I'm not to familar with tire setups. I won an ebay auction for some rx7 rims for my 1g AWD and they came with decently-shaped tires. Only problem is the fronts are 225/50/16, and the rears are 245/45/16. Now I know that on an awd car, the tires have to be generally the same size. Are those 2 sizes close enough to be ok? Well...if I have to replace one pair, which pair should I keep if the 1g is mostly used for straight lines/dynoing?

thanks,

my_gst95

my_gst95 said:

Now I know that on an awd car, the tires have to be generally the same size.

Click to expand...

No, they have to be specifically the same size.

Required information for determining tire fit:

Tire size: you got that one
Wheel diameter: I think I can guess
Wheel offset:
Wheel width:
Will car be lowered?:

This really depends on the type of tires that you have. From the sizes that you have posted those two should have very similar OID (24.5” is my best guess) and that is what is the most important here. You should measure diameter of those two sizes and if they are pretty close, you will be fine from the center diff VC point of view.

Since stock RX7 has 8” wide rims either tire is fine for that. Although, 245s are at the lowest edge for that size, but 225s are right where they are the best.

The one strange thing is that RWD cars like to have wider rear tires, but AWD cars have a lot of work requested from the front ones instead. From that logic, you would be better off with wider tires in the front and narrow ones in the rear. If you would have a FWD, that would make even more sense.

At the end, if you want to get rid of one set, you would be better of with 225s for every day driving (especially with snow in mind). If you are for some racing use with those, then keep your 245s!


Good luck!!




Fedja

Hey thanks for the help guys Since we don't see much snow here and its mostly for racing, i'll use the 245's.

my_gst95

A 225/50/16 is about .7% larger than a 245/45/16. So the question comes down to whether a .7% difference will cook the center VC. And the answer - after searching [drum roll please] - is: no-one seems to know.

Therefore, I, personally, would not do it.

- Jtoby

jtmcinder said:

A 225/50/16 is about .7% larger than a 245/45/16. So the question comes down to whether a .7% difference will cook the center VC. And the answer - after searching [drum roll please] - is: no-one seems to know.

Therefore, I, personally, would not do it.

Click to expand...

JT, you can not specify any numbers for the OID without knowing the type/brand of the tire. They all have different ways to do this and they are always different. The basic math that you can do here with the numbers from the sidewall just don’t reflect the reality out there! Just think of the width of the latest Hoosiers. Once compared 225/50/16 from Hoosier was as wide as 245/45/16 from Kumho (I think that was AS03 vs. Victoracers, but not 100% sure)

Another thing in specifying 0.7% up there doesn’t go along with the thread wear that might be in there. If one tire is a brand new and other one used just 60% or even more (even if they would be the same originally) worn out one would be more then 1/4" shorter and this is more then 0.7%.

As long as the tires are the same OID, they are fine regarding to the center diff and it’s VC. As a mater of fact, once calculated transmition ratio between the front and the rear wheels wasn’t exactly the same either.


Fedja

Actually, if you measure some tires or get the specs from a good source, such as the Tire Rack, you will find that manufacturers are very good about keeping the diameter within about one-quarter percent of what it should be according to the sidewall numbers. Where tires often differ from the sidewall numbers is in terms of width, although this depends so much on wheel width as to give them a good excuse. Similarly, treadwear plays a rather small role, with a worn-down tire remaining within 1% of a new tire in nearly all cases. Yes, people claim to be able to detect the torque advantage of a shaved tire, but that is not only butt-dyno data but it is also confounded with the weight of the tire (which drops when shaved) and the reduction in slip percent.

All in all, I still believe that the core question in this thread is whether a center VC can survive with a .7% difference. If what really needs to be known if it will survive with a .5% or .9% difference doesn't matter very much to me. You get the idea.

- Jtoby

In theory a 225/50/16 is 24.86 inches in diameter. Therefore the circumference is 78.06 inches. A 245/45/16 is 25.65 inches in diameter, and circumference is 80.53 inches.

