RWD car pulling to left-right. LSD to blame?

[cantfindausername]

I've got a KAAZ 2-way diff fitted on my car, and have covered about 5k miles since installation.

I've just completed a 3300 mile trip around europe and today was the first time I've driven the car since being back.

I noticed that the car is pulling to the right when I accelerate, and then pulling to the left when I let off.

Its not just a gentle glide to each side either, its quite a promanent steering in that direction, even noticable by passengers.

Is this likely to be a problem with the Diff? I followed the break in procedures, and currently run the box on Redline MT90 gear oil.

Advice and opinions welcome as always.

Thanks,
Ant

[cantfindausername]

Problem sorted. Turns out that the rear right tyre was down to 14psi (slow puncture) - Never saw this when running the stock diff as it didn't lock so well.

Guy, please feel free to remove this topic if you feel it's now irrelevant.

[Guy Croft]

Ant, hi

no, it's a useful thread, it should stand, OK?

GC

[cantfindausername]

Thats fine with me Guy.

[Colin Haggett]

This why the checking of tyre pressures is so important, Can cause real handling problems on both road and competition vehicles.

[Guy Croft]

Quite so Colin, but of course unless you have manufacturer's data on tyre size and pressure it can be tricky. Change the wheel/tyre type and you're on your own..

It hasn't been mentioned here and a lot of folk won't know how to set pressures 'from scratch'. Would anyone care to talk about the use of a tyre pyrometer to set the pressures for optimum tyre contact patch? The influence of corner-weight settings would be useful too.

GC

[Mats]

I think there are probably as many ways to do this as there are drivers/mechanics. I can start with how I do it and maybe someone can pick up the ball and add to the thread.

Car:
Alfa Romeo GTV2000 (Alfetta)
900Kg/130Hp
I mostly race on tight, twisty tracks but they are still quite fast (120-130Km/h average)

Tyres are 195/50-15 or 205/50-15, "R" compound. I.e. Yokohama 032R, 048, Bridgestone 540S, Kumho V70A or similar. Most of these tyres in this size seem to enjoy a hot pressure of around 2.0bar.

Depending on the weather situation you will need to guessimate the cold pressure but it will range from about -0.6 to -0.4bar from hot pressure. In my experience (and this I can't explain) different tyres have different pressure rise for the same hot temp. This is very counter intuitive but as it comes to tyres not everything is working in line with what you would expect.
So if you have a nice warm day and aim for a hot pressure of around 2.0 bar 1.6bar would be a good starting point. Maybe now is a good time to talk about tools. I use:
1 Good quality tyre pressure gauge.
1 Tyre pyrometer (probe type, not IR/surface).
1 Setup sheet to write down the initial setup and weater + changes.
1 Laptimer and/or datalogger. Laptimer is important.
(option)
2 Friends to measure temp and write it down on the setup sheet.

Begin with a few warm up laps to get the car/driver up to operating temp and to put some base temp in the tyres, if this is the first time on a new set you might want to do a quick check in the pits so the pressure looks ok, as I said some tires raise the pressure more then others.
Go out and make sure you do a couple of really fast laps and then make a F1 style entry into the pits and have your gear/mechanics as early in the pitlane as possible. It's important you really drive hard all the way to the measuring site because the track will cool the tires down if you coast around to save the brakes (by the way, stay off the brakes in the pit!).
Record inner, middle and outer temps per tyre and the pressure, if the pressure is too high adjust down to just under what you aim for, say 1.95 in this case.
Leave the pits and repeat this procedure until the temps/pressure have stabilized or there is a big temp variation across the tyres.
Do some cool-down laps to cool the brakes, engine/gearbox/diff and tyres, heat soak can be pretty bad. Next you need to analyze your data.

Load = temp.

If your outer temps are constantly higher then the inner ones you need more negative camber.
If your inner temps are higher you need less negative camber.

