Help me decide which tires...
02-26-2017, 02:57 PM
#16
Racer
Thread Starter
As I covered in my previous post the wrong sized tires means the front tires are turning more revs per mile than the rear tires which is the *opposite* of what happens when factory sanctioned tires/wheels are used.
The result is while the viscous coupling fluid still gets heated up because different rotational speeds of the two sets of discs in the coupling the discs that are connected to the rear diff turn slower than the discs connected to the front diff.
There is then no torque transfer (5% to 40%) from the rear diff to the front diff. Instead the slower turning discs connected to the rear diff act to brake the discs connected to the front diff. The front wheels/tires then can experience some considerable braking effect as speed increases. Nominally there is 5% to 40% torque transfer from the rear diff to the front diff. I do not know at what speed 5% occurs and I have not come upon a chart/graph that shows how the torque transfer increases with speed only that Porsche in its reference material writes that the 40% transfer occurs at around 150mph.
Maybe one could assume the amount of torque is linear to speed and assume that at say 75mph 20% torque is transferred.
Regardless instead of torque being transffered from the rear diff to the front diff some considerable breaking action is being transmitted.
Around town on dry/good roads even on the highway perhaps this probably won't make itself known, although fuel mileage will probably suffer some. At higher speeds the car may (I say may) experience/manifest weird behavior.
The front end of these cars is pretty lightly loaded and can get even lighter under some conditions. Thus the braking affect arising from the mismatched front and rear wheels/tires can have more effect.
The basic problem is you have configured the car in a way it was not intended to be configured. Thus the PSM and ABS systems are having to operate under conditions they were never intended (or even tested possibly) to operate under by Porsche. You are charting new ground.
Maybe your usage will never have the car misbehave. (Probably helped by the fact the revs/mile difference is so low.) If so that's great. But I see no reason tempt fate with an incorrect selection of tires and I caution people about this when the opportunity arises.
The result is while the viscous coupling fluid still gets heated up because different rotational speeds of the two sets of discs in the coupling the discs that are connected to the rear diff turn slower than the discs connected to the front diff.
There is then no torque transfer (5% to 40%) from the rear diff to the front diff. Instead the slower turning discs connected to the rear diff act to brake the discs connected to the front diff. The front wheels/tires then can experience some considerable braking effect as speed increases. Nominally there is 5% to 40% torque transfer from the rear diff to the front diff. I do not know at what speed 5% occurs and I have not come upon a chart/graph that shows how the torque transfer increases with speed only that Porsche in its reference material writes that the 40% transfer occurs at around 150mph.
Maybe one could assume the amount of torque is linear to speed and assume that at say 75mph 20% torque is transferred.
Regardless instead of torque being transffered from the rear diff to the front diff some considerable breaking action is being transmitted.
Around town on dry/good roads even on the highway perhaps this probably won't make itself known, although fuel mileage will probably suffer some. At higher speeds the car may (I say may) experience/manifest weird behavior.
The front end of these cars is pretty lightly loaded and can get even lighter under some conditions. Thus the braking affect arising from the mismatched front and rear wheels/tires can have more effect.
The basic problem is you have configured the car in a way it was not intended to be configured. Thus the PSM and ABS systems are having to operate under conditions they were never intended (or even tested possibly) to operate under by Porsche. You are charting new ground.
Maybe your usage will never have the car misbehave. (Probably helped by the fact the revs/mile difference is so low.) If so that's great. But I see no reason tempt fate with an incorrect selection of tires and I caution people about this when the opportunity arises.
This is good stuffs. Very informative. Thank you.