Since the center vc has been brought up, I'll discuss what it means to have different tires on front and rear of an AWD. Let's say someone had 225/50/16's on the front and 245/45/16 on rear. Since the front two tires are 2.47 inches smaller than the rear two, the center diff and vc will be working constantly. Every 32.6 front tire revolutions will yield 31.6 rear tire revolutions. So, every 2544.92" or 212.08' or 0.04 miles the plates in the center vc spin 360 in relation to each other. So, every mile the vc rotates 24.89 times. If you are traveling at 1 mile per minute (60mph) the vc is rotating 24.89 rpm, or approx, 0.4rps. On a side note from dsmtuners member "dynatos" turning the vc 90-110 rpms should yield 65-72ftlbs. of torque. I know it isn't linear, but even at 24.89rpms there is bound to be some significant power transfer/drag through the center diff.

This is absolutely critical on AWD's with welded center diff. or spool. Since the center is locked the front/rear wheels WILL turn the same rate, but want to turn different rates because of different tire circumference, and consequently scrub rubber off the tires at all times.

As far as tread wear goes, (generally) 10/32 is standard tread. If you wear it off entirely there is 5/8" diameter tire difference between new and bald. On a 225/50/16 that comes out to a 1.93 inch circumference difference between new and bald, so you can see that tread wear is rather important as well.

If anyone missed the connections I made to get to my conclusions, PM me and if there is enough interest I'll explain.
I hope this helps!

Personally, I'd buy 4 new tires. But if you are the thrifty minded type of fellow, you could take a piece of string and measure the circumference of the two tires you want to keep and then go to the tire store and measure some other tires. I (my 02 cents) would keep the CIRCUMFERENCE within 1/4". Remember to deflate your tires before you measure them. The ones at the tire store will be deflated when you measure them and you want your readings to have similar parameters. There is the chance that the tires will not swell/grow the same amout when inflated or even run exactly the same in the real world even if they are the same circumference (still assuming different age/brand/size tire here)

Good luck,

You forgot to include gearing ratios for the above calculation. With all of that, math looks a little bit different. Also as you have said your self, in ideal world 225/50/16 tire would be of the size you calculated. Unfortunately that is far from truth. The same size tires from two different manufacturers will always be different...

Fedja

jesepes said:

On a side note from dsmtuners member "dynatos" turning the vc 90-110 rpms should yield 65-72ftlbs. of torque.

Click to expand...

An isolated piece of data, such as this, is really not that useful without a lot more background information. A VC gains heat at a rate determined by the speed difference between the two shafts. A VC dumps heat at a rate determined by the difference between its current temp and the temp of the stuff around it. Because of this - and unlike, e.g., a Quaife - VCs rarely behave consistently even when the difference in shaft speeds remains constant. Usually a VC is heating up or cooling down, so its effect is always changing.

The question really comes down to whether the difference in shaft speeds that will be caused by having these particular tires on the car will put the VC into a state where it is heating up. In other words, the question is whether the VC can dump the heat that it will be gaining from the speed difference. If it can dump the heat, then the car should be alright. If it cannot dump the heat, then the VC will be cooked.

- Jtoby

ps. your post included the claim that a spool causes the front and rear wheels to turn at different rates, which is not correct; you might want to fix that

MrAWD said:

You forgot to include gearing ratios for the above calculation. With all of that, math looks a little bit different. Also as you have said your self, in ideal world 225/50/16 tire would be of the size you calculated. Unfortunately that is far from truth. The same size tires from two different manufacturers will always be different...

Fedja

Click to expand...

Maybe you could site an example of where gear ratios come into play? The scenario I described has no bearing on gear ratios. (I assumed that tcase/rearend/trans all belong together, and aren't swapped 1g tcase for 2g tcase or something like that.)

Yes ideally 225/50/16 is 24.86" diameter, no it's not far from the truth, and I'd bet that the AVERAGE diameter is close to 24.86. Wiseman "jtmcinder" says: ...you will find that manufacturers are very good about keeping the diameter within about one-quarter percent of what it should be according to the sidewall numbers, or .0025. That's ~ +/- 1/16" diameter on above stated tire. (I would not be surprised to find a larger discrepancy than that though, say MAYBE 1/2" between an ultra-high performance, asymmetric, directional tire and hohum all season symmetric tire. (This is why I encourage the physical measuring of tire circumference!))

jtmcinder said:

jesepes said:

On a side note from dsmtuners member "dynatos" turning the vc 90-110 rpms should yield 65-72ftlbs. of torque.

