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[Drivaar]

I've got Pilot Super Sports, and they are great... but I've got family in town and the ride is just very harsh, especially in the area I live because of all the potholes and uneven roads. And after driving sport package BMWs for a few years on these roads, I'm getting a lot more interested in having a more comfortable ride.

Today I lowered the tire pressures from 35 front / 35 back to 32 front / 34 back, and the ride is noticeably more comfortable. I was thinking of lowering to 30 psi all around -- any issues with that? Especially in summer, water evacuation isn't a big problem, so lower just seems like a nice option if all I'm concerned about is comfort, no?

On another note: I'd also love to get a more comfy set of tires when my Super Sports wear out, but when I look at the braking distances of those other tires, I'm losing 10 feet from 50-0 mph (and exponentially more as speed increases)! Any recommendations for a nice comfy set of tires?

I'm grateful for any advice!

Thanks!

[gtaccord]

Probably can't go much lower than that without your tpms going off (assuming you still have your sensors installed). Also know that tire wear will be accelerated with low pressures and mpg will suffer.

[Drivaar]

I can just reset the TPMS -- just want to know what is safe, really... Sounds like people go pretty low before track days (some threads mention 30 or 31 psi all around cold)...

At this point I'm plenty happy to trade of tire wear and mpg for passenger comfort. I feel pretty bad about how rough the ride is for older folks. (And personally I've also realized that Oakland is just not the right place for 18" tires with stiff sidewalls, high psi, and sport shocks!)

[kdbryce]

Buy a 750 and get out of here, old man.

Just joking.

Wouldn't this compromise gas mileage, though? I'd care more about that. Despite my love for spirited driving, I've found myself doing everything I can to get that extra tenth of a mile lately.

[alexwhittemore]

Originally Posted by kdbryce

Buy a 750 and get out of here, old man.

Just joking.

Wouldn't this compromise gas mileage, though? I'd care more about that. Despite my love for spirited driving, I've found myself doing everything I can to get that extra tenth of a mile lately.

Yeah, it will. I couldn't tell you by how much, but the hypermiling crew virtually always over-inflates to some degree, and I've seen as high as almost 25%. This also tends to reduce wear - lower contact patch and lower power lost due to rolling resistance, lower tire decay. The hypermilers tend to report tires lasting very long, as well. Additionally, you'll notice a ">100mph" pressure spec on the car that's 10-15% higher - presumably that's so that at high speed, the tire doesn't wear as quickly, putting you in danger of a blowout, but honestly that's only an educated assumption and I'd love to hear a real answer.

The downside of those things is rougher ride as you've mentioned, and less grip in the corners. Now, by how much? Good question. And in the opposite direction, you CAN run down to zero psig, the question is how long your tire lasts. If you're not on a run flat, you can certainly ride a 0 psi tire, but only for maybe a half mile before it's totally shredded .

So certainly underinflating gets you a softer ride at the expense of tire wear, and you can probably move that slider as low in pressure as you like. Spitballing, though, greater than maybe 10% under-inflation will seriously compromise tread life and mileage.

[Drivaar]

Apparently fueleconomy.gov states that MPG drops 0.3% for every 1 unit of psi drop. Not too bad, really, for a considerable gain in comfort...

[MrSteak]

BMW's recommended tire pressure is LOW enough. The numbers BMW provide are for a balance of performance & comfort. The recommended max PSI on the tire sidewall is based purely on the tire manufacture's performance testing. So the tire's rating, performance, and safety specs are all based on that max PSI... so the lower you drop below that, the further away you get from what the tire was designed for.

But if you want to keep going lower for "comfort"... you'll be sacrificing safety and performance, along with tire durability (including rim protection).

[williamf]

I am not sure, but I think there is a point at which a lower tire pressure will reduce smoothness. You don't want the car bouncing up and down with any kind of resonance. I am guessing but this might happen if you get below 28 to 26 psi.

[Roleez]

Lower tire pressure to run on roads with potholes. Let's think about that for a second ...

[fazman]

Wouldn't it make more sense to toss on some 17" rims/tires and go for as narrow width as possible and a taller Aspect Ratio?? If you still feel the MPSS are rough on the 335... maybe the 335 is the problem... time for you to look into a 5er or 7er maybe an X5???

I didn't recall if you were driving on 19" or 20" rims?

[Drivaar]

Absolutely -- the problem is the 18" rims, the stiff tires, and shocks... But I'm trying to find a low-cost and easy solution.