@02996ttx50: Yes V12 Hankooks
02-26-2017, 03:50 PM
#17
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well either way if money is of little conesquence then of all the choices for a street tire, the ps2's have to be at the top of any list..and they have far less sidewall flex than the v12's - which you'lll notice instantly when pushing thru corners..
as to tread life.. think i recall macster saying he gets something like 12k ( or even more?! ) from a set of rears... so you'll get a good return in mileage for paying top dollar for the michi's.. i just never get more than 4k miles so "sticky" and less forgiving sidewalls matters more to me,.. less so treadwear/life.
i dont know how its been down in SD for you? but up here my rear tires have been squirrelly as hell after all these rains esp when i hit standing water! r888's in the rear are LOUSY for water, unless hydroplaning is fun
also, dont know if you've given the pirelli p zeros consideration? i think theyre pretty decent, and at a little bit less $ than the ps2's..
just another available option you have...
as to tread life.. think i recall macster saying he gets something like 12k ( or even more?! ) from a set of rears... so you'll get a good return in mileage for paying top dollar for the michi's.. i just never get more than 4k miles so "sticky" and less forgiving sidewalls matters more to me,.. less so treadwear/life.
i dont know how its been down in SD for you? but up here my rear tires have been squirrelly as hell after all these rains esp when i hit standing water! r888's in the rear are LOUSY for water, unless hydroplaning is fun
also, dont know if you've given the pirelli p zeros consideration? i think theyre pretty decent, and at a little bit less $ than the ps2's..
just another available option you have...
02-27-2017, 10:21 PM
#19
Racer
Thread Starter
well either way if money is of little conesquence then of all the choices for a street tire, the ps2's have to be at the top of any list..and they have far less sidewall flex than the v12's - which you'lll notice instantly when pushing thru corners..
as to tread life.. think i recall macster saying he gets something like 12k ( or even more?! ) from a set of rears... so you'll get a good return in mileage for paying top dollar for the michi's.. i just never get more than 4k miles so "sticky" and less forgiving sidewalls matters more to me,.. less so treadwear/life.
i dont know how its been down in SD for you? but up here my rear tires have been squirrelly as hell after all these rains esp when i hit standing water! r888's in the rear are LOUSY for water, unless hydroplaning is fun
also, dont know if you've given the pirelli p zeros consideration? i think theyre pretty decent, and at a little bit less $ than the ps2's..
just another available option you have...
as to tread life.. think i recall macster saying he gets something like 12k ( or even more?! ) from a set of rears... so you'll get a good return in mileage for paying top dollar for the michi's.. i just never get more than 4k miles so "sticky" and less forgiving sidewalls matters more to me,.. less so treadwear/life.
i dont know how its been down in SD for you? but up here my rear tires have been squirrelly as hell after all these rains esp when i hit standing water! r888's in the rear are LOUSY for water, unless hydroplaning is fun
also, dont know if you've given the pirelli p zeros consideration? i think theyre pretty decent, and at a little bit less $ than the ps2's..
just another available option you have...
I have look into other brands and it came down to Michelin & Yoko. It's the quiet and long life tread I'm willing to spend a little more on.
Just curious if I need to take off my 17mm spacers for the 315.
@911mhawk: Did your 315 rubbed? And did you rolled your fenders? Thanks.
02-28-2017, 01:20 AM
#20
^^ This also solves any revs per mile front-rear size mismatch you'd get with the 235s. 245+315 is my AWD track combo too FWIW and the car loves it.
02-28-2017, 02:03 PM
#21
Originally Posted by 996tnz
^^ This also solves any revs per mile front-rear size mismatch you'd get with the 235s. 245+315 is my AWD track combo too FWIW and the car loves it.
02-28-2017, 05:26 PM
#22
Instructor
235/40/18 produces 818 revs/mile.
315/30/18 produces 817 revs/mile. With the Turbo the rear tires want to make more revs per mile than the fronts so the viscous coupling fluid heats up and there is a transfer of torque from the rear axle to the front axle.
If you use the above set of tires while the viscous coupling fluid will get hot from the revs/mile difference (though the difference is just 1 rev/mile which is quite a bit less than the difference with factory sanctioned wheel/tire sets) the front axle tries to transfer torque to the rear axle. This will act to effect some braking action on the front diff and thus the front wheels/tires.