Click to expand...

An isolated piece of data, such as this, is really not that useful without a lot more background information.

Click to expand...

I emailed dynatos about this info he posted. It turns out the exact spec numbers don't necessarily apply to DSM's, but I believe the general test procedure does. Example: put VC in lathe and spin at X rpm. Measure for minimum of Y torque. Link to link Dynatos sent me Seeing how this appears to be a tech article, I can't imagine that they'd leave out heating if it was a factor in their test. (I know this is for Subaru's. It's not the car that is the point, it's the procedure for testing this type of machine. That is the point.)

I am aware of the heat and how it affects VC's.

jtmcinder said:

The question really comes down to whether the difference in shaft speeds that will be caused by having these particular tires on the car will put the VC into a state where it is heating up. In other words, the question is whether the VC can dump the heat that it will be gaining from the speed difference. If it can dump the heat, then the car should be alright. If it cannot dump the heat, then the VC will be cooked.

Click to expand...

I agree, and at the same time, I don't have answer, but want to know it.

Sorry about the bad post wording before. What I posted and what I meant to post were two different things. Did the modified version clear it up?

All in all, I still believe that the core question in this thread is whether a center VC can survive with a .7% difference. If what really needs to be known if it will survive with a .5% or .9% difference doesn't matter very much to me. You get the idea.

Click to expand...

I agree. mygst95, would you care to perfrom some real world tests and let us know what you find? (I know this is unpractical, but) Run those tires, and let us know how your car/vc likes/dislikes it. I've already worked out what rate the vc will be working at 60mph (assuming your tires actually measure what my estimates were.) Let us know what you find out! This seems to be the only way to get realistic results for the questions above that need answered. (I don't expect you to actually do this test, but it would be a sacrifice for the cause of greater knowledge.) (The % difference in my scenario is 3.067% difference though.) My "guess" is that the vc will survive. And, if you put the bigger tires on the back, then the center diff/vc will keep the drivetrain to the rear's under slight torque and may (slim chance here) allow vc to lock up faster in low traction conditions. You may however see accelerated wear on drivetrain and maybe overheating of rearend as well. In any case with tires that don't match I'd expect to see accelerated drivetrain wear.

Will you test out that tire combo. and let us know?????
Thanks,

This post turned into some pretty damn good info. Thanks guys

would you care to perfrom some real world tests and let us know what you find?

Click to expand...

Yeah, I guess I could drive around with it for a bit, but I am planning on installing a VCE on my 1G awd soon as well as just replacing 2 of the tires so everything matches up. Dunno how 'technical' I could be, seeing as it would mostly be guestamations and how it 'feels'. But yeah, I'll post what I find in the week that I breakin my new turbo setup.

my_gst95

you can take guesses at the tire diameter all you want.
It has been stated that these tires are used, so that throws all the "new tires" comparison numbers to shit in a hand basket.

So that being said...
If they are the same brand, and have the same "worn in" OD, or pretty damn close (under 1/8" difference) you'll be fine.
However, the footprints of the 2 tires are EXTREMELY different. The 245 is MUCH LARGER. And THAT can make a BIG difference. you will have the rears hooking up much better then the fronts, which can lead to mechanical failure.

But overall yes, I would go with the 245 as the full set.

Glad to hear it. Be careful, take it slow for the first while. I advise you to check the temperature of your drivetrain ( trans, tcase, and esp. rearend ) after you drive around by putting your hand on those parts. Yes, that means climbing under the car to do so. Also listen for abnormal whirring/gear noise. Your driveshaft may wiggle a bit and vibrate due to your setup. I probably won't and if it does it shouldn't be bad.

I also expect a few replies on here telling you not to do what you are going to do. (And probably that i'm an idiot for suggesting it.) Please understand fully that there is a potential for damaging parts if you are not careful. Disclaimer: I cannot take any responsibility for what you do with your car. I think you'll be fine, just be careful. If you have any big problems, don't keep driving it that way!

If you actually plan on installing VCE, go to no end of trouble to ensure tires are "as close as humanly possible!."

