Newbie car related questions... (new question in post #7)

[magburner]

After being advised to create a separate thread for a question I had regarding how my car operates, I thought I'd start one for this. I (think) that this question might get a few replies, anyway, here goes.

Yesterday, I was driving my car at around 60mph, and as I got to the top of a rise in the road (which then swung down sharply to the left), I noticed a very weird feeling.

My Vectra SRi has stiffened and lowered suspension, so it makes it good at taking corners and stuff like that. I know that when I turn the car will, and at the speeds I've driven, I've never lost grip or traction.

Yesterday, as I got to the top of the hill, I felt the car roll ever so slightly towards the outside of the corner, or rather reseat its position. At first I wasn't sure what it was, so I got to thinking.

It couldn't of been the suspension, because the car had been steady up until that point, and I was more or less going straight at the time. The road was wet, but I never felt like the car had lost grip, and I wasn't turning erratically. The only other explanation I could surmise was that maybe the car had 'rolled on the tires' if that is at all possible.

So the question is, can you feel the car move on the tires, that is can you feel the tires flex as the car moves?

Any and all advice will be greatly appreciated. Thanks. 👍

 

[PeterJB]

Did you turn the wheel and the car still went straight forward? Because that would be a lock-up. Or maybe the car went airborne momentarily since you were going quite fast.

 

[mustangGT90210]

Since you were at the top of a hill at a nice rate of speed, when the road suddenly flattened or started going down, your car was still going upwards, making less weight on your tires, greatly reducing your grip.

If that makes no sense, I think it's Internation A License test 10 in GT2 that shows what I'm trying to say

 

[wfooshee]

Car got light at the crest, and with slight loss of grip slid towards the downhill side of the road, since the road is crowned.

Most cars (before ABS was prevalent) were designed to lock the fronts before the rears in hard braking, as a locked front end will generally keep its direction, while a locked rear end will try to swap ends. If you're driving along somewhere and you see skid marks, they'll generally curve towards the edge of the pavement, which is a result of the slding wheels, not haveing grip, following gravity downhill.

That downhill burp is what you felt as the car was light, and if I understand your post correctly, you applied steering at or immediately following the crest, while the car was lightest. Further, if the suspension is overdamped on rebound, the momentum of the car could lift the wheels off the pavement.

 

[ExigeEvan]

It's the same negative G feeling you get when you come over a fast crest ona roller coaster. You'll lift out of your seat slightly.

That's all that happened here, except your car's weight lifted off the road slighlty.

 

[Stevisiov]

After being advised to create a separate thread for a question I had regarding how my car operates, I thought I'd start one for this. I (think) that this question might get a few replies, anyway, here goes.

Yesterday, I was driving my car at around 60mph, and as I got to the top of a rise in the road (which then swung down sharply to the left), I noticed a very weird feeling.

My Vectra SRi has stiffened and lowered suspension, so it makes it good at taking corners and stuff like that. I know that when I turn the car will, and at the speeds I've driven, I've never lost grip or traction.

Yesterday, as I got to the top of the hill, I felt the car roll ever so slightly towards the outside of the corner, or rather reseat its position. At first I wasn't sure what it was, so I got to thinking.

It couldn't of been the suspension, because the car had been steady up until that point, and I was more or less going straight at the time. The road was wet, but I never felt like the car had lost grip, and I wasn't turning erratically. The only other explanation I could surmise was that maybe the car had 'rolled on the tires' if that is at all possible.

So the question is, can you feel the car move on the tires, that is can you feel the tires flex as the car moves?

Any and all advice will be greatly appreciated. Thanks. 👍

When you say 'move on the tyres' do you mean the tyre sidewalls temporarily deforming? if so I have never really noticed it as it tends to be marginal and only really happens under hard cornering on high grip surfaces, something I personally have little experience of It will be the car temporarily 'going light' over the crest, as previously stated by others.

It has made me think about a scenario which I am sure people here will be able to sort out.

Assume normal dry conditions, and lets say on a closed road, in a mid engined rear wheel drive car of 50:50 weight distribution. You approach a corner that you need to slow down for. The corner starts fairly large radius but tightens towards the end of the corner, there is a slight crest during the large radius part to the corner.

Assuming the correct racing line for this car and corner, which would be the quickest way to get round this corner with regard to braking?

Now normally on a perfectly flat tightening corner I would assume trailbraking would be the best way. Squeeze the brake fairly hard with little turn in initially, reducing braking and increasing turning simultaneously as the corner tightens. Then easing the throttle slightly at the apex before acclerating harder on the exit.

The problem is with the crest. Braking fairly hard and turning slightly over the crest will want to make the car oversteer. So should the trailbraking be interrupted for the crest so that the car is hardly turning or braking?

