Tire pressures


#1

I just happen to look at my son’s tire plate info on his 05 VW Golf and was shocked to see that the recommended pressures were 30psi for front and 41psi for the rear. I have a real problem with those numbers and he has kept it inflated at 32psi for all four.

What is the logic for that wide difference between front and rear


#2

What is the logic for that wide difference between front and rear

Test track testing.  They run the car on various conditions on a test track, including wet pavement, dry and check out handling characteristics particularly under emergency conditions.  I would suggest sticking with the manufacturer's recommendations.

Most recommendations have two sets of recommendations, one for normal load and one for maximum load.  

Going up a couple of PSI (assuming it does not go over the maximum limit indicated on the tyre) may be a good idea, if you don't mind a slightly stiffer ride.  Those couple of PSI can help you out if you forget to check the pressure and it has lost some.  A little too much is usually a better idea than too little.

#3

That tire set-up (low pressure in front - high pressure in rear) is used to decrease oversteer. It’s probably a fun car to drive (when the tires are inflated to specs!).


#4

[b]quote:

What is the logic for that wide difference between front and rear?[/b][quote]quote:Test track testing.
[/quote]
Test track testing? What does that mean?


#5

I doubt oversteer is much of an issue in a FWD golf, it appears to me that making the rears harder would actually reduce understeer and make the car more neutral (if that’s possible on a FWD car). It would be interesting to see how the car handles with the correct pressure, it might be a little looser in the rear (and more fun to drive).


#6

I read on carbibles.com a good starting point for tire pressure is minus 10% of the max.
Any thoughts on that approach?
mk


#7

quote:

I doubt oversteer is much of an issue in a FWD golf, it appears to me that making the rears harder would actually reduce understeer and make the car more neutral (if that’s possible on a FWD car). It would be interesting to see how the car handles with the correct pressure, it might be a little looser in the rear (and more fun to drive).
I’ve never driven a FWD Golf and I probably should have left out the remark “more fun to drive;” it’s probably the opposite.

Having said that, I’m pretty sure that “making the rear wheels harder” (by higher inflation) reduces oversteer, not understeer. Making the rear wheels “stiffer” reduces the slip angle of the rear tires (i.e., it aligns the tire/tread contact patch on the road more with the plane of the tire itself). When the slip angles of the rear tires are larger than the slip angles of the front tires, the rear tires “walk sideways” a bit more than the front tires in a curve, This leads to a larger turning radius for the rear of the car than for the front: oversteer. “Stiffening up” the rear tires reduces the rear slip angles and therefore tightens up the turning radius of the rear: oversteer is reduced.

BTW, the term “slip angle” has nothing to do with the car sliding or slipping on the road surface. It’s just that all four tires always “walk out” on a turn (that is how centripetal forces are created), and depending upon whether the front or the rear tires “walk out” more leads to either understeer and oversteer.

Therefore, higher pressure in the rear and lower pressure in the front reduces oversteer, as I said in my previous post. But that probably makes for less fun driving rather than more. I agree with you there. But it makes for a safer car and VW probably wants to avoid lawsuits.


#8

There was an article I came across years ago that says tire will add about 5 PSI when hot. So if you go over the manufacturer recommended PSI by 1 or 2, I don’t think it will make a big difference.


#9

Making the rear wheels “stiffer” reduces the slip angle of the rear tires (i.e., it aligns the tire/tread contact patch on the road more with the plane of the tire itself).

I understand what you are saying, but consider that the rear wheels are unlikely to be exactly perpendicular to the road when cornering at the limit. At some point, increasing the rear tire pressure will tend to increase the slip because the tire tread will be less able to conform to the road. I’m not sure if 40 psig is sufficient to reduce the rear grip on a FWD car, I was assuming that it would. It’s a little hard to predict what FWD cars will do at the limit because their handling is so flaky (they have an annoying habit of lifting the inside rear wheel off the road), and they have such a large understeer by default. It also helps to increase the front roll stiffness and decrease the rear roll stiffness to try to keep the rear tires relatively flat on the road. Personally, I just drive RWD cars.

Overall, I agree that the recommended pressures are intended to make the handling relatively neutral. I would stay pretty close to the VW recommendation unless I had a good reason to change it (I might increase them all by a couple of PSI, but I certainly wouldn’t run with the rears under-inflated).


