New tires on rear...why?

[Propflux01]

Why do tire installers recommend putting new tires on the rear if you only need two new tires? I was looking at tires and saw a poster showing a car skidding around a curve (animated drawing) with old tires on rear and new in front, and below that was the car going around same curve normally with new tires on rear and older on front. Seems to me with only two new tires in back, you would be just as easy to have under steer with new in back, VS oversteer with new in front. What's the logic in this?

 

[MaximusD]

Car I bought used had two new tires on front and I didn't check the back tires. Same brand so I figured all the same. Wrong. Rear tires were legal, with about 4/32 tread left. However, last winter I hit a patch of slush, car did a complete 180 spin due to the back tires losing grip. It's a front heavy car, not as much weight on back wheels and WHOOSH. Changed all 4 corners to better brand and no more spinning.

 

[d00df00d]

Originally Posted By: Tim H.
Why do tire installers recommend putting new tires on the rear if you only need two new tires? I was looking at tires and saw a poster showing a car skidding around a curve (animated drawing) with old tires on rear and new in front, and below that was the car going around same curve normally with new tires on rear and older on front. Seems to me with only two new tires in back, you would be just as easy to have under steer with new in back, VS oversteer with new in front. What's the logic in this?

Understeer is safer and less scary than oversteer for the average driver.

 

[Quattro Pete]

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=52

 

[dparm]

Oversteer is easier to control, in my opinion.

 

[d00df00d]

Originally Posted By: MaximusD
Car I bought used had two new tires on front and I didn't check the back tires. Same brand so I figured all the same. Wrong. Rear tires were legal, with about 4/32 tread left. However, last winter I hit a patch of slush, car did a complete 180 spin due to the back tires losing grip. It's a front heavy car, not as much weight on back wheels and WHOOSH. Changed all 4 corners to better brand and no more spinning.

Minor point: Having less of the car's weight on the back doesn't make the tail more likely to come out in a corner. If anything, it makes it less likely. If the rear lost grip first, it was because of tires and/or suspension.

 

[bullwinkle]

I always considered it "gluteus maximus" covering, just like the tire stores always want to sell you 4 snow tires on a RWD vehicle instead of the 2 on the drive wheels. How anybody ever made it through winter on 2 studded snow tires back in the day is beyond me!

 

[d00df00d]

Speaking of front-heavy cars and where to mount new tires, a few things just occurred to me after watching the video that Quattro Pete posted.

First, hydroplaning resistance is a function of ground pressure and tread depth. In a front-heavy car, the front tires will have more ground pressure; thus, they can resist hydroplaning with less tread depth.

Second, most cars simply aren't set up to be communicative. Given that, I can easily understand why rear grip levels are more important than front grip levels.

Thus, I think it's completely understandable that the good tires should be in the back for most people.


As an aside, I think this whole discussion shows how much we lose to the fact that most car buyers are completely unaware of and uninterested in vehicle dynamics. The flip-side to this whole idea of putting good tires on the back is that it compromises braking distances in favor of handling stability. We wouldn't need to make such compromises as much if we bought better-handling cars with low centers of gravity and good weight distributions. I know my car still has pretty idiot-proof handling even with great new tires up front and worn ones in the rear. Don't ask me how I know. But I digress...

 

[SwampSurvivor]

I think it's completely unsafe to do, especially on a front wheel drive car, in the snow.

Yeah, the rear won't swing out - but the car also won't be able to stop, start or turn. All it can do is understeer.

 

[Astro14]

Originally Posted By: dparm
Oversteer is easier to control, in my opinion.


You mean...like in an old 911??

Sure, lots of those wrapped around trees when they had that "controllable" lift-throttle oversteer...

And understeer is felt through the wheel...more warning = safer...

 

[scurvy]

Originally Posted By: Tim H.
What's the logic in this?

It's simple physics and complicated lawyers.

With new tires on the front & old tires on the rear, all other things being equal you are more likely to oversteer. With new tires on the rear and old tires on the front, all other things being equal, the car is more likely to understeer. This is just freshman physics - draw free body diagrams for each scenario and it will be obvious.

As previously mentioned, understeer is most likely easier for most appliance drivers and could be considered safer by the lawyers.

