mx5cartalk.com

This may sound strange but bear with me. I drive over the gateway bridge a few times a week, which for those of you who don't live in South East QLD is a fairly long. very steep bridge on the gateway motorway in Brisbane with an 80 km/h hour speed limit.

I usually drive my Octavia RS down to uni but last week needed to take my MX5. In the RS, if I clutch in or put the car into neutral at the top of the bridge, I need to brake to ensure I don't go over 80, I believe I could possibly get to 100 km/h but I am of course a law abiding citizen. Driving the MX5 last week, the car was struggling to get up to 60 km/h rolling, probably getting to around 55 before I put it back into gear.

People would argue that it is because the Octavia is heavier, which is true, but my Fiesta XR4 used to roll faster than 80 km/h and weighs the same as the MX5.

Why do you think the 5 rolls more slowly than other cars? Is it just my 5 or do others have similar experiences?

With my fairly limited grasp on aerodynamics, I would imagine that a car that rolls more slowly expends more fuel wastes more power than a car that rolls more freely.

As an endnote, I understand that it isn't good to roll down hills in neutral, and this is not something I usually do. I just sometimes like to conduct highly scientific experiments such as this to see what the results are

Jack the car and check for brake drag if it is apparent on the rear axle it may be a badly adjusted hand brake.
Greg.

Weight doesnt change anything, gravity is 9.8m/s/s regardless of mass.

Its rolling resistance, part of it is poor aerodynamic drag performance, and your brakes are dragging. tyre pressure comes into play with resistance so check that.

Dann

NitroDann wrote:Weight doesnt change anything, gravity is 9.8m/s/s regardless of mass.

Its rolling resistance, part of it is poor aerodynamic drag performance, and your brakes are dragging. tyre pressure comes into play with resistance so check that.

Dann

Ah, that was silly wasn't it. Should have remembered that weight doesn't have an effect.

Tyres are currently 26 PSI as per factory door cards, wheels are stock. Will have a look at increasing this.

Brake calipers were replaced during roadworthy just before I bought the car 4000 KM ago and the discs were "refurbished" according to the receipt I have here. Not sure what the mechanic meant, machined maybe? I will get somebody to have a look at the brakes for me, I am not very mechanically-minded (although I am learning)

I do drive in S.E. Queensland, And know the gateway bridge.
I think wind can be a major player here, I have dríven my Motorcycle over that bridge many times
and you can really feel the wind on a bike.
Perhaps a Highly Scientific experiment would involve getting off the gateway, and going over the bridge in the other direction.
None the less, I think your car should gain speed going down hill in neutral.

I'm going to have to agree and say that you were probably driving into the wind.
Interesting to think about rolling resistance, I guess it have effects spill over into your regular 'throttle open' driving too.

You're usually better with your physics than this, Dann.

The car is coasting down a slight incline. Let's assume that rolling resistance is a trivial factor. The main thing here is the balance between two opposing forces: the force of the car being accelerated by gravity versus the force of the air against the car as it moves through it.

The aerodynamic force is wind pressure times area. Wind pressure is dependent on speed and aerodynamic efficiency, so l let's just assume that we're talking about what happens in a very narrow band around 80kmh with the same car.

The gravity force is the old F=ma: force is mass times acceleration. Acceleration is constant n this case, being g times a slope factor. If you increase the mass then you increase the force the car exerts on the air.

You should be able to add sandbags to the car until it holds a steady 80kmh on that hill in windless conditions without resorting to stuffing about with brakes, etc.

A different way to look at it is to look at variations in wind pressure, which is most easily done by varying speed. On the same slope each car will ultimately reach an equilibrium point where gravity forces equal wind forces and the car remains at steady speed. The same MX5 carrying 0, 2 or 4 sandbags may achieve steady state at perhaps 60, 70 or 80 kmh respectively.

I know that force increases with mass, hence mass increases weight. You are right in pointing out that saying that weight doesnt change anything, is incorrect.

Dann

I do this all the time. If I thought it was in any way dangerous I wouldn't do it.
Obviously there are a bunch of considerations: other traffic, cross streets, traffic lights etc. You use your common sense.
I have often thought about the differences in 'free-wheeling speed' attained between the different vehicles I drive.
In most cases, I concluded that it was related to the exact speed of the vehicle when placed in neutral and the exact point on the road where this was done, rather than anything about the vehicle.
I assumed wind conditions were also important, particularly I would imagine on the Gateway, although it would be a cross-wind rather than a headwind/tailwind wouldn't it.

AntHarmer wrote:Why do you think the 5 rolls more slowly than other cars? Is it just my 5 or do others have similar experiences?

It's just your MX-5. You have some frictional forces that should not be there - tyres (but pressures would have to be extremely low) , brake drag and/or some aerodynamic drag - hands above windscreen as speed brakes

In both our cars (BMW hatch and NC), in 5th or 6th gear and engine braking (I never use neutral unless stopped), I need to brake to stay under 80 km/hr. I think I need to be in 4th in either car to hold 80 km/hr using engine braking.

Dann's physics is correct - mass (weight) has no theoretical effect in the same environmental conditions.

The wind on the Gateway bridge is often stron, and seldom a straight crosswind, So yes it can get you on an angle.
Most of the time, None the less the gateway bridge is fairly steep, And I always have to brake to keep from building speed.
No matter what I am driving.

Chuck a fat chick in the car and see if that makes a difference. and put some more pressure in your tyres. 26 is a bit low.

AntHarmer wrote:This may sound strange but bear with me. I drive over the gateway bridge a few times a week, which for those of you who don't live in South East QLD is a fairly long. very steep bridge on the gateway motorway in Brisbane with an 80 km/h hour speed limit.

Back in the day, The gate way is a private road, you could knock yourself out,it may of changed now. But back when I was working on it. You couldn't get booked for speeding on it. That was less than 3 years ago.

Important thing here is that I wasn't behind some crazy old scientist going down the gateway at 55kmh. That would have pissed me right off.

Just out of interest, how does driving in neutral sit with you in safety terms? And what does the law say?