Remember to deflate your current tires, measure circumference, then go and buy your new tires after you measure their circumference. Shoot for <1/4" difference (w/o VCE) between tires. (My 02.) Exact same is ideal, of course.
Good luck, be careful

jesepes said:

Maybe you could site an example of where gear ratios come into play? The scenario I described has no bearing on gear ratios. (I assumed that tcase/rearend/trans all belong together, and aren't swapped 1g tcase for 2g tcase or something like that.)

Click to expand...

When I had to go through the mess of replacing the trainy and the T-case, I did some calculations and gear ratio between the front and the rear is not the same. I forgot how much of the difference was in there, but there was some. If someone would run through the calculator and get the numbers you would see that too!

jesepes said:

Yes ideally 225/50/16 is 24.86" diameter, no it's not far from the truth, and I'd bet that the AVERAGE diameter is close to 24.86. Wiseman "jtmcinder" says: ...you will find that manufacturers are very good about keeping the diameter within about one-quarter percent of what it should be according to the sidewall numbers, or .0025. That's ~ +/- 1/16" diameter on above stated tire. (I would not be surprised to find a larger discrepancy than that though, say MAYBE 1/2" between an ultra-high performance, asymmetric, directional tire and hohum all season symmetric tire. (This is why I encourage the physical measuring of tire circumference!))

Click to expand...

Hmm, quick search on TireRack showed the following:
1. BFG g-Force T/A KD had 25” OID
2. Bridgestone Potenza RE050 had 25” OID
3. Bridgestone Potenza S-02 had 24.8” OID
4. Bridgestone Potenza S-02 had 24.7” OID (this one is for Porsche)
5. Bridgestone Potenza S-03 had 24.9” OID
6. Continental ContiSportContact 2 had 24.7 OID
and so forth…

So, even the same manufacturer didn’t have the same OID for their tires, which was my point at the beginning about using ideal numbers.


Fedja
ps. all of the above numbers where for the 225/50/16 tires

MrAWD said:

When I had to go through the mess of replacing the trainy and the T-case, I did some calculations and gear ratio between the front and the rear is not the same. I forgot how much of the difference was in there, but there was some. If someone would run through the calculator and get the numbers you would see that too!

Click to expand...

Whoa! After all this time, you're saying we don't have a 50/50 torque split. Heavy.

- Jtoby

1997 W5M33:

front: 3.800

transfer case: 1.074
rear end: 3.545
total rear: 3.807

Looks like they're darned close to me.

- Jtoby

So, one end is 3.8 while the other is 3.807!

If you would shift into the gear that has 1.000 ratio and spin the car at 3000 RPMs and using 25.3" OID for wheels (EVO wheels but, should serve the purpose here as well), that would give you a speed difference of 0.109 mph. That comes to 0.18% of the difference from the actual speed.

So, what I said was that numbers are off. Difference is small but it is there and that is what I was saying at the beginning.



Fedja

MrAWD said:

So, one end is 3.8 while the other is 3.807!

Click to expand...

I can't wait until someone posts that we have a 50/50 center, because I am going to be all over his a$$, pointing out that we really have a 49.96:50.04 torque split.

- Jtoby

Wow, MrAWD, I'm glad I asked you about the gear ratios. Neither jtmcinder or I knew about this. I think the people who know this nice fact has tripled in the past few posts. Thanks for showing me something new!

If you would shift into the gear that has 1.000 ratio and spin the car at 3000 RPMs and using 25.3" OID for wheels (EVO wheels but, should serve the purpose here as well), that would give you a speed difference of 0.109 mph. That comes to 0.18% of the difference from the actual speed

Click to expand...

Is there a chance you could elaborate on this. I don't understand what you meant. Your car doesn't have "the gear that has 1.000 ratio." Is this a hypothetical ratio, or were you trying to get the wheels to turn the same as the engine: 3000rpm? Also how fast was your car travelling to get 0.109mph discrepancy?

I worked out my scenario with your ratios on a 2g, it looks like factoring that in reduces the problem of vc rotation from 24.89rpm to 23.42rpm.
Note, the ratio difference (assuming tire size that is EXACTLY the same front and rear) yields a vc rotation of 1.4665 revs/mile, and where tire discrepancy in my scenario yields vc rotation of 24.89 revs/mile. Er, 23.42 revs/mile after factoring gear ratios!