Anyone have a clear response about how low pressure can go safely? (That said, it sounds like 30 psi front, 32 back is not a problem at all.)

Roleez: I agree there is a risk with potholes... I had actually raised tire pressures up previously in the past after getting a bubble. I'm trying to balance...

I confirmed yesterday that there is a very considerable improvement in ride quality just by dropping 3 psi or so! That seems like a nice trade-off.

[ChipB]

I would absolutely avoid reducing tire pressure more than 10% (about 3 PSI) from the recommended values printed on the driver's door jamb sticker. Going much lower will not only adversely affect tire wear and gas mileage as others have pointed out, but also negatively affect the balance and handling of the car. With lower pressure the tire side wall will roll over more in the turns, potentially increasing understeer (and oversteer at the same time), and hence you may find the car doesn't turn as you expect it to in a panic situation. The problem here is not the PSS's - it's the suspension on the car, and having bought a 335i you most definitely made a choice for a fairly sporty ride. So either (a) trade the car in for a Lexus, (b) rent a plushy car while the family is in town, or (c) live with it. I would choose C.

[insane79]

well i think 30psi will be ok if the car is not fully loaded, over here in India we have pathetic roads, many people in sedans put 30-32psi here, doesnt matter if its a bmw,audi or merc. I personally fill 32psi all around & find it a ok compromise but then i m on 17" wheels, 18" wheel over here is asking for a rally cross ride I think you should stick to 32psi all around.

[ed330D]

You really are playing with fire. If nobody else could be involved I'd probably just leave you to get on with it, but your passengers and other innocent road users could suffer from your meddling.

Tyre pressures are critical to safety: too low and they provide insufficient rim protection, risking a shattered rim; too low can split the tyre sidewall, both giving rapid deflation. And wrong pressures can make for an evil handling car.

BMWs are set up to understeer. For the road this is desirable - it's far safer than oversteer. By changing the pressure differential between the axles, ie dropping the rear pressure by more than the front, you are likely to induce oversteer. You really don't want to do this on a road car. It's difficult to drive a car with oversteer on a race track, just plain stupid on the road.

Yes, those who go on the track do lower the pressures as already mentioned, but, and it's a big but, if things go wrong there usually plenty of run off, and unlikely to involve anyone else. And those low pressures rapidly go up on the track, which is the main reason for setting them lower when cold. It's actually the running pressure that matters, not the cold pressure.

So do yourself a favour, do some research on slip angle, its effect on oversteer/understeer, and in particular how changing tyre pressures effects the slip angle.

Apologies for the rant, I just can't believe the number of posts where members change sway (roll) bars, dampers (shocks), springs, tyres, in ignorant bliss.

[ChipB]

Originally Posted by insane79

well i think 30psi will be ok if the car is not fully loaded, over here in India we have pathetic roads, many people in sedans put 30-32psi here, doesnt matter if its a bmw,audi or merc. I personally fill 32psi all around & find it a ok compromise but then i m on 17" wheels, 18" wheel over here is asking for a rally cross ride I think you should stick to 32psi all around.

I've been to India several times, and given that the typical top speed around town is less than 10MPH (or more like 2 MPH in typical traffic) having lower pressure isn't too much of an issue. But don't do this if you expect to hit anything north of 50 MPH.

And just to add to thec omment about people running low pressure at the track: if you're running on street tires typical advice is to start with normal air pressure and see what the build up in pressure is with the high heat from the track. It's not unusual for pressure to build from, say, 36 PSI to 48 or higher in a 20 minute track session. So it's common to lower the cold pressure by 3-4 PSI, so that when hot it doesn't get too high. And if running R-comps (dedicated track-only tires) most are designed to work best in a narow range of pressures when hot, typically in the mid 30 PSI range. Which means starting cold with presure in the mid- to high-20's. In any event do not take the practice from the track and use it to rationalize what pressures you should run on the street!

[Drivaar]

All I'm really asking is whether it's okay to run something like 30 psi front / 34 rear instead of what's in the door jamb: 32 psi front / 38 rear.

Seems like this shouldn't be an issue, right? We're taking about 10% or less drop in tire pressure to get some comfort, that's all.

(And yes, next car will not be a BMW, definitely... that's another topic... I did the sporty thing, enjoyed it for many years on the right roads, and now would prefer something more comfy. )

And thank you all as always for the many responses!

