Inertia

[Barry Cade]

Just in from a breakdown, which involved driving up and down the M90 again, quite a hilly motorway. Anyway, I was thinking about the motors I've had which seemed to just waft along, and freewheel for ages, or so it seemed, without needing to keep "on the gas". Kinda hard to explain but some of the cars I've had seemed to just roll with hardly any drag. Obviously binding brakes or crap aerodynamics ruin this, but there doesn't seem to be any rhyme or reason to which cars do and which don't. My W124 did, my W126 didn't.. Carina doesn't ,Swift did?

 

Can someone please explain what the hell I'm on about and if you even understand what I mean...

[Ben_L]

Just in from a breakdown, which involved driving up and down the M90 again, quite a hilly motorway. Anyway, I was thinking about the motors I've had which seemed to just waft along, and freewheel for ages, or so it seemed, without needing to keep "on the gas". Kinda hard to explain but some of the cars I've had seemed to just roll with hardly any drag. Obviously binding brakes or crap aerodynamics ruin this, but there doesn't seem to be any rhyme or reason to which cars do and which don't. My W124 did, my W126 didn't.. Carina doesn't ,Swift did?

 

Can someone please explain what the hell I'm on about and if you even understand what I mean...

 

I get that feeling. The Fiesta tends to carry it's speed well once you dip the clutch but the Cavalier didn't (I think the rear brakes played a part).

My works Diesel and 12valve Corsa's roll really well but the Supervisors' Focus's dont.

[Spiny Norman]

Know what you mean, my Merc W210 seems to 'drag' quite a bit compared to most of the other cars I've had recently, I've always put it down to the rwd and friction in the autobox, which doesn't seem to 'disconnect' itself from the engine on a long downhill over-run the way the ZF boxes on BMWs do.

[wackywacerwill]

I reckon tracking plays a major role here, all my vehicles require a fairly planted foot

[CreepingJesus]

When you hold the clutch in, it's only the aerodynamic drag, tyres' rolling resistance and (even assuming your brakes are tip of the toppermost) the natural tendency of disc brakes to drag slightly, that's slowing you down.

With the clutch engaged, the frictional losses in the transmission, and the engine braking come out to play. Broadly, if your gearing is 'long' (so the engine's turning over slowly at cruising speed), and the flywheel's heavy, it'll retain more of the momentum you put in to get you there. It takes more energy to put that momentum there in the first place, so there's a trade-off. A Ferrari, with a flywheel like a circular saw blade, will tend to accelerate more easily, but will also decelerate more easily too.

Autos and two-strokes tend to have less engine braking effect (or even have an actual freewheel), so they'll roll better. Maybe. (Conveniently ignores increased frictional losses in planetary auto boxes...) Hope that makes some sense.

[michael1703]

Is there a tyre width difference between the mercs?

[Barry Cade]

When you hold the clutch in, it's only the aerodynamic drag, tyres' rolling resistance and (even assuming your brakes are tip of the toppermost) the natural tendency of disc brakes to drag slightly, that's slowing you down.

With the clutch engaged, the frictional losses in the transmission, and the engine braking come out to play. Broadly, if your gearing is 'long' (so the engine's turning over slowly at cruising speed), and the flywheel's heavy, it'll retain more of the momentum you put in to get you there. It takes more energy to put that momentum there in the first place, so there's a trade-off. A Ferrari, with a flywheel like a circular saw blade, will tend to accelerate more easily, but will also decelerate more easily too.

Autos and two-strokes tend to have less engine braking effect (or even have an actual freewheel), so they'll roll better. Maybe. (Conveniently ignores increased frictional losses in planetary auto boxes...) Hope that makes some sense.

 

thanks CJ,I know all the mechanics behind what causes drag, but it doesn't seem to explain why some cars do and some don't. Some cars just seem to roll much better than others, and just need a whiff of throttle to keep them moving along. Small heavy aerodynamic cars with skinny tyres should be the best, but haven't been, in my experience. I'd love to see a drag race in reverse, where both cars cross the finish line, engage neutral and switch off the engine, and see who goes furthest.

[Pillock]

Some days my Focus seems to "waft" nicely with little power input, other days it needs a bit more poke and the fuel economy figures seem to suggest the same, perhaps it has a dragging brake or something.

 

My old Mk3 Golf once felt a bit "held back" and wouldn't coast nicely up the A46 to Lincoln, took loads of right foot to hold a steady 80. The glowing rear disk explained that one quite nicely though.