#10

I understand what you are saying, but consider that the rear wheels are unlikely to be exactly perpendicular to the road when cornering at the limit.
Well, yes, rear wheel camber affects handling, but that’s another topic.
At some point, increasing the rear tire pressure will tend to increase the slip because the tire tread will be less able to conform to the road.
I have no idea what you mean here. Increasing tire pressure always reduces slip angle. And by “slip angle” you know I don’t mean the point at which the tire starts to skid along the road, right?
I’m not sure if 40 psig is sufficient to reduce the rear grip on a FWD car, …
I’m not sure why you are bringing up grip again. I am talking about cornering with no slipping or sliding. Once that occurs, the car is out of control and all bets are off.
It’s a little hard to predict what FWD cars will do at the limit because their handling is so flaky (they have an annoying habit of lifting the inside rear wheel off the road), …
That is what rear roll bars are for. Also: “at the limit”: I am not discussing limiting conditions here. I am dicussing how the car responds (oversteer or understeer) to agressive but safe driving (all four wheels on the ground, no sliding).
… and they [FWD] have such a large understeer by default.
That is true, due to the relative weight differential front to rear, plus the fact that driving a pair of wheels (front or back) tends to increase the slip angle of the driven set.
It also helps to increase the front roll stiffness and decrease the rear roll stiffness to try to keep the rear tires relatively flat on the road.
? Run that one by me again.
Overall, I agree that the recommended pressures are intended to make the handling relatively neutral.
I disagree. For a FWD, the recommended pressures tend to move the car further from neutral ? more stable but less fun. But that may depend on what you mean by “neutral.”


#11

Manufacturers have good reasons for the tire pressures they specify. They are carefully chosen to balance handling and wear. I am very uncomfortable about deviating from them.

VW may have an E-mail address to which you can send questions. If so, give it a try.


#12

Also: “at the limit”: I am not discussing limiting conditions here. I am dicussing how the car responds (oversteer or understeer) to agressive but safe driving (all four wheels on the ground, no sliding).

Now I understand what you are saying, I think we are talking about two different things. I was addressing cornering at maximum speed (i.e., on a track) that does involve considerable “sliding” of both ends of the car. “At the limit” the angle of the car in the corner is primarily controlled by the throttle, not by steering. If a car is set up to be neutral, more power will tend to generate understeer and less power will generate oversteer. Most street cars are set up to understeer at the limit (i.e., not spin), so that someone entering a corner to fast will just scrub of some speed while traveling in a straight line. Higher performance cars are normally set up to be more neutral, and capable of faster cornering without the front tires “washing out.” I’m not sure how to apply these terms if we are not discussing the limits.

Regarding roll stiffness (again at the limit), the softer end of the car will tend to maintain better grip than the stiffer end. One can adjust the roll stiffness (and aerodynamic down-force at higher speeds) at each end of the car to control the degree of over/understeer.


#13

The pressure of the rear is not going to change drastically as the tires warms up. I set the pressure of the rear as high as possible with FWD, 30/35(in cold) sounds normal to me.
I have never driven a 05 VW Golf before, so cannot comment on the 30/41. It is because of the OEM tires?


#14

Very possibly, two words – Questionable Engineering

I’m not an automotive engineer, but I’m more familiar than most people are with how things get designed and built. My guess would be that there is some problem with the vehicle design that was found after the design was locked in – something that couldn’t easily be fixed by a minor design change. My guess is that the engineers were trying to overcome a suspension or steering problem by tinkering with tire pressure.

Those with long memories will recall that a large tire pressure differential was used on the early Corvairs in the 1960s to try to overcome what turned out to be occasionally lethal consequences of the rear axle design (http://en.wikipedia.org/wiki/Chevrolet_Corvair).

Nine psi seems to me a pretty large differential between front and rear. I’d follow VW’s recommendation on this one. It may be no big deal, but keeping that differential could be important.


#15

Something sounds fishy here.

First, I have no first hand experience with VW Golf cars, so I’m casting about in the dark here. But lets use some logic.

First, a range of 30 to 51 pounds of pressure is extreme, both from a tire wear standpoint, and from a sidewall flex standpoint. I am a big advocate of the 5% less than max pressure (or 4 lbs.) inflation adjustment. This minimizes sidewall flex which reduces heat build up and firms up handling.

So I would look at a couple of things here. First, what is the max pressure of the tire molded into the sidewall? 51 lbs. would seem to exceed the tire manufacturer’s max. pressure rating, which is dangerous. On the other hand, logic would tell me that IF the tire is a 55 lb. max pressure rated tire that 30 lbs. is way too soft on the other end.

