Originally posted by kb58
quote:
Originally posted by smart51
...Front engined cars can be front heavy giving poor turn in and poor traction but better braking...
Rear-mid engined cars can have a 50/50 bias or can be rear heavy. This leads to good traction but can give poor turn and braking due to the lightly loaded front.
quote:
Originally posted by Kowalski
A mid engine setup should be quicker in a straight drag, just because more of its weight is on the driven wheels so it has more traction, and the same will be true accelarating out of every corner.
[Edited on 12/12/05 by Kowalski]
Actually the opposite is true. Having all the weight on the front tires means the rears are doing nothing, unable to help stop the car. The coefficient of friction is not linear, that is, doubling the weight on the tires does not double the braking ability. A mid-engine car will decelerate better then a front-engine car because all four tires are better able to aid braking.
Compare the braking distance of a Porsche and a Acura ITR. The rear engine placement of the Porsche keeps the rear tires on the ground, allowing them to help stop the car.
smart51 - 12/12/05 at 03:38 PM
quote:
Originally posted by kb58
Actually the opposite is true... Having all the weight on the front tires means the rears are doing nothing...
Read my post again, especially the bits about weight distribution. A front bike engined seven can have a rear weight bias a bit like a mid engined car. BMWs aparently have good weight distributiuon as well and they are front engined.
quote:
Originally posted by kb58
A mid-engine car will decelerate better then a front-engine car because all four tires are better able to aid braking.
Really? Putting the engine at the back aids braking compared with a front engined car, if they both weigh the same, have the same weight distribution and the same tyres etc?
A car with too much rear weight may not reach 50/50 even under hard braking. For best braking you want >50% brake force at the front. Tyre grip is non linear with weight, I agree, but you will still want >50% front weight for best braking effort.
[Edited on 12-12-2005 by smart51]
nre - 12/12/05 at 04:10 PM
Yep, I think you are both right! To get the most out of the tyres under braking, ideally you want to share the vertical load equally between the 4 tyres. So the ideal for braking would be a static weight distribution that resulted in a 50:50 split at max braking. From some rough calcs, I reckon my Mojo (60% rear weight bias) gives a 50:50 weight distribution at about 0.6g decel, so at max braking the front axle has more of the weight on it than the rear. So to optimise braking you'd probably want something with approx 70% rear static distribution. A car with a static distribution biased to the front will end up with a very light rear end under max braking, with the fronts doing most of the braking. Obviously this has to be balanced against handling, where a 50:50 split will generally be best...
kb58 - 12/12/05 at 04:58 PM
quote:
Originally posted by smart51
A car with too much rear weight may not reach 50/50 even under hard braking.
True, but while it's possible to design a car with a lot of rear weight bias, how much is too much? Say, 65% at the rear? Time for an example.
We have a 1700lb car with 65% rear/35% front weight distribution, a CG height of 15" and a wheelbase of 100". This gives 1105lbs on the rear axle and 595lbs on the front.
Assuming 1G deceleration, 255lbs is tranferred from the rear to the front axle, so now there's 1105 - 255 = 850lbs at the rear and 595 + 255 = 850lbs a the front. Perfect 50/50 on the tires.
Of course tire section size plays an issue, and typically RWD cars have larger rear tires, so in the above case the rear tires will be doing more of the braking.
Having said this, I have no real-world data for stopping distance to back this up, but for now I still feel a rear weight bias helps braking.
Chippy - 13/12/05 at 12:58 AM
Automotive design is I feel not an exact science, you can look at various good handling cars and find them far from the same in design. A case in
point would be the 911, design totaly wrong, handles very well. VW beetle, same principle, but handle like shite. Most, not all, later Beemers handle
well, similar Fords, (Granada), not so well. It does seem to me that the more effort that is put into the chassis, and suspension designs, together
with trying to get a nice 50_50 split on the weight, and keeping the COG as low as pos, and you will/may have something that handles.
locost_bryan - 13/12/05 at 02:59 AM
Slightly off topic, but the only manufacturer I can recall who had similar packages in very different configurations, would be Toyota back in the
mid-80's.
The Corolla was raced in Group A as the rwd liftback or as the fwd hatch - about the same weight and size, same 4AGE engine, similar brakes and
aero.
The rwd was faster on tighter circuits, but the fwd was faster on sweeping circuits, but on most circuits there was no difference.
Perhaps the mid/rear cars are faster because they're more designs with better sorted suspension and aero, and the mid/front cars are slower
because they are older designs with compromised suspension and aero?
kreb - 13/12/05 at 03:20 PM
Rear-engined Porsches handle well because they've had so many years to wean the Treachery out of them.
I'm a little surprised at the wonky tech being bandied about on this one. Cornering, braking and accelerating all happen best under certain
weight distributions. Since racing is a dynamic, not static event, handling will be a function of how those factors interrelate along with other items
such as downforce and polar moment of inertia. That said, it has long been established that a true mid-engine layout (as opposed to front-mid-engined)
gives one the best platform to address those various issues.
A Locost can come pretty close though
andygtt - 13/12/05 at 06:35 PM
50/50 weight distribution isn't the be all and end all of handling....
using this theory alone a bumdbell would be perfect, but in reality it would be far from.
Getting the weight withing the wheelbase if more important IMO.
That aside I think one of the major advantages of mid engine is aero.