Replacing the Odd Man Out - 2015 Audi A3 Long-Term Road Test

Edmunds.com

Replacing the Odd Man Out - 2015 Audi A3 Long-Term Road Test

Our long-term 2015 Audi A3 needed a new pair of matching tires after a blowout left us with one odd tire.

Read the full story here

nate001

Am I the only one confused about what happened here? Did you replace the Nitto Motivo with a Continental ProContact or did the new two tires replace two other tires on this car, which would still leave one old tire.

texases

I thought most AWD systems need 4 matching tires, not 2 and 2.

socal_eric

On a strictly mechanical system, such as using helical or conventional differential between the front and rear axles it's usually recommended to have all four tires be around the same diameter/wear so they rotate at the same speed since there is a fixed ratio and some power is applied at all times since the front and rear axles are mechanically meshed/engaged via the diff's gearset (or locked in case of a transfer case on some trucks that don't have a full-time 4WD system). The Haldex system used in this Audi has a power take-off unit mounted to the transaxle which constantly sends power to the rear, but instead of a mechanical center differential to bias torque front-to-rear it instead uses a clutch pack at the rear drive module to engage and use the power being sent from the front.

Because of this electronic clutch pack being able to accurately measure precise wheel and axle speed it can engage the clutches a certain amount to transfer whatever required torque is needed on the rear. Since the front and rear aren't mechanically tied at all times through a fixed gearset or transfer case and the system is designed to run with a slip or certain level of engagement in the clutch pack it should be fine to just ensure the tires on each axle match (probably most important on this car for the rear axle as the front differential in the transaxle should be an open unit and wouldn't have any wear issues like what could be experienced side-to-side through the rear drive module's diff and clutch pack assembly.

dldave

nate001 said:

Am I the only one confused about what happened here? Did you replace the Nitto Motivo with a Continental ProContact or did the new two tires replace two other tires on this car, which would still leave one old tire.

Yeah, I was confused as well, but just assumed they replaced with the original tires.

s197gt

they popped one continental. replaced with a nitto. they wanted all matching tires. instead of buying a new continental to replace the nitto, they bought two continentals for the rears replacing the nitto and the older continental. thus, two older continentals on the front and two new continentals on the rear. all continentals are matching models and tread designs; thought different tread depths as the new ones are... newer.

nomercy346

not sure about quattro but the general consensus was always that you need matching rolling diameter front and rear on most AWD systems.

My old ZJ has a viscous coupler in the t-case to transfer power to the front axle. as long as both axles spin the same speed it's a relatively loose connection and only transfers 20% of power to the front. As soon as both axles spin at different speeds the fluid within the coupling heats up and, using the changing propertys of that fluid, more power is being sent to the front.
If you have constantly different speeds (as in different tread depths) you burn out that coupler and it cakes together. Basically you'll end up with a locked drivetrain, same as a part-time 4wd. So, no more tight turns, lots of binding, worn tires, drivetrain components etc.

Actually I believe it would be better to have one worn and one new tire per axle (as long as none of your diffs are limited slips), that way front and rear axle still turn the same speed

socal_eric

Viscous couplings would be another type where you'd want to keep the rolling diameter and tread depth as close as possible on both axles. Keep in mind that "quattro" is essentially a brand for marketing and doesn't define the technology used. While many of the older longitudinally-mount engines used Torsen (now JTEKT owned) mechanical, helical gear torque biasing diffs, cars like this A3, the Golf, Jetta, etc. with transverse mounted powertrains have been using Haldex (now BorgWarner owned) systems also branded quattro.

Since the Haldex system in the A3 uses a wet multi-plate torque coupling to actively engage a wide-range of torque available on the rear axle, it isn't relying on a strictly fluid-based coupling to sense slip and transfer power, and *should* be perfectly fine with slightly more wear on one set of tires between front-to-rear. If they were using Haldex's Cross-Wheel Drive (XWD) with the electronic limited slip (eLSD) in the rear drive module to vector torque side-to-side on that axle, it also uses wet pack clutches and might be worse than running two similar tread tires.

That's just a high-level technical overview of how some AWD and part-time 4WD systems work but the best bet is to follow the manufacturer's guidelines for tire wear, rotations, and what to do if a tire is damaged.