Can of worms

[Battle_axe]

Right weight why run it form all my physics lessons and common sence i cannot see the point in people running weight in there car when running lipo i understand the need to bring the weight up to the min limmit but thats it surely all that needs to be done is put the weight needed (to bring it up to the limit) in places to restore the origanal weight distrbution and thats all. any people have other ideas? hope tony gets in on this too.

[Marvin]

It can make the car easier to drive. Added weight that has been placed low down can actually lower the centre of gravity, and aid grip. Also, on a bumpy track, or a loose track it can make the car less 'skittish' as it'll be affected by the ruts and bumps less than a lighter car.

[Chrislong]

Cos a heavier car is easier to drive consistently fast.

A light car is capable of being driven faster (within good track conditions and relatively smooth track), but not many can drive it to its limit at the speeds we reach as it will be harder to drive (and setup being nailed is more critical).

Think of it by scaling up our scale. The cars reach a scale speed of 300-400mph or thereabouts, reaching top speed in less than a second, and with a scale weight of 16-18kilos = hard to drive fast! (insane)

[Agt26]

Adding a little extra weight can help keep the car settled/planted on a rough track. It can also help with getting the balance of front and rear grip right.

(someone else could probably give a better explanation)

Andrew,

[Battle_axe]

i ran my 2wd last week on dirt with lipo and the car cornered allot quicker and i was much faster round the tighter sections of the track the only place i truely strugled was over the double double section of our track where i strugled to keep the nose down the car all up wth a 3800 lipo was 1370g i think if i put the lead in it to take it up to the limit in the front it sould drive ok? wrong or right?

[tymill]

I can imagine a few ways that increased mass in the car would help handling.

As mentioned earlier by Marvin, the centre of gravity can be manipulated and moved closer to the ground, which can help keep all four wheels more evenly loaded when under acceleration either laterally or forward/back. The forces acting on the car to accelerate it (friction between the wheels and the track) are on the ground, and the closer the c of g is to the ground, the less of a torque is induced to roll the car (either sideways or into a wheelie/endo).

Also, increased mass increases the inertia of the whole car, so it is more stable and "unwilling" to be accelerated in weird directions by bumps etc.

I suppose you can also manipulate the polar moment of inertia a bit - that is how "centred" the mass is in the car. If the polar moment is reduced, (mass in the middle) the car will change direction more easily than it would do if the mass was at the ends of the car. Think of how it's easier to spin a 500g mini-football than a 500g hula hoop...

Further, it improves the unsprung mass ratio so the suspension works better. That is the fraction of the car's mass that is not "suspended" by the suspension (e.g. wheels, tyres, and a fraction of the suspension parts) is lower. Think Hornet!

But hey, I'm certainly not a top-flite driver, but I've got a pretty good head for these general concepts. I'm totally willing to be corrected though!

Cheers,
Ty

[jhammond]

^That's a very good explanation!

Bravo.

[MRD]

A thumbs up to Tymill Hawking

[Battle_axe]

Originally Posted by tymill

I can imagine a few ways that increased mass in the car would help handling.

As mentioned earlier by Marvin, the centre of gravity can be manipulated and moved closer to the ground, which can help keep all four wheels more evenly loaded when under acceleration either laterally or forward/back. The forces acting on the car to accelerate it (friction between the wheels and the track) are on the ground, and the closer the c of g is to the ground, the less of a torque is induced to roll the car (either sideways or into a wheelie/endo).

Also, mass generally increases the inertia of the whole car, so it is more stable and "unwilling" to be accelerated in weird directions by bumps etc.

I suppose you can also manipulate the polar moment of inertia a bit - that is how "centred" the mass is in the car. If the polar moment is reduced, (mass in the middle) the car will change direction more easily than it would do if the mass was at the ends of the car. Think of how it's easier to spin a 500g mini-football than a 500g hula hoop...

Further, it improves the unsprung mass ratio so the suspension works better. That is the fraction of the car's mass that is not "suspended" by the suspension (e.g. wheels, tyres, and a fraction of the suspension parts) is lower. Think Hornet!

But hey, I'm certainly not a top-flite driver, but I've got a pretty good head for these general concepts. I'm totally willing to be corrected though!

Cheers,
Ty

this is all a verry good explanation and i agree with what you are saying in the end i cant argue with physics as for the foward drive this is surely something that can be corrected with differnt springs oils and as the car is now lighter running a softer car should be easyer. i am soo new to driving and racing these things but if people are adding lead beond and ontop of the weight with nimh then to be honest i cant see the improvment this can add to the car and i will save my money

[super__dan]

If it feels good do it!