While the car proably won't end up in the weeds or up against a guard rail you have modified the car's dynamics and not in a qood way if you go with this wheel/tire combination.
315/30/18 produces 817 revs/mile. With the Turbo the rear tires want to make more revs per mile than the fronts so the viscous coupling fluid heats up and there is a transfer of torque from the rear axle to the front axle.
If you use the above set of tires while the viscous coupling fluid will get hot from the revs/mile difference (though the difference is just 1 rev/mile which is quite a bit less than the difference with factory sanctioned wheel/tire sets) the front axle tries to transfer torque to the rear axle. This will act to effect some braking action on the front diff and thus the front wheels/tires.
While the car proably won't end up in the weeds or up against a guard rail you have modified the car's dynamics and not in a qood way if you go with this wheel/tire combination.
I don't believe this to be true. I have often read opinions in this forum (and others) that a larger rear will cause some sort of braking force or more generally damage the viscous coupling, but have yet to read any authority for that position. A differential in speed causes the fluid to heat and transfer power, but under no circumstances can this drive train send power from front to rear. If the car is moving forward, then the vanes in the VC are spinning in a forward direction. The 996tt is a rear drive car with assist from the front when the rear spins, regardless of tire size.
I think it is fair to say that excessive difference in tire size (front v. rear) may be damaging to your 996, however, it does not matter whether it is the front or rear is that is slightly larger.
My sources/authority for this statement are:
1) Published Porsche manufacturer specifications (summer spec has front slightly larger, winter spec has rear slightly larger);
2) the service manager for Porsche Bellevue (WA) who confirmed this to me in writing;
3) noted Porsche 996 expert Adrian Streather who states in his book "Porsche 996 Essential Companion" (Veloce, May 2016) at page 259 "There is no evidence to suggest however, that PAG attempted to use different wheel and tyre assembly rolling diameters... No data is provided anywhere regarding the impact of rolling diameter variations and front drive percentages. There is nothing in any factory Porsche AG or ZF [mnfger of coupling] company documentation either. Anything on the internet, in books or in magazines about rolling diameter differentials and which tyres to use is just speculation because it's not supported by any technical documentation."; and
4) an email exchange I had with Mr. Streather where he stated in direct response to this very question: "So to answer your question it doesn’t matter which set of wheels is rotating faster as long as one set is and rotating at a fast enough speed to heat up the VC’s silicon fluid. If the fluid is not heated up you get no drive transferred forward regardless of the tyre dimensions differential. In deepest darkest cold winters this might take a while…"
Until I see some source beyond "it's physics, trust me," I continue to maintain that you needn't worry about whether front or rear is larger, as long as they are close.
I think the counterpoint to the incessant posting that a larger rear will damage the VC deserves to be made.
By the way, I run a larger rear (+0.8%) in winter and stock sizing in summer, and have observed absolutely no ill effects (for whatever a one off anecdote is worth, which isn't much).
02-28-2017, 06:18 PM
#23
There's absolutely nothing wrong with running a 235/40 and 315/30 and anyone saying otherwise is wrong to do so.
02-28-2017, 07:03 PM
#24
NT01s. No issues. I do have rolled guards and 5mm spacers but they are more a legacy of me testing 265s on 10 inch rims once. Can't remember if I first ran the 245s while still fully stock, but at worst they may just need a 5mm spacer for some added clearance to the factory struts.
The front-rear wheel diameter argument becomes an 'angels on the head of a pin' discussion for sizes within a few percent of each other IMO, though I have found torque transitions to be a bit jerkier when the fronts are smaller, versus having smaller rears. I have a few quiet 'reference corners' on the drive to work where I tend to pick up small differences more easily. That makes sense to me too, as smaller rears are essentially already overspeeding the input plates of the VC a little, so a greater degree of slip as they lose traction is just ramping up something that's already in progress, making for a quicker and smoother process than one that also reverses the relative travel of the discs in the VC when slower turning rears suddenly become faster turning rears as they start to spin up.