 

[Astro14]

Originally Posted By: Miller88
I think it's completely unsafe to do, especially on a front wheel drive car, in the snow.

Yeah, the rear won't swing out - but the car also won't be able to stop, start or turn. All it can do is understeer.


Then don't cheap out and buy just 2 tires...replace all 4 if the fronts are that worn...otherwise, put the new ones on the back, the car will be safer.

This topic comes up about every 3-4 months on BITOG...and the same points are made...

I've watched cars lose control so many times (Vermont winters)...that I know that: what people think they can handle, and what they can actually handle, are two very, very different things...

 

[Quattro Pete]

Originally Posted By: Miller88
I think it's completely unsafe to do, especially on a front wheel drive car, in the snow.

Yeah, the rear won't swing out - but the car also won't be able to stop, start or turn. All it can do is understeer.

Just because it says to put new tires on the rear, it does not automatically mean that it's OK to have completely bald tires on the front. You still need to have tires on the front that have reasonable amount of tread left. And with that, you should not have too much of a problem stopping, starting, or turning. Also, most cars these days are equipped with traction control which will further help avoid wheel spin when accelerating.

 

[Russell]

Originally Posted By: MaximusD
Car I bought used had two new tires on front and I didn't check the back tires. Same brand so I figured all the same. Wrong. Rear tires were legal, with about 4/32 tread left. However, last winter I hit a patch of slush, car did a complete 180 spin due to the back tires losing grip. It's a front heavy car, not as much weight on back wheels and WHOOSH. Changed all 4 corners to better brand and no more spinning.


Many years ago, my daughter wrecked a 92 camry V6 on icy roads because the rear end came around. I put the new tires on the front. Rear had older tires with minimal tread. Live and learn.

 

[dla]

What would happen if you stagger the new tires? Say you put one on the passenger front and one on the driver rear.

 

[MNgopher]

Been there, done that, got the T-shirt. Drive a front wheel drive car with worn out rear tires on ice or snow, and the reason for placing the better tread on the rear tires becomes apparent quickly. One spinning 360 through an icy intersection was enough to convince me of that.

For the average driver (and even an experienced driver depending on how fast the rear end snaps out), understeer is much, much more predictable.

And I agree with Astro. The real point here is to have 4 good tires on the vehicle, particulary for adverse weatehr conditions like ice and snow. We have the same arguement every few months and I know what my answer is from the school of hard knocks...

 

[Propflux01]

Well, reason I asked the question was I needed to replace the fronts on one of my cars. The rears were new when the car was purchased, but the fronts were worn and older. The rears have 5k miles on them now. I put the new ones on the front, as fwd vehicles tend to wear the fronts quicker, and figured by next rotation time they will be fairly equal in tread depth. I didn't feel that 5k would make that much difference in traction on the rear, but installer had asked if I wanted them on the back. I said no, the front and let it go. When I was in the waiting room I saw the poster but had trouble figuring understeer vs oversteer ( I've experienced them both with about the same control result) and why they were so adamant about new tires in the rear.

 

[rslifkin]

IMO, new tires belong on the front, provided the driver knows what he's doing. I would MUCH rather deal with oversteer than understeer.

 

[SteveSRT8]

you say that now, but I have personally experienced the infamous snap spin of a FWD vehicle.

They can really let go, and it is not always a "catchable" spin.

 

[rpn453]

Originally Posted By: Tim H.
Well, reason I asked the question was I needed to replace the fronts on one of my cars. The rears were new when the car was purchased, but the fronts were worn and older. The rears have 5k miles on them now. I put the new ones on the front, as fwd vehicles tend to wear the fronts quicker, and figured by next rotation time they will be fairly equal in tread depth. I didn't feel that 5k would make that much difference in traction on the rear, but installer had asked if I wanted them on the back. I said no, the front and let it go. When I was in the waiting room I saw the poster but had trouble figuring understeer vs oversteer ( I've experienced them both with about the same control result) and why they were so adamant about new tires in the rear.


Not a problem if the tires have similar tread depth. You just don't want a new tire on the front with a worn-out tire on the back because you could spin if you lose only the back end in snow or in a hydroplane situation while cornering. The sort of sudden oversteer you could get if only the rear end hydroplanes cannot be controlled even by an elite driver.