of the speed comparisons is between front wheels and rear wheels. So you can change by 50 but you cannot go down by 50 at the front, and up by 50 on the rear, if you understand what I mean. This will give you a differential of 100.
Try to stay in the 831-851 range front and rear. This is quite easy if you read the tyre specs with what is on the market today.
Ciao,
Adrian
911C4
851 is the standard pre-programed level in both the C2s ABS control unit and the C4s PDAS/ABS control unit. Front and rear. You have a tolerance of 6%. However one of the speed comparisons is between front wheels and rear wheels. So you can change by 50 but you cannot go down by 50 at the front, and up by 50 on the rear, if you understand what I mean. This will give you a differential of 100.
Try to stay in the 831-851 range front and rear. This is quite easy if you read the tyre specs with what is on the market today.
Ciao,
Adrian
911C4
