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    How does fuel sloshing about affect handling?

    If at all...? It just occurred to me that the instability that I was describing in the winter tyre (mpg) thread was just like trying to push a half-full bottle of water across a table.... I hope you can understand what I mean!

    So, with the tank being in the front of the car, what stops the fuel sloshing about and compromising the handling? I'm guessing the design of the fuel tank has baffles or something in it??? Anyone with technical knowledge help me out?

    Re: How does fuel sloshing about affect handling?

    Quote:
    wtsnet said:
    Anyone with technical knowledge help me out?




    Sorry, no idea But you might ask Ferrari's F1-engineers. AFAIK they have invented some kind of high tech pump in order to resolve the problem for Schumi's weekend-toy

    Re: How does fuel sloshing about affect handling?

    If sloshing is a problem, the tanks have baffles. These are simple plates that stop the fuel surging across the width of the tank. Some cars don't need them and I'm not sure about Porsche. However, 30-40 litres of fuel surging across the car in a high g manoeuvre would probably be felt by the driver (soon after the turn is initiated) so I would guess the Porsche tank does have baffles.

    Re: How does fuel sloshing about affect handling?

    Quote:
    GR said:
    If sloshing is a problem, the tanks have baffles. These are simple plates that stop the fuel surging across the width of the tank. Some cars don't need them and I'm not sure about Porsche. However, 30-40 litres of fuel surging across the car in a high g manoeuvre would probably be felt by the driver (soon after the turn is initiated) so I would guess the Porsche tank does have baffles.




    An interesting question.

    Since this is the Boxster Board, let's base our thoughts on the Boxster:

    - This car's fuel tank is a plastic blow-moulding, so it is a one-piece moulding with a narrow neck, like a detergent bottle , so no access to install baffles as could be done with a pressed steel tank made of two shells welded together.

    - When tank is full, there is no room for fuel to slosh around, so no problem.

    - When tank is almost empty, the weight of the remaining fuel which could slosh around is negligible, so no problem.

    - Let's assume that worst case is when tank is half full, therefore containing 30 liters of fuel, which would weigh roughly 24 kilograms. Imagine a 1.0 g left hand turn on road tires (the highest lateral acceleration you are likely to achieve on normal asphalt). The half of the fuel (12 kg) on the left-hand side of the tank will tend to slosh over to the right hand side. The fuel surface in the tank will be at a 45 degree angle to the ground, due to 1.0 g (gravity) acting downwards and 1.0 g lateral acceleration due to cornering. So only half of the 12 kg (= 6 kg) will slosh over to the right hand side of the tank).
    Assuming 600 kg of the Boxster's total weight of roughly 1300 kg act on front wheels, the "normal" load per front wheel would be 300 kg per side. Assuming that full weight transfer of fuel acts only on the front wheels, because tank is ahead of center of gravity of the car, then the weight transfer of fuel to the right hand wheel would increase wheel loading from 300 to 306 kg.

    I think it is safe to assume that the weight transfer of 6 kg due to fuel sloshing around in the tank would hardly be noticeable compared to the other effects on wheel loads arising during hard cornering.

    The above "analysis" includes several assumptions and approximations for the sake of illustration, but I think the conclusion will not be too far from the truth.

    Re: How does fuel sloshing about affect handling?

    Fritz, good analysis

    Re: How does fuel sloshing about affect handling?

    Quote:
    fritz said:
    Quote:
    GR said:
    If sloshing is a problem, the tanks have baffles. These are simple plates that stop the fuel surging across the width of the tank. Some cars don't need them and I'm not sure about Porsche. However, 30-40 litres of fuel surging across the car in a high g manoeuvre would probably be felt by the driver (soon after the turn is initiated) so I would guess the Porsche tank does have baffles.




    An interesting question.

    Since this is the Boxster Board, let's base our thoughts on the Boxster:

    - This car's fuel tank is a plastic blow-moulding, so it is a one-piece moulding with a narrow neck, like a detergent bottle , so no access to install baffles as could be done with a pressed steel tank made of two shells welded together.

    - When tank is full, there is no room for fuel to slosh around, so no problem.

    - When tank is almost empty, the weight of the remaining fuel which could slosh around is negligible, so no problem.

    - Let's assume that worst case is when tank is half full, therefore containing 30 liters of fuel, which would weigh roughly 24 kilograms. Imagine a 1.0 g left hand turn on road tires (the highest lateral acceleration you are likely to achieve on normal asphalt). The half of the fuel (12 kg) on the left-hand side of the tank will tend to slosh over to the right hand side. The fuel surface in the tank will be at a 45 degree angle to the ground, due to 1.0 g (gravity) acting downwards and 1.0 g lateral acceleration due to cornering. So only half of the 12 kg (= 6 kg) will slosh over to the right hand side of the tank).
    Assuming 600 kg of the Boxster's total weight of roughly 1300 kg act on front wheels, the "normal" load per front wheel would be 300 kg per side. Assuming that full weight transfer of fuel acts only on the front wheels, because tank is ahead of center of gravity of the car, then the weight transfer of fuel to the right hand wheel would increase wheel loading from 300 to 306 kg.

    I think it is safe to assume that the weight transfer of 6 kg due to fuel sloshing around in the tank would hardly be noticeable compared to the other effects on wheel loads arising during hard cornering.

    The above "analysis" includes several assumptions and approximations for the sake of illustration, but I think the conclusion will not be too far from the truth.



    I almost had to start a spreadsheet to get all that down

    Re: How does fuel sloshing about affect handling?

