RE: Light cars are not the answer: Tell Me I'm Wrong

[Furyblade_Lee]

Derek Chevalier said:

anything fast said:

I know an Elise can Eat a Mustang round a track etc etc.

Elises are actually pretty slow round a track considering their power/weight

Are they??? Unless there are long 120+ mph straights, you cannot go much faster around a track than an Elise or Caterham with sub 160bhp.

You need quite a bit of extra power to overcome lightweight, especially in the braking advantage.

[nickfrog]

Derek Chevalier said:

nickfrog said:

Correct. As explained by others, higher static mass means LESS lateral grip and MORE longitudinal grip (traction)

I'm not sure how this could be true

Counter intuitive isn't it ?

Think about it. When you turn the steering wheel the force you fight against is the centrifugal force (the one that makes your car "wanting" to carry on in a straight line). The more mass, the more weight transfer and the greater the centrifugal forces that your tyres have to work against to resist losing grip.

Stick 100kgs in the front boot of a Boxster. Choose a corner where you can normally turn the car in at 50mph on the extreme limit of front grip with an empty boot, turn the car in with the added 100kgs on board. Do you reckon you're going to start understeering at 45mph or at 55mph ?

http://www.autozine.org/technical_school/handling/...

Edited by nickfrog on Thursday 8th January 20:19

[chelme]

kambites said:

There's certainly a place for both.

+1

[T0MMY]

Derek Chevalier said:

Elises are actually pretty slow round a track considering their power/weight

You need to consider that straight line performance is not based entirely around power:weight so very light cars may have a bigger disadvantage on the straights than you might think.

On paper a 500kg kitcar with say 400bhp/tonne may have the same power to weight ratio as a 600bhp car weighing 1500kg but will only really keep up when the speeds are fairly low. Accelerating off a 100mph corner will see the heavier car pulling away massively as it has 3 times the power but similar or (in 7 style kitcars) even lower drag.

In reality an Elise is losing out down the straights to any heavier car that has the same power:weight ratio so it may have lots to do in the corners.

[nickfrog]

No it's simply down to gear ratios which are not optimised for straights. They hit a barn door at 120mph.

[Derek Chevalier]

Furyblade_Lee said:

Derek Chevalier said:

anything fast said:

I know an Elise can Eat a Mustang round a track etc etc.

Elises are actually pretty slow round a track considering their power/weight

Are they??? Unless there are long 120+ mph straights, you cannot go much faster around a track than an Elise or Caterham with sub 160bhp.

You need quite a bit of extra power to overcome lightweight, especially in the braking advantage.

Yes, first noticed it when it was on a par with a 106GTI back in the 90s on a handling day issue of a magazine.

[Derek Chevalier]

nickfrog said:

Derek Chevalier said:

nickfrog said:

Correct. As explained by others, higher static mass means LESS lateral grip and MORE longitudinal grip (traction)

I'm not sure how this could be true

Counter intuitive isn't it ?

Think about it. When you turn the steering wheel the force you fight against is the centrifugal force (the one that makes your car "wanting" to carry on in a straight line). The more mass, the more weight transfer and the greater the centrifugal forces that your tyres have to work against to resist losing grip.

Stick 100kgs in the front boot of a Boxster. Choose a corner where you can normally turn the car in at 50mph on the extreme limit of front grip with an empty boot, turn the car in with the added 100kgs on board. Do you reckon you're going to start understeering at 45mph or at 55mph ?

http://www.autozine.org/technical_school/handling/...

Edited by nickfrog on Thursday 8th January 20:19

Centrifugal force? Not sure how that is relevant in the example.

I was questioning the increased longitudinal grip - not sure why adding weight will increase it.

[oversteerer]

Aren’t heavier cars more stable at higher speeds? For example, I found my M3 to feel safer and more stable than my S2000 when driving at over 130 mph.

[StottyEvo]

A great article with a well thought out opinion. Although for me personally I love nothing more than a raw lightweight car. As I read I was thinking that when I get to around 40 I'll probably agree with the entire article, the last paragraph made me snigger

I agree to some extent though, heavy doesn't necessarily mean lumbersome. A decent chassis setup on a heavy car will be more fun than a poor chassis on a light car IMO anyway.

Hopefully soon I'll have the best of both worlds, I'm looking to buy a stripped 106GTi with a cage&harnesses to use as my daily whilst the considerably heavier yet equally impressive Evo can be used on occasions.

[Terminator X]

In my old Spec C WR Ltd they even deleted the passenger sun visor to save weight, boom! Lighter battery, thinner windscreen etc too.

TX.

[errek72]

They may not be the answer. But perhaps that is because of the question you are asking. Strange to start a reasoning with the outcome, no?

[otolith]

T0MMY said:

In reality an Elise is losing out down the straights to any heavier car that has the same power:weight ratio so it may have lots to do in the corners.

Good power:weight, poor power:drag.

[errek72]

bnracing said:

I am a massive Alfa fan but they have never been known for building cars with correctly setup dampers for English roads. Also another down fall of the 4c is its massive heavy wheels and low profile tyres neither of which suit a lightweight car.

