stray_pube

I blew out a tire last week, put on the spare, then drove to a tire center to get it patched. Turned out the tire developed a bubble, probably because I had to drive on it for like 60 seconds while it was going flat so I could pull off the highway and into a safe area, so I had to replace it. Had ~ 27k miles on the set of Conti Pure Contacts ECO Plus whatevers, and just over 7/32 tread left. The tire center said I could probably get away with replacing just one, but I didn't want to play that game so I bought 2 and had them installed on the front. I usually rotate every 8k but I am going to skip a rotation so the fronts wear down quicker to match the rear.


While driving with the spare on I could definitely feel a difference in handling. Good thing it was only for ~ 35 miles to the tire center. Can't imagine that was any good for the drive train but short of a tow what choice did I have? I noticed the RDX manual recommends replacing all 4, but 2 is fine if replacing 4 isn't possible. My Acura service manager said not to worry if the fronts are only 2/32 more than the rears. He did go on to say that if it was a CRV then changing all 4 is a must. Is there something about SH-AWD vs. that other system, which is what I presume the 2013+ RDX uses, that makes it "ok" to just swap out 2 tires instead of 4?

XLR8R

I just today had non-reparable tire damage and bought only one tire. The tire shop shaved it down to match the other 3 at 7/32. Shaving only costs about $20, compared to the cost of 2 or even 4 tires.

Not many tire shops have shaving machines (also called tire truing), but it is much less expensive and saves the other 3 tires.

You can also order a shaved tire from Tire Rack. They trim it to your spec and ship it to you.

dcmodels

Quote:

Originally Posted by stray_pube

... I noticed the RDX manual recommends replacing all 4, but 2 is fine if replacing 4 isn't possible. ...

As best as I was able to determine, all SH-AWD owner manuals say that, at least according to my local Acura dealer (both of them): RDX, TL, MDX, etc.

Quote:

Originally Posted by stray_pube

... My Acura service manager said not to worry if the fronts are only 2/32 more than the rears. ...

I damaged one tire, and replaced two, on my 09 RDX, when the tires were at 6/32 remaining. If interested, you can read more here, post #7:

https://acurazine.com/forums/tires-wheels-suspension-362/replace-two-four-tires-787555/

Quote:

Originally Posted by stray_pube

... Is there something about SH-AWD vs. that other system, which is what I presume the 2013+ RDX uses, that makes it "ok" to just swap out 2 tires instead of 4?

Yes. There is a basic difference between the two systems, but I don’t exactly understand why it matters for tire replacement. Here is the *idiot’s* explanation. I spent about 1.5 hours once with the service manager at my local Acura dealer, looking at and reading the official Acura description of the two different systems, on his PC, and discussing it with him.

The SH-AWD system uses two clutch packs in the rear differential. There is one clutch pack controlling the output to each of the two rear axles. In other words, power is always delivered, via the rear drive shaft, to the rear differential. Then, electrical power is used to generate a magnetic field, in each clutch pack, to determine how much power (torque) is delivered to each of the rear axles, individually. This allows the rear diff to act like a locked diff, only with controlled power to each axle.

Understand that the clutch packs are located *after* the power is split by the rear diff gears to each rear axle. These gears are not true helical hypoid gears, as typically used in a rear diff, and as used in the front gen-1 RDX transfer case, which looks like a typical rear diff (helical gears and gear lube used). Because the gen-1 rear diff gears are not true helical, ATF can be used, but is also a good reason to change the ATF often.

The later gen-2 RDX version, uses a system basically like that used in the Honda CR-V. There is only one clutch pack, not two. The single clutch pack is located just inside the *entry* point of the rear drive shaft, inside the rear diff, and before the power is *split* to each rear axle using a gear set.

As best I could understand, the gen-2 RDX rear diff is an open diff. This is basically like what happens with my Chevy 4x4 (sort of in reverse). That is, the Chevy rear diff is limited slip, and when power is delivered to the front diff, the front diff is open, allowing one of the front wheels to slip.

So, when power is delivered to the gen-2 RDX rear diff, because the diff is an open type, one of the wheels can slip relative to the other wheel.

So, on the gen-1 RDX, one of the front wheels can slip relative to the other front wheel, but the rear wheels can be locked together, or the amount of rear wheel slip relative to each other, is controlled (limited), as well as the amount of power being delivered to the REAR vs the FRONT wheels.

But, on the gen-2 RDX, one of the front wheels can slip relative to the other front wheel, and also one of the rear wheels can slip relative to the other rear wheel, and power is only controlled between the FRONT and the REAR, rather than between the two rear wheels.

mrgold35

I had the same issue when I ran over road debris and put a non-repairable rip in the side of my tire. I had to argue with the Discount Tire guy because he wanted to replace all four tires because of sh-awd instead of only two tires and keep them on the same axle like I recommended. I had to pull up a copy of the owner's manual I had on my Dropbox and show him so he could proceed. I usually cross rotate my tires; but, I kept the newer tires on the fronts longer and just did front/back rotations only. Front/rears are very close to 4/32 now, trying to time out the purchase of new tires and bad weather in NM.

Tomtwtwtw

This is just my speculation, but since the CR-V and Gen2 RDX AWD systems are not full time, and only engage when they detect front wheel slippage, perhaps that is another reason they recommend replacing all 4. If you have worn tires in the front, depending on how sensitive the computer is, the fronts would technically be rotating faster than the rears and the ECU may think there is slippage and re-route power.