On my 1g m/t, the ratio difference is front: 3.8666...... to rear: 3.86776859504132. A difference of .001102. That's a lot less than a 2g m/t's .007.

Do either of you (MrAWD or jtmcinder) have a spool or welded center diff?

I can't wait until someone posts that we have a 50/50 center, because I am going to be all over his a$$, pointing out that we really have a 49.96:50.04 torque split.

- Jtoby

Click to expand...

I come up with a different torque split. I come up with 49.907:50.092. Here's how i did it:
3.807/3.8

Answer: 1.00184210526316*50
Answer: 50.092105263158-100

Answer: -49.907894736842
Did I do this right? I can't think of how this could be wrong.

Hmm, quick search on TireRack showed the following:
1. BFG g-Force T/A KD had 25” OID
2. Bridgestone Potenza RE050 had 25” OID
3. Bridgestone Potenza S-02 had 24.8” OID
4. Bridgestone Potenza S-02 had 24.7” OID (this one is for Porsche)
5. Bridgestone Potenza S-03 had 24.9” OID
6. Continental ContiSportContact 2 had 24.7 OID
and so forth

Click to expand...

I averaged these, and came up with 24.85 inches. That's pretty close to the ideal size of 24.86. I would say .01" isn't MrAWD:"far from the truth" but more like Jesepes: "close."

In closing, I'd like to give you another thank you for bringing something very cool to this thread, MrAWD.
Thanks,

jesepes said:

Do either of you (MrAWD or jtmcinder) have a spool or welded center diff?

Click to expand...

Of course not. We're autocrossers.

.

jesepes said:

I come up with a different torque split. I come up with 49.907:50.092. Here's how i did it:
3.807/3.8

Answer: 1.00184210526316*50
Answer: 50.092105263158-100

Answer: -49.907894736842
Did I do this right? I can't think of how this could be wrong.

Click to expand...

The ratio of 3.807 to 3.800 is 1.0018.

1.0018 = X / (100-X)
100.18 - 1.0018X = X
100.18 = 2.0018X
100.18/2.0018 = X
50.045 = X and 100-X = 49.955

I just happened to round a little.

- Jtoby

Yeah, my first "Answer" is the ratio. The same one you used. The ratio is 1.0018 to 1.0000 .
Since we want to find where 100% torque goes, we need to take that ratio and convert it to add up to 100. Originally it adds up to 2.0018. We need to make it = 100. To do do this we must find out what to multiply each side(meaning each side of the "X:X" ratio) by. To do this divide 100 by the ratio total (2.0018). Answer: 49.9550404635828. Now, this is the number that we will now multiply both sides of the "X:X" ratio by. 49.9550404635828*1.0000, and 49.9550404635828*1.0018. Answers, respectivly: 49.9550404635828: 50.0449595364172.

Final ratio=49.9550404635828: 50.0449595364172 or, the same thing you said. I made a mistake in my calculations. And the above paragraph is my second rework of the equation in my head that I wrote down. I got it right this time.

Hm, my original equations probably don't make sense to anyone but me. I should have written them more clearly. I'll do that next time I post equations. My math is rusty because I've been out of school/real math classes for 6 years.
Thanks for the answer jtmcinder.

Yeah, I have a welded center in my 1g awd, and I autocross it. Man, it's a blast to drive. If you ever get the chance to autox one with a spool, do it just to feel the difference. It is sloppy, but man it's fun! When you push it to the limit, there is no time between over and understeer. It changes instatly from one to the other, and it's very predictible and controllable(between the two.) I'm not big into the competition part, just doing my best in my car and getting to know how my car handles. I'm on all stock suspension and tires. Tires are really old and hard. Front has some +camber while rear retains -camber. It's far from setup to autox, but it's a blast. I have some incar vids I took with my camcorder. And I log with DSMLink while I make my runs. I love it.

Thanks again,

Oh, I've tried a car with a spool. A 1G with about zero camber, AGX shocks, a 20g, and no power steering. (I'll let you guess what the car was really set up for.) It was probably the worst day I've ever had autocrossing. I wish I could say it was fun, but it was mostly frustrating.

- Jtoby