If your tyres are hotter in the middle the pressure is too high
Hot on the edges (both) you need to inflate.

This sounds very simple but as you can have both too much negative camber and too much pressure at the same time it can be a bit tricky to catch all the variations.

There are also other issues that can make things a bit difficult, my car for instance almost lifts the inner front wheel in the corner and therefore heats the tyre a bit uneven and makes the tyre wear strange looking. This is very difficult to catch without having someone looking at the car in the corner or in my case have someone take a series of photos during a hard turn. Never underestimate the importance of the double check/reality check. Normally you might dial in less neg. camber to deal with the wear pattern and temps but the laptimer indicated that this was not the right way to go and the pictures proved why.

In the end the laptimes must be what you aim to get as low as possible (for sprint racing) and sometimes that makes you set up the car a bit differently then what the temps tell you.

Since all changes to the setup (toe-in, corner weight, fuel load and so on) will make load the tyres differently you will always have to play around with the pressure and camber to get the most from the tyres, this is why it's extremely important to have a setup sheet record to have all the used setups recorded and what worked best for that particular setup. It will shorten the dial-in time a lot!

This is my experience, the pressures are not the same for othertyre sizes and you need to figure out what works on your car/setup. Don't be afraid to experiment, sometimes going to extremes will open up a whole new world of experience!

Sorry for the long post.

[Kev Rooney]

The backyard mechanics version for road cars with occasional track use requires a stick of chalk or light coloured crayon. Mark a single line across the full width of the tread front and rear and drive the car gently for a mile or so. Inspection of how much of the telltale remains will tell you whether the tyre is over or under inflated ie All worn from the middle with edges still remaining =over inflated and vice versa .

Tyres pressures need to be adjusted in conjunction with shock absorber dampening rate if they are adjustable .Also if the spring rate is too high excessive tyre pressure will make this worse and 'launch' the whole wheel at every bump.

The larger the tyre and rim for any set given weight of vehicle the lower you will need to set the initial pressure.

[cantfindausername]

Great information guys. Made for a very interesting and worth while read!

[mtbr]

The different rates of pressure rise with temperature, for different tires, Mats mentioned, is usualy due to varying amounts of moisture trapped in the tire. If the regs allow, filling the tires with Nitrogen or "dried" air makes this aspect of set-up easier, as the pressure variations are much reduced.
Sometimes you will find one tire, on the car, that goes way higher than the others; this is frequently the first one of the set fitted.
Fully deflate it and pump it up again, (before you spend too much time looking for something wrong with the car.)

The moral here is purge the airline properly (and maintain the filter drier) before you inflate the tires.

Mark Wilson

[cantfindausername]

So would this moisture be a more promenent problem from the bif compressor type of air sources, as opposed to a foot pump say?

I've seen the adverts for nitrogen filled tyres in the Demon Tweeks catalogue, and have often wondered about how goot this was. Does it need refilling/gas changing regularly or anything like that?

[mtbr]

So would this moisture be a more promenent problem from the bif compressor type of air sources, as opposed to a foot pump say?

I've seen the adverts for nitrogen filled tyres in the Demon Tweeks catalogue, and have often wondered about how goot this was. Does it need refilling/gas changing regularly or anything like that?

It is unlikely you will introduce a significant quantity of water into the tire using a foot pump. The problem usually occurs when tires are fitted track side and inflated with a compressor mounted in a van. Such arrangements rarely have adequate filters or traps.
Theoretically, tires fitted on a warm, humid day will have more moisture in them, than tires fitted on a cold dry day. This may acount for the differences in the behaviour of different sets of tires, however the situation where one tire responds to temperature in a significantly different way to the rest, is usually due to condensate, from the airline, being introduced to the tire.