[alexwhittemore]

Originally Posted by Drivaar

All I'm really asking is whether it's okay to run something like 30 psi front / 34 rear instead of what's in the door jamb: 32 psi front / 38 rear.

Seems like this shouldn't be an issue, right? We're taking about 10% or less drop in tire pressure to get some comfort, that's all.

(And yes, next car will not be a BMW, definitely... that's another topic... I did the sporty thing, enjoyed it for many years on the right roads, and now would prefer something more comfy. )

And thank you all as always for the many responses!

Almost certainly yes. But you'll get quicker wear and slightly lower economy for sure. Though based on the info above of something like .3%/psi, not MUCH lower economy.

[insane79]

just wanted to know why bmw has recommended more air in the rear? i basically drive my car alone or max with 1 passenger so should i still keep my rear tyre pressures more then the fronts? is it cause the car is rear wheel drive?

Thanks..

[alexwhittemore]

Originally Posted by insane79

just wanted to know why bmw has recommended more air in the rear? i basically drive my car alone or max with 1 passenger so should i still keep my rear tyre pressures more then the fronts? is it cause the car is rear wheel drive?

Thanks..

According to ed330d, they try to limit oversteer under street conditions (which an RWD car would be more prone to), and with a higher rear tire pressure and slight negative camber of maybe -2* max, that all would seem to fit.

http://www.tirerack.com/tires/tirete....jsp?techid=58

[insane79]

Originally Posted by alexwhittemore

According to ed330d, they try to limit oversteer under street conditions (which an RWD car would be more prone to), and with a higher rear tire pressure and slight negative camber of maybe -2* max, that all would seem to fit.

http://www.tirerack.com/tires/tirete....jsp?techid=58

interesting info thanks, so basically if i put the fronts 32 & rears 34psi then the car will have less tendency to oversteer but will cause the the back bouncing around more i guess, will try it & see how it feels..

Thanks.

[jadatis]

The tire-and car makers use a formula to determine the advice pressures.
Goal is to get a deflection of the tire , with wich no damage to the tire(sidewall) when driving the speed and having the alighnment used.
I once got hold of that formula used in Europe , wich America swiched over to in 2006 and went running with it.
Half of my public map of skydrive that belongs to my hotmail-adress with same username as here, is filled with maps with spreadsheets to calculate it.
But in the proces of gathering information , I got much to know about tire-pressure , maximum load and reference pressure( wich is not the maximum pressure) and how car axles are loaded mostly.

https://skydrive.live.com/?cid=a526e...E092E6DC%21128

So to calculate your situation I need to know more , and then I can calculate How low you can go without damaging the tires.
If you have low hight/width tires I have to compensate for the to high given maximum load of the tire , so doing it yourselves with one of my spreadsheets is dangerous, will come to to low pressures for such a tire. Could be then that the save pressure for the tires , is to high for comfort and gripp.

What I need is from car :GAWR= Gross Axle Weight Ratings front and behind.
GVWR=Gross Vehicle weight Rating.
maximum used speed of car.
camber angle of mostly only rear wheels ( wheels like this on the axle /-\ instead of this|-|)
If you can give the empty weight and where the motor is placed , and how you load it normally ( persons and load), I can estimate the axle-loads and give a lowest pressure for that.

From the tires I need: Maximum load and kind of tire to determine the reference-pressure. But also the exact sises and speedcode.
Example: If you read from tires 235/40ZR18 92W XL , then the 40 is that hight width division in % and is called the aspect ratio.
92 is the loadindex wich means 630kg/1389lbs maximum load , but mostly that is written on the sidewall too.
W stand for speedcode up to 270km/???m/h.
XL stands for XL/reinforced/Extraload and needs a higher pressure for the maximum load of 41 psi for American tires. If you cant find this or P before the sises its a standard load tire with 35psi reference-pressure.

So to give a save lowest pressure I need all this.

[driverman]

OP: Regarding your 2nd question on "more comfy" tires...

Conti DW's are a much softer than the best Max Perf Summer tires, so you get a smooth ride AND they are cheap, BUT at the expense of responsiveness and stability, especially when they are new.

I've got about 4,000 miles on a set of Bridgestone S-04's and I love them. They are responsive, stable, very quiet and firm (not harsh).

I suggest you check tirerack's and Car & Driver's comparison tests of Max Perf Summer tires. They are very helpful.