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Comment by cowsandmilk | Hacker News Reader

Why not just move the tires from the front wheels to the back?

Just so the laymen here don’t think this is some kind of hack, this is what is referred to as “rotating your tires”, which should sound familiar.

The problem is that tires are often not rotatable due to staggered setups where the front and back tires are different sizes (very common) or due to directionality of tires (less common).

Different sizes means you can swap left to right and right to left. Directionality means you can swap front to back and back to front. Directional tires of different sizes means you can’t swap a damn thing (as drivers of Porsche and some BMW configurations find out quickly enough.)

In commercial tire shops in the US the tires with the most tread always go on the back regardless of any other consideration.

This is because losing traction in front causes understeer, which is usually easily managed by typical drivers. On the other hand losing traction in back causes oversteer, which is often very rapid in attack and severe in extent. For some vehicles it is essentially impossible for a non-professional driver to maneuver out of an oversteer event.

So for safety sake the best tread always goes on the back. Most shops will refuse to do anything else.

Then there is my eldest daughter.

“Dad, car has been a little bumpy for the past month”

I hop in go down to road. Experience is best described as driving on square tires. https://youtu.be/QF7odK55gkI

Both front tires had massive bulges sticking out.

She had been driving like that for a month.

She's your daughter...

Oh I have my own stories of being completely oblivious, or not wanting to bother others.

However she was a special needs adoption due to age. Only had her ~4 years at that point.

This is one of those myths that just needs to die.

"Put the good tires on the back" is a terribly inaccurate rule of thumb. Whether you want the good tires on the rear or the front is gonna depend on how the vehicle handles. Most modern mom-mobile type crossover are so biased toward under-steer (usually from a combination of rear weight and stiff front sway bar) that they need all the help they can get up front and putting the better two tires on the rear is just wasting traction because that end was never going to break traction anyway.

Also, Firestone happily puts fronts only on my junk.

The problem is that vehicle handling is dependent on equal grip from all 4 tires. When you introduce differential in traction front to rear, you can throw the "understeer or oversteer bias" directly out the window. Modern cars do a great job compensating for less than ideal conditions but there is a limit. If you still think the new tire on the rear rule of thumb is inaccurate, watch this:

https://youtu.be/gSz7cm6MwH0

Re-read what I said. I'm specifically talking about the ass-heavy FWD crossovers and minivans that dominate the roads today (though I suppose there's a few FWD sedans out there with similiar handling). They all have a very respectable sway bar up front for good "car-like" handling and they all have a lot of weight out back. This results in a class of vehicles that mostly cannot go sideways even if you try (and believe me, I have tried). Someone with one of those cars comes in with four nearly bald tires and wants two good ones put on and you put them on the rear then you have improved their handling by exactly zero because the fuse in the system is still the front traction. Adding more rear traction doesn't help because handling/performance is determined by the weakest link in the chain and that's still the front which is unchanged . If you put the tires up front then sure, there's a 2% chance of going sideways rather than a 1% chance but you've also improved the handling greatly by addressing the weakest link which is end that was lacking in traction. The vehicle will corner and stop much better because the threshold for loss of traction is much higher.

Does this make as much sense for front wheel drive cars? Both of my FWD cars get front tires to wear more, and if I wanted to keep the better tread on the back, I'd never rotate them, and fronts would wear out much faster than the backs.

Not to mention that front wheels also have bigger brake rotors, so worse tread up front would also mean worse breaking.

Definitely makes sense on FWD cars, common throughout Europe. And indeed we don’t rotate tyres here, as you point out, the front ones wear out faster anyway, so when they’re dead you move the back ones to the front and put new ones on the back

Far better to lose some steering and braking power from worn front tires than to have your back flip out on you

But back wheels only rotate as a result of moving against the ground. Grip on the back tires seems most important for balanced braking.

I am likely missing on some obvious basic physics here, but I wonder how does better grip in the back on a FWD car help reduce oversteer. Any pointers?

I guess the easiest way to see it in practice is in winter when FWD cars put snow chains on the front wheels. As long as they’re driving uphill it’s OK, but as soon as you drive down the mountain you’ll see cars spinning out of control. At the slightest turning circle, The inertia of the car will push the back out, and 99% of drivers don’t know how to deal with that situation- so they brake and try to turn harder, which makes things even worse.

Obviously if everyone was used to driving RWD and countering oversteer , you wouldn’t need to put the best tires at the back. But the reflexes for the vast majority of drivers are to brake of things get tricky (which is perfectly fine for normal FWd driving)

Does any of that matter for cars with ESC?

Yeah isn't this why you should rotate your tires regularly?

You get downvoted but yes, that's the reason for swapping front to back and vice versa.

Might have different tire size front and back - https://tiresize.com/tires/Tesla/Model-3/2020/

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