[Richard Lowe]

I've always liked running my cars light, but then I also generally prefer a car with a stiff chassis to a floppy one unlike most.

One of the biggest gains from moving to lipo (especially with 2wd) IMO is the ability to move the weight to the extremities of the chassis - as tymill already mentioned. It's much better to get up to weight with things like brass bulkhead/squat blocks which give you the calming effect through bumps without the car having to heave lots of lead about letting you keep straight line performance.

[tymill]

Cheers dudes - looks like I've just been outed as being a science teacher!

And hey, for what it's worth I'll never have to worry about being underweight racing my gold tub RC10!

It's all good...
Ty

[Spoolio]

At the end it comes down to what feels right for YOU. If you can drive a light car on softer springs and lighter oil then that's great. That the majority prefer a heavier car is one of those things, I know I prefer my car to be a chubby little porker as it makes it easier to handle and god knows I have enough trouble going around a lap without fighting a wayward car as well .

Like Tymill says, it's all good, wouldn't do for everyone to be the same would it.

[Battle_axe]

well i hope to get my orion lipos for christmass or before if the bank lets me then we will see how it all goes

[Big G]

I added weight to the back of the car because it nose dived horribly over jumps.

but adding a few grammes to the left or right to balance out the electrics has to be beneficial

[Rocking Donkey]

just a quick note, if you scale the mass of a 1/10th scale car to 1/1 size, then you have to take into account that it is becoming 10 times larger in all directions. hence 1.5 kg for a 1/10th car would be 1.5 tons for a 1/1 version. (1.5 kg x 10 x 10 x10) .

Also if you consider the grip you can generate. Assuming that you have a given coefficient of friction for a given tire, the friction generated is proportional to the weight, hence:
friction = mu*m*g , where m*g is just the weight of the car. Actually depending on the surface the effect might be even better than linear as the coefficient of friction changes if the tire spikes dig into the surface, helped by a little extra weight.I'm not actually sure if this is a valid arguement or not, I'll have to think about it a little more

We also are lucky enough, to have more power than we can handle ;-) This is pretty rare in motor sports and there is little to be gained by having the most powerful motor available. This is coupled to having an almost unlimited energy source (compared to our usage) in the LiPo battery. When I fist started racing finishing a race with the batteries we were using was a real issue, 1400 mAh is not a lot. Then it was important to have a light car to minimse the energy use when accelerating for a given force (motor), being light was the only way to be fast and last the distance. This is all now not an issue.

[James]

Times also relative remember, so it needs to be adjusted too

[tymill]

Good points - esp about how we've got essentially limitless motorage these days!

Yeah, I thought about that grip issue with the coefficient of friction etc, but if you think about it, it's not a big advantage at all unless that relationship is actually "better than linear". Because the force of friction = mu * m * g you'd have proportionally more frictional force with more mass, yes. But on the other hand you'd also need just as much more force to accelerate the car in any direction, since f = m * a. Hmm. I think I'll also go into ponder-mode...

As you point out it's also interesting to remember that when we say the cars are 1:10 - we're really only taking about length - one dimension. A 1:1 car will cast a shadow 100x bigger - two dimensions. And have a volume 1000x greater - the third dimension. So if that 1:1 car is made made of all the same materials, it'll have 1000x more mass!

[Spoolio]

OMG, I thought I'd logged onto oOple but clearly I've miskeyed and ended up on an Open University online lecture . Seriously, it's nice to know the depth of knowledge we have available on here. If I stick with the whole thread, do I get a Certificate at the end .

[AntH]

.....this debate will run and run! Tyres (even model car ones) are generally non-linear, as in they do not obey F = Mu*R. Prove this to yourself by putting stiffer springs on the front of your car. This will increase the vertical forces acting on the front tyres giving more front grip, right? Nope. The sum total of the grip available (side force) from the front wheels relative to the rear will reduce due to this non-linearity. The front outside tyre 'stalls' and cannot generate any more side force.

Despite this non-linearity there is an argument that launch traction could be improved by sticking weight over the rear, up to a point. Corner entry and mid corner I think the physics would 100% say that adding weight is not good.

Over bumps? Yes more sprung mass, but more mass/energy to control....

Unfortunately due to the general randomness of my driving I can't prove any of this out anyway! Have fun.