Rustler, thanks for that research and I guess my above considerations matter little in the real world and it is not a safety issue. Heck, I've even still gone out to qualify while thinking I had mismatched front pads left to right (race versus street - but it turned out the shop had accidentally left a street backing plate on the car on one side only), so I certainly wouldn't lose sleep over it. But I'm also a bit of a tech junkie and just feel better knowing I've optimised a system so I still prefer the same or smaller rears where possible. But 818 vs 817 revs I'd call as even anyway.
The front-rear wheel diameter argument becomes an 'angels on the head of a pin' discussion for sizes within a few percent of each other IMO, though I have found torque transitions to be a bit jerkier when the fronts are smaller, versus having smaller rears. I have a few quiet 'reference corners' on the drive to work where I tend to pick up small differences more easily. That makes sense to me too, as smaller rears are essentially already overspeeding the input plates of the VC a little, so a greater degree of slip as they lose traction is just ramping up something that's already in progress, making for a quicker and smoother process than one that also reverses the relative travel of the discs in the VC when slower turning rears suddenly become faster turning rears as they start to spin up.
Rustler, thanks for that research and I guess my above considerations matter little in the real world and it is not a safety issue. Heck, I've even still gone out to qualify while thinking I had mismatched front pads left to right (race versus street - but it turned out the shop had accidentally left a street backing plate on the car on one side only), so I certainly wouldn't lose sleep over it. But I'm also a bit of a tech junkie and just feel better knowing I've optimised a system so I still prefer the same or smaller rears where possible. But 818 vs 817 revs I'd call as even anyway.
Last edited by 996tnz; 02-28-2017 at 07:25 PM.
02-28-2017, 07:55 PM
#25
Rennlist Member
I did have some slight rubbing on the inside rear with 315/30-18 but that could have been avoided by using GT-2 liner brackets and mounting the liners a little further away.
Phound and others have posted many times about this mod.
New rear liners are cheap so not a real big deal as long as you're not rubbing so much you get into the turbo inlet tubes.
02-28-2017, 09:10 PM
#26
No rolled fenders.
I did have some slight rubbing on the inside rear with 315/30-18 but that could have been avoided by using GT-2 liner brackets and mounting the liners a little further away.
Phound and others have posted many times about this mod.
New rear liners are cheap so not a real big deal as long as you're not rubbing so much you get into the turbo inlet tubes.
I did have some slight rubbing on the inside rear with 315/30-18 but that could have been avoided by using GT-2 liner brackets and mounting the liners a little further away.
Phound and others have posted many times about this mod.
New rear liners are cheap so not a real big deal as long as you're not rubbing so much you get into the turbo inlet tubes.
02-28-2017, 09:47 PM
#27
i have rolled fenders, partially eaten gt2 liners, and 12" wheels.
rubbing is just part of the equation if you run hard tight canyon corners. the wheel bounce on hard compression alone will guarantee that.
tracking might even be more forgiving, in terms of what minimal clearance exists, until momentarily airborne.
rubbing is just part of the equation if you run hard tight canyon corners. the wheel bounce on hard compression alone will guarantee that.
tracking might even be more forgiving, in terms of what minimal clearance exists, until momentarily airborne.
03-04-2017, 02:42 AM
#28
Instructor
Since you are a daily driver (like me) I switched to Continental DW in a 295/35z18 and the matching fronts. The diameters are .1 different but 1" taller, scrapes my driveway a little less. The main reason I changed was to get some sidewall so it would quit beating the **** out of me on the way to work. That and a little drop in tire pressure helped a lot and I think it looks better. I have Contis on my BMW and they have been fine. Ultra performance, maximum cornering, heat cycling and that are not my principal interest, just getting to work with my fillings in is.