To use Nitrogen effectively, you need access to the equipment whenever you need to add gas to the tires. Having tires filled once and then having to add air in service partially defeats the purpose. The main benefit is that pressure rise with temperature, is more consistent and smaller in magnitude, than with air. This makes adjusting tire pressures far more simple, with the added advantage that the car doesn't need to start the race on underinflated tires, in order to avoid serious overinflation later in the race. Suppliers of Nitrogen filling equipment also claim that the tires don't oxidise, when filled with Nitrogen and therefore stay more flexible. Frankly, this claim is of little importance, given that most race tires will be well passed their best due to heat cycling, long before the air in the tires makes them go hard!

Many club level race series have banned the use of nitrogen (and tire warmers) to help keep costs down. In this situation, the best you can do is make sure the air you fill your tires with is as dry as possible.

[cantfindausername]

I see your points about it being more a "where every second counts" type of concern. For your average track day driver, there would be very little reason for having this sort of equipment on hand, and chances are I'd only use a foot pump to pump the tyres up anyway.

Great info to have though, always keen to expand my knowledge of stuff like this!

Ant

[TR-Spider]

I was always wondering about this Nitrogen thing.

The air we breathe is aprox. 78% Nitrogen and 21% Oxygen, where there is no mayor difference in the expansion behaviour between the two gases.
So the main argument remaining is the water-content.
If we fill our tire with ambient air 20°C of 60% humidity, it contains 8g water per kg air.
Aprox. tire air mass 0.150 kg -->so 1.2 g water.
Now, if the tire heats up to 70°C this 1.2g water will change its volume...a bit.
Actually the volume difference (pressure difference) seems to be more in the 1/10 percent range.
The very worst case, pressurized air with 100% humidity at 70°C makes a bold 3% volume difference compared to dry air.

Sounds like another modern myth to me.

Thomas

[mtbr]

The presence of moisture can be significant.
Consider some typical data from a Lotus Elise race car running on Yokohama A048R tires Operating temp for this tire is between 60 and 90 deg C. For the front tires, we found that we achieved the best temperature distribution, across the tread width, with a hot operating pressure of 24-25 psi.

By measuring the tire temps after one lap and then taking further measurements as the tire cools, we are able to estimate the tire temperatures with the car on the grid after the parade lap. These are typically around 35 deg C. Although they can vary a lot, as this phase of the pre race is often subject to delays and disruption.

If the air in the tires were perfectly dry (or nitrogen) and the mass of gas correctly adjusted to achieve 25 psi at a mean operating temp of 75deg C then we would expect the tire pressure on the grid, before the start of the race to be 22psi. (pressure is proportional to temp at constant volume) This would present no particular problems, the car would respond normally, albeit with less grip than when the tires are fully warm and there would be little risk of damaging the tire sidewall due to under inflation.

Now consider the situation where sufficient water has been introduced into one of the tires for a saturated vapour to exist at full operating temp. ( Although this is an extreme example, this situation can occur when condensate is sprayed from the air line into the tire.) The difference between the saturated vapour pressures at 75 deg C and 35 deg C is around 4.5psi (estimated using Bridgeman & Aldrich coefficients for this temp range)

This would mean the car is now waiting to start the race with one tire at 17.5 psi.
This car will require careful handling on the first lap and will probably concede some places on track until the tire comes up to operating temp/pressure. Later in the race, the situation can go then other way. If the tire is overheated, then pressure increases above the other tires, the centre of the tread gets even hotter due to the over inflation and so on.

Moving back from this extreme example, it should be noted that in practice, the pressure change with respect to temperature usually exceed that indicated by proportion alone and that pressure variations due to the presence of moisture are not linear where a saturated vapour can exist.

Whether the use of Nitrogen (or dried air) is justified or not, is a matter for individual teams and series organizers. However, I stand by the practical point in my initial post, the presence of moisture in tires introduces yet another variable into an already complex relationship and if you have one tire exhibiting marked pressure changes relative to the others, then it’s a good idea to check that there is not some water in it, before you go looking for broken dampers, bent wishbones or other problems.

Mark.