[Bobthebeard]

No science behind this statement whatsoever, but on a sunny* day, when the car has been washed and polished, quality time spent cleaning it inside and out, pleasant bit of tinkering, etc. Then taken for a run, the car usually seems to waft along much more pleasantly.

Clearly a state of mind in my case, but often seems to happen when all "is well with the world"

 

(Binding brakes excluded, obviously.)

[SambaS]

My Samba Style does not roll well at all, its a binding brake feel but comes from the engine. Hard to explain, as if the engine isn't run in or something. 20% lower top speed than it should have too. Most strange.

[dollywobbler]

Citroen BXs don't seem to roll well. I've had a few and have always felt that there's a brake binding. I've learnt to live with it as the brakes are clearly not binding! 2CV is held back by its super-low gearing. Dip the clutch and it rolls very merrily indeed.

 

But some vehicles feel laboured at motorways speeds while others don't. Oddly the BX non-turbo diesel seems quite happy to stay at 80mph for hour after hour, but the turbo diesel seems happiest at 70. The TD has taller gearing, but that means that 70mph feels very relaxed (about 2600rpm). The non-turbo diesel seems to hit a sweet spot at an indicated 80mph.

[Lankytim]

The saab seems to coast for ages, so much infact that its easy to end up in the arse of the car infront sometimes. The astra coasts well too, now i've changed the sticky front caliper!

[Barry Cade]

Citroen BXs don't seem to roll well. I've had a few and have always felt that there's a brake binding. I've learnt to live with it as the brakes are clearly not binding! 2CV is held back by its super-low gearing. Dip the clutch and it rolls very merrily indeed.

 

But some vehicles feel laboured at motorways speeds while others don't. Oddly the BX non-turbo diesel seems quite happy to stay at 80mph for hour after hour, but the turbo diesel seems happiest at 70. The TD has taller gearing, but that means that 70mph feels very relaxed (about 2600rpm). The non-turbo diesel seems to hit a sweet spot at an indicated 80mph.

 

Yes Someone gets what I mean

 

I did a couple of hundred miles in the Carina today and it bugs me how I always seem to have to power it along. MPG is normal, no binding brakes, no dodgy wheel bearings, no stupid spoilers or a sail on the roof. but it doesn't roll like a Cavalier... One of the few cars I've owned too that doesn't have a "sweet"spot.

[jonny69]

Inertia is the resistance of an object to change its motion. In space, the higher an object's mass, the higher its inertia.

 

For a car it's not that clear cut. You have mass, rolling resistance from the tyres and aerodynamic drag. At a given speed, technically a heavier car will roll on further because it has more momentum, but more mass means more rolling resistance from the tyres which is a high contributor to the total drag. You also put more energy in to get it to that speed in the first place. On top of that there's aerodynamic drag - anything large and brick-like in shape causes a lot of drag and will slow quite quickly from high speed. As an example I collected a roll of loft insulation on the roof from Southampton to London and at the same speed I noticed a couple of mpg less with the roll on top.

 

Slow speed stuff is slightly different. You can pretty much discount aero drag below 30mph due to the inverse square law. Take something like a Ford Pop, a light little car with tall skinny crossplies and simple efficient drivetrain; I could roll that thing in and out the garage and parking spaces outside my flat with one hand. Compare it to her nearly new Pug 207, which I can bare shift with all my strength and it just rolls back to a stop almost immediately; you start to understand why modern cars are no more fuel efficient than 50 years ago. It's mainly down to the weight and tyre resistance.

[HMC]

My old fairway taxi was great in this respect, almost felt like it had a freewheel as it would waft along with occasional throttle inputs. The wife s freelander 2 is terrible, its an auto and unless you keen the throttle at least partly open it immediatedly begins to slow on the flat, almost like some sort of engine braking is at work. Appartently thats what they all do, but it is annoying.

[ChinaTom]

My XJ12 maintained any speed under 50mph with no help from me even with the handbrake shoes binding. Actually at 30 mph, it drifts up to 35 by itself. I have my brakes on through all 30mph villages. If you purposefully drop the speed to 20, it won't gain any speed, but just around that 30 mph area, it will get you 3 points if you aren't careful. Not up hill though. Hills make it slow down thankfully

[catsinthewelder]

I could coast for miles in my old Tipo and often did to save fuel, the Megane and Clio wern't bad either.

 

 

The HiJet pickups that I've used at work are hopeless for it though