    Quote:
    ajcastaneda said:
    I almost had to start a spreadsheet to get all that down



    I'll e-mail mine to you so you can check whether I cheated by entering the wrong formulae.

    Re: How does fuel sloshing about affect handling?

    Cool Fritz - some good response there.
    Interesting about the construction of the tank. I knew it was plastic, but didn't know that detail.
    How about this - the handling affect that I've noticed (that might be down to tyres, fuel, driver ... ) is most pronounced when....

    When driving down a straight road at 80mph, drive like BA Baracus in The A Team () ie, make small but rapid left-right-left movements with the steering wheel. If you get the right frequency, then you get a situation that just as you are steering left, the car is starting to go right... a sort of resonance starts to happen, a bit like the Millennium bridge in London Docklands. (Before they fixed it).

    Or, to put it in more real-world terms, if you had to perform a high-speed lane-change manoeuvre, then (on a bad day) I find that I'd have to noticeably counteract the wobble that happened after I'd switched lanes. Damn thing behaves like an SUV!

    Might be:
    a) Me
    b) Tyres
    c) PASM
    d) Half a tank of fuel.

    If (as per the thread title) it is d)... Fritz - what happens if the tank isn't baffled and you have half a tank and you set up a resonant slalom type movement? What sort of frequency of slalom would build a resonance effect?

    If anyone would like to reproduce my little experiment and post their findings, then be my guest. Drive like BA people!


    Yes, I'm bored!

    Re: How does fuel sloshing about affect handling?

    Incidentally, fritz, I don't quite agree with your physics - that's kind of a static 'after things have stabilised' situation.

    What I'm talking about is when you are going straight, then you suddenly hit a left bend of a constant radius. You steer, the 12kg mass suddenly moves across the front of thecar, hits the right side of the car and stops. Isn't it more like someone 'dropping' a 12kg bag of potatoes on the side of your car, then removing it? (as the wave of fuel bounces off the right hand side of the car?)...
    It's more of a fluid dynamics issue, than a steady-state issue, surely?

    Have you ever rocked from side to side in an elevator? How much do you weigh compared to the mass of the whole elevator?...

    Re: How does fuel sloshing about affect handling?

    I think that what you have in mind is caused by the so-called hysteresis of the tires, ooops, tyres.

    The contact patch of the tyre does not quite move in synch with the turning of the road wheel, due to flexing of the tyre sidewall. By the time the contact patch does catch up with the wheel, you've overshot the path you were really aiming for with your steering input, so you have to correct by turning the wheel back slightly past the center position.

    You could counteract it by "anticipating" this effect, eliminating the need to correct.

    It might feel like a "resonance" if you drive a slalom path, but it will only happen once if you do not repeat the opposing steering inputs.

    I'm going to have to call it a day now, because I'm tyred, ooops, tired.

    Re: How does fuel sloshing about affect handling?

    fritz - love your sense of hunour

    Yeah, I'd thought it was down to the soft winter tyres, but Percymon reckons the Alpins have a relativley stiff sidewall... didn't manage to find a GT Purely Porsche today to check out that article.

    So then I thought about the fuel thing - I'm keeping an open mind... I just find it interesting! If it is the tyres, then it's odd that sometimes they feel better than at other times, and I'm not sure it can be correlated with the ambient temperature.... When it's bad, it's really noticable, and I want to give the Alpins the 'benefit of the doubt'.

    I have filled half a tank, but haven't had time to give it a good high-speed jiggle test yet. You can't really tell at low speed....

    Re: How does fuel sloshing about affect handling?

    I should have added that tank shape can minimise the problem too. My dad has an old MG with a slab tank on the back. If you corner hard (more of a lurch really) and then stop, you can feel the car gently rocking. But with live axles, I don't think the handling is overly compromised.

    Re: How does fuel sloshing about affect handling?

    Thanks GR, makes sense. Anyone else?

    Re: How does fuel sloshing about affect handling?

    Racing fuel cells have open-cell foam inside them to reduce sloshing (and improves fire safety).

    Re: How does fuel sloshing about affect handling?

    Yes, the cells disrupt and stop the flame front if there is a fire. Armoured vehicles have yet another system that works on the same principle.

    Re: How does fuel sloshing about affect handling?

    FYI, on Sunday eve, with half a tank, handling was pretty good. Lateral grip seemed better too, though 9c temp - I'm veering towards the opinion that it's just all the salt on the roads at the moment... we had rain, and they didn't re-grit.

    Getting back on topic though, as the fuel dropped to 1/3 to 1/4, I felt that I could feel the affects of sloshing. Admittedly I was trying to provoke by deliberately setting up a resonant left-right wave in the remaining fuel slosh. After several small left-right-left steering inputs, I'd return to the straight ahead and hold the wheel firmly. I swear I could then feel two tugs on the steering, one to the left, and another to the right. Both seemed to be step impulses, which leads me more to the fuel rather than tyre hysteresis explanation.

    Additionally, though I might be imagining it, when going round a fast left bend and twitching the steering (Ie, follow the curve in a series of step corrections, rather than smoothly), there seems to be more of an impulse force hitting the right side of the car than when you do the same thing on a right hand bend. This might be something do do with the shape of the tank - the fuel filler being on the right hand side of the car.

    This is all very much my own musings - I may be right, I may be barking up a non-existent tree, it may be more noticeable because of my current tyre setup... who knows! I'm still learning. I must attend a driver ed track day sometime!

     
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