Oh there we go, the imfamous English roads.

[nickfrog]

Derek Chevalier said:

Centrifugal force? Not sure how that is relevant in the example.

I was questioning the increased longitudinal grip - not sure why adding weight will increase it.

Centrifugal force is 100% relevant when it comes to lateral grip. It's the prevalent force when turning.

I didn't realise you didn't agree with weight benefiting traction though, sorry. Simply because the added mass exert more vertical load on the tyre, which in a straight line means more friction. Think typical 911 traction for example.

Or think of a couple of big blokes jumping on the back of a stuck RWD car stuck in the snow to compress the springs, they try to apply transient weight that will be enough to generate traction to make the car move off its position.

[errek72]

T0MMY said:

Derek Chevalier said:

Elises are actually pretty slow round a track considering their power/weight

You need to consider that straight line performance is not based entirely around power:weight so very light cars may have a bigger disadvantage on the straights than you might think.

On paper a 500kg kitcar with say 400bhp/tonne may have the same power to weight ratio as a 600bhp car weighing 1500kg but will only really keep up when the speeds are fairly low. Accelerating off a 100mph corner will see the heavier car pulling away massively as it has 3 times the power but similar or (in 7 style kitcars) even lower drag.

In reality an Elise is losing out down the straights to any heavier car that has the same power:weight ratio so it may have lots to do in the corners.

You're right.
Question now is which car is more fun and if that can be related to weight or not.

[71tuscan]

iwantcheese5 said:

Yes, but for a given shape car the lighter one will use less fuel. At the end of the day all areas of the car need to be optimised.

An easy way to reduce the weight of cars would be to make them smaller...

Noticed this a while ago when my Chimaera was parked next to an Opel Insignia in the office car park. The Chimaera was short in it's day, but certainly not a narrow car. Next to the Insignia it looked like a nimble toy and the Insignia looked like a passenger ferry. No wonder new cars are that heavy...
If you compare an 80s BMW 5 series, Mercedes SL or Porsche 911 with their current versions, they,ve more ore less doubled in size.

[Kipplemaster]

The Alfa 4C is ugly. Tell me I'm wrong.

[errek72]

71tuscan said:

iwantcheese5 said:

Yes, but for a given shape car the lighter one will use less fuel. At the end of the day all areas of the car need to be optimised.

An easy way to reduce the weight of cars would be to make them smaller...

Noticed this a while ago when my Chimaera was parked next to an Opel Insignia in the office car park. The Chimaera was short in it's day, but certainly not a narrow car. Next to the Insignia it looked like a nimble toy and the Insignia looked like a passenger ferry. No wonder new cars are that heavy...
If you compare an 80s BMW 5 series, Mercedes SL or Porsche 911 with their current versions, they,ve more ore less doubled in size.

Oh yes

[otolith]

nickfrog said:

Simply because the added mass exert more vertical load on the tyre, which in a straight line means more friction. Think typical 911 traction for example.

That's weight distribution, though, not weight. An Elise has bags of traction compared to something heavier like an MX-5 for the same reason.

[thegreenhell]

nickfrog said:

Derek Chevalier said:

Centrifugal force? Not sure how that is relevant in the example.

I was questioning the increased longitudinal grip - not sure why adding weight will increase it.

Centrifugal force is 100% relevant when it comes to lateral grip. It's the prevalent force when turning.

I didn't realise you didn't agree with weight benefiting traction though, sorry. Simply because the added mass exert more vertical load on the tyre, which in a straight line means more friction. Think typical 911 traction for example.

Or think of a couple of big blokes jumping on the back of a stuck RWD car stuck in the snow to compress the springs, they try to apply transient weight that will be enough to generate traction to make the car move off its position.

But in those examples you are talking more about weight distribution, not increasing overall vehicle weight. In the second example you are adding a large additional load, perhaps even double, to a lightly loaded axle, while adding a proportionally much smaller load to the total vehicle mass. If you got those two same burly blokes to sit at the car's centre of gravity so that overall weight and individual axle loading were increased proportionally then you wouldn't see the same increase in traction.

Newton's second law (F=ma) tells us that the force required to accelerate a given object is directly proportional to its mass.

The tyre friction curves tell us that the friction force does not increase proportionally with load.

Longitudinal traction does not increase with increasing vehicle mass.


As with the lateral grip discussion, there is some confusion and mixing of terms appearing in some posts. Tyre friction force is not the same as grip and traction. Grip and traction are merely the resultant forces experienced by the car and its driver.

Lateral friction force of the tyre increases with increasing vertical load (vehicle mass), but in a non-linear fashion. See the tyre friction curves already posted in this thread.

Centrifugal force, that which is required to hold an object on a circular path (ie a corner), increases linearly with increasing mass. F=(mv^2)/r, or F=mrw^2

Grip, as felt by the driver as cornering G force, decreases with increasing vertical tyre load, because the lateral friction force of the tyre increases at a lower and ever decreasing rate compared to the required centrifugal force.