Second, pull out the owners manual and confirm the pressures that are shown there (and put your bifocals on when you read them). If 51/30 is still the recommended balance, I’d send an e-mail to VW and ask someone in engineering to explain why the front is way soft and the rear is so hard. Sounds backwards to me in addition to being excessive.

Then re-read the door sticker with your glasses ON and make sure you saw it correctly while waiting for VW’s answer as to why they ask you to exceed the maximum pressure rating of the tires.


#16

Oops!! Didn’t have my bifocals adjusted correctly when I read 51 instead of 41, sorry. Comments still stand, but 41 should be safe pressure on most tires…


#17

Thoughts?

How does the tire know what vehicle it is on?


#18

Now I understand what you are saying, I think we are talking about two different things. I was addressing cornering at maximum speed (i.e., on a track) that does involve considerable “sliding” of both ends of the car.
And I am addressing spirited street-driving, but NO sliding.

“At the limit” the angle of the car in the corner is primarily controlled by the throttle, not by steering. If a car is set up to be neutral, more power will tend to generate understeer …
In a RWD car, more power to the rear wheels tends to generate oversteer, not understeer. When a driver is racing his RWD through a curve, he accelerates coming out of the curve to help swing the rear end out. He is using what is called “power oversteer.” On the otherhand, accelerating a FWD through a curve increases understeer, just the opposite from RWD cars.

The reasons for the above is that increasing power to a pair of wheels in a curve increases the slip angles of those wheels. This leads to increased oversteer or increased understeer, depending upon whether the power is transmitted through the rear or front tires respectively.

Regarding roll stiffness (again at the limit), the softer end of the car will tend to maintain better grip than the stiffer end.
The camber angles change more at the softer end which reduces the size of the tires’ contact patch with the road. This would make the softer end more prone to side-slipping, I would think. Why do you feel the softer end (all else being equal) maintains a better grip?

???

Getting back to the original question, I think running high pressure in the rear tires is a safety precaution on VW’s part. High pressure in the rear increases understeer, which is a more stable driving condition for most people than oversteer. One extra safety feature: The risk of hydroplaning on wet roads decreases with increasing tire pressure (front wheel hydroplaning occurs less often because the driver can sense the approaching dangerous condition through the steering wheel.) Volkswagen lawyers must have had a hand in the final design.


#19

In a RWD car, more power to the rear wheels tends to generate oversteer, not understeer. When a driver is racing his RWD through a curve, he accelerates coming out of the curve to help swing the rear end out. He is using what is called “power oversteer.”

Again it depends on the type of car and how the car is set up, but if you try that in an early 911 you will be sorry (don’t ask me how I know). In a properly balanced rear or mid-engine RWD car with decent power, you can force understeer (plow the front tires) before the apex with more throttle due to the weight transfer to the rear (this allows you to power into a corner without losing the rear end). Letting off the throttle rapidly, or touching the brakes (god forbid) in a corner will result in you going backwards before you know what happened (due to the weight transfer to the front). I agree that you can also kick out the rear end by using too much throttle after the apex, but if you try that in a well balanced car, you are also likely to end up in the weeds (don’t ask about that either). The trick is to control the amount of front/rear drift with the throttle throughout the corner so you end up losing the minimum speed and are still pointing in the correct direction for the next straight (easier said than done). Ideally you will have some understeer before the apex while you scrub off speed and keep the engine power up; a very little oversteer after the apex will work (just don’t overdo it or you will lose the rear end).

Using “power oversteer” in a front engine RWD car (with 60 or 70% of the weight in the front) may work, depending how the suspension is set up, but I certainly wouldn’t want to try it in anything with a reasonable weight distribution. It’s also not a very quick way to get through a corner (unless you’re dirt track racing). Just thinking about driving a FWD car near the limit scares me.

As I think about it, my comments about roll stiffness are really only applicable to a well balanced RWD car with the camber adjusted appropriately (significant positive camber at the soft end of the car). In the case of a rear/mid-engine RWD car, making the rear roll softer will tend to keep both rear wheels planted as the body rolls. the inside front tire will tend to loss some grip and the outside front tire will tend to plow (understeer). Have you ever seen a rear engine car exiting a corner with the inside front tire off the ground?

For street use, just follow the car manufacturer’s recommendations for tire pressure and alignment specs, and you will usually stay out of trouble.


#20

There are some technical explanations that have been offered here that made my eyes glaze over when I started to read a couple of them; might try again later.

At any rate, I have found that our front drive cars steer better on the freeway with more pressure in the rear tires. I have deviated from the mfr’s specification but so far, tire wear seems to not be adversely affected as long as the tires are rotated regularly per the mfr’s spec.