Driveline Noise - Tire Pressure

[belize1334]

I've got some pretty serious driveline decel noise so I periodically search the internet for something new. I came across a Subaru thread where SOA recommended decreasing pressure in the rear tires to achieve more uniformity in the tire diameter front to rear. So, I went out and pulled some pressure out of the rear tires. Now running 36psi front, 30psi rear. I have to say, the grumble is MUCH IMPROVED. I still have some nasty gear rattle/scrape coming from the front but the loud growel from the rear diff is now significantly diminished. So, if you've got loud driveline growl under decel, try taking some pressure out of the back tires and report the results. Once we have more data we can discuss why this might be a valid/good/bad solution.

 

[xaisomboun]

Holy crap, I'm going to try this out right now, I'll report back on the results!

 

[Barnes]

This is interesting, but at the same time worrisome. I *think* I know how this works, and it isn't a fix. I suspect that by changing the rear tire pressure you reduce the rolling diameter. The rear wheels now are trying to turn faster than the fronts. So what you in effect have done is change the way the ring and pinion are loaded. I think effectively creates a situation that reduces how much deceleration load the rear tires transmit through the rear diff. I'm not sure is a good thing or not.

 

[RayH]

I've played around with the pressures in the past with the same effect. I also suspect it's due to diameter differences between front and rear when you play with the pressure. The slightly different speeds and resulting torque loads is not as much a concern as where it ends up which is the center diff. The lsd is what's trying to even out the torque load and removing the slack. The question is whether it's enough to heat and damage the lsd. If you are playing with the pressures, I suspect the lsd would be taking up any load difference between front and rear even when you're not off throttle it's just that there's no slack in those situations to hear the noise.

That said, I've was running like this for a few years with no problems until I stopped driving the car for other reasons. I'm not going to recommend it but if you do do it, you want to adjust the pressure a little bit at a time until the noise goes away and no more than that.

 

[belize1334]

My understanding is that the increased weight in the front of the car causes the tires to sag more for the same inflation pressure. By pulling a bit of pressure out of the back you even out the deflection which in turn evens out the rolling diameters. Ideally you'd have the pressure set so that sitting still the tires had identical profiles. Then when you decelerate you transfer a little weight to the front but no more than is transferred back when you accelerate. That'd minimize the overall difference. All I know is the car is more pleasant to drive with a little less air in the back.

 

[Barnes]

Ahh, that does make a lot of sense.

 

[IncorpoRatedX]

I had some very similar noises, pulled my clutch and this is how it looked:

click