 

[magburner]

I had totally forgot about this thread, but another serious question has prompted me to remember... Duh

When you say 'move on the tyres' do you mean the tyre sidewalls temporarily deforming? if so I have never really noticed it as it tends to be marginal and only really happens under hard cornering on high grip surfaces, something I personally have little experience of.

Yeeah, thats exactly what I meant. The corner in question did have that light brown 'high grip surface' that you sometimes see on tight bends. I'll go with the general consensus that it was because the car had lifted off the road, as it seems the most likely explanation.

Onto another question, one that is a lot more serious, and nearly caused me to wipe out at speed yesterday.

Other than a puncture, what other reason would cause a tyre to lose pressure? I'm not talking about slight pressure loss, as all car tyres lose pressure over time, I'm talking about extreme pressure loss (compared to the other tyres).

Here is a diagrammatic example of the tyre pressures I use, with the pressures I just discovered at the garage in brackets. pressures are in psi.

...............Left.............Right

Front........35 (34)........35 (34)

Rear.........38 (12)........38 (37)

As you can see, three of the tyres have lost a realistic amount of pressure, whilst the left rear, has lost a ridiculous amount. What is causing this? I first noticed the problem about a month or so ago, and put it down to a slow puncture, so I too my car to the garage and replaced both rear tyres with brand new ones.

Now, I have noticed the exact same problem again, What can be causing it? I only have two theories, as to why this would be so, I will detail them now.

Theory one

The left rear alloy wheel is either damaged or deformed in some way. This could be possible, but unlikely, as when my tyres were replaced, the wheels were rebalanced. I'm sure the mechanics would of noticed if there was a problem with the alloy wheel.

Theory two

This theory is based on pure speculation, but it is the only other feasible explanation I can come up with. Its a little 'out there', but it is the only other explanation I can come up with.

I am a fan of PGR (Project Gotham Racing), and I like the way that the game splits a city into various tracks along street plans. I do the same in my home town as a fun way of getting around town. I have constructed various 'circuits' around my town based on the quality of the corners that are available, and my need to get from one place to another (but not necessarily on time to destination or fuel economy).

One thing that my 'circuits' have in common is that the are all clockwise. Could this clockwise motion be causing the left rear tyre to deflate more than the rest? Like I have said, this is pure speculation, but it is the only other explanation I can come up with.

My car is a Vauxhall Vectra 1.8i SRi (new model) with 17" alloy wheels and Goodyear Eagle NCT tyres all round. Any help or advice would be greatly appreciated. Thanks.

 

[Greycap]

Theory three

The valve on the wheel is faulty and leaking. Perhaps?

 

[TB]

Your positive it's not a puncture?

It could be a bad seal between the tire and the rim or the valve stem could be leaking.

Edit: Tree'd.

 

[Slicks]

A leaking valve stem would be the most likely culprit. On street tires it's extremely rare to pick up more than 10 - 15 PSI due to heating, and that's in a track environment.

 

[wfooshee]

It leaks.

Driving will not vent the tire, unless it's catastrophically. The bead won't unseat unless it really really unseats, like from a curb impact or something, and that won't be a slow leak.

Use kitchen dishwashing detergent (the kind for hand washing, not the dishwasher appliance) in water to make a weak soap solution.Wipe that around the bead and the valve. No bubbles, then apply it on the tire's surface. Still no bubbles, then it's the wheel. Sometimes a wheel is too porous, or damaged from an impact, and not airtight. Only way to be sure is to go through the trouble and expense of having the tire mounted on another wheel, dismount both left side tires from their wheel, put them on the opposite ones, see of the problem follows the tire or the wheel.

 

[ExigeEvan]

Theory three

The valve on the wheel is faulty and leaking. Perhaps?

I've had the same issue with mine.


Give the valve a little flick, if it's leaking there should be a slight 'Psss' as some air is released.

If that isn't it, fill a small paddling pool with water and put your wheel into it and play spot the bubbles.

 

[magburner]

Theory three

The valve on the wheel is faulty and leaking. Perhaps?

Hmmm... Never thought of that one! LOL, is the valve part of the tyre, or the alloy wheel, and how do I get it replaced? Thanks. 👍

Use kitchen dishwashing detergent (the kind for hand washing, not the dishwasher appliance) in water to make a weak soap solution.Wipe that around the bead and the valve. No bubbles, then apply it on the tire's surface. Still no bubbles, then it's the wheel. Sometimes a wheel is too porous, or damaged from an impact, and not airtight. Only way to be sure is to go through the trouble and expense of having the tire mounted on another wheel, dismount both left side tires from their wheel, put them on the opposite ones, see of the problem follows the tire or the wheel.

Gonna try this one when I got some spare time on Wednesday. If there are bubbles, then I will go and play holy hell with the garage in question! Thanks. 👍

Thanks to you all for the good advice. 👍