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d000hg

Cars in real life

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I have a problem developing a car simulation that I know very little about how real cars work. I'm down with the engine/torque thing now and can bodge a gear box and clutch I believe, but I'm unsure about the brakes. As I understand it the handbrake/emergency brake basically locks the rear axle. The normal brakes apply a 'frictional' torque to all the wheels. So how do you do wheelspin without moving? Is the engine torque just much greater than what the brakes can achieve so the drive axle still spins? Or do the brakes on the drive axle relax when you accelerate? Also with steering - I know in real life the front wheels don't turn exactly the same amount when you steer since the outside of the car turns in a bigger circle than the inside, but is this likely to have a noticeable affect in my simulation - and if so to what order is the difference of how much the wheels turn? What's the name for the bit which connects the drive axle to the output of the gearbox? Finally, my cars all have locked differentials ie a solid axle with a wheel at each end. Is this a realistic model used in any cars at all or are limited-slip diffs used at all? If so what are some good types to search for and attempt to simulate? Many thanks for your knowledge and advice.

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A good place to start when trying to understand stuff like this is at www.howstuffworks.com, theres lots devoted to car mechanisms.

As for your questions...

Not quite sure what you mean in your first paragraph, you can't get wheel spin when applying brakes, but I think I'm misunderstanding what you've written.

As for your steering, it wont have much of an effect really, but it of course depends on how much accuracy you want. Ackerman steering geometry is what you are refering to, whereby the outer wheel has a bigger radius of turn than the inner wheel. Do a google for ackerman steering and it will elaborate on what exactly it is, it's a bit hard to explain in words! It can be implemented with a fair bit of trigonometry.

What's the name for the bit which connects the drive axle to the output of the gearbox? Hmm, not quite sure which bit you are referring to, but to my knowledge a basic system for a 2WD car is..

Engine Driveshaft -> Flywheel -> Clutch(es) -> Gearbox -> Differential -> Wheel axles

Locked differentials aren't used in real-world road cars as effectively there is no differential so it makes cornering much harder, LSDs are the norm I believe. There are different types for different uses. Try looking at Salisbury and Viscous differentials and it'll probably turn up more for you.

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Hi,
I think you may find Haynes "Car Builder's Manual" interesting, it should give you a pretty accurate and detailed picture of how a car is put together. Actually, there's a bunch of pictures in it...

There are also books such as "Race Car Chassis design and construction" and "Chassis Engineering" but they are focused at the structure of the car (including suspension and steering) and does not contain much info about the engine and drive-train...

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Quote:
Original post by d000hg
As I understand it the handbrake/emergency brake basically locks the rear axle. The normal brakes apply a 'frictional' torque to all the wheels. So how do you do wheelspin without moving? Is the engine torque just much greater than what the brakes can achieve so the drive axle still spins? Or do the brakes on the drive axle relax when you accelerate?

First off, braking force is not evenly distributed between front/rear of the car. Typically 80% will go to the front. This is because weight transfers forewards under braking causing there to be significantly more traction available up front. If the 'brake bias' as it is called, is set to 50/50, then the rear tires will lock up early and the car will be difficult to control. Also you would be giving up potential stopping power as the front tires would not be giving 100% of their grip.

As for your burnouts, typically engine torque will be enough to overcome the rear brakes as they really dont do a whole lot in the first place. Hardcore drag racing cars may use a 'line lock' setup in which the front brakes can be activated independantly from the rear.

Quote:

Also with steering - I know in real life the front wheels don't turn exactly the same amount when you steer since the outside of the car turns in a bigger circle than the inside, but is this likely to have a noticeable affect in my simulation - and if so to what order is the difference of how much the wheels turn?

There is ALOT that goes on in the geometry of front suspension/steering systems that is not obvious at first. I'll list most of the basic things, and let you research what you feel is important for your particular sim.

1) ackerman steering: (sometimes called parrallel steer). This is what you describe as "the inside wheel turning more than the outside". Pretty much all cars have some amount of this, but how much DOES have an effect (especially on a race car, where it may be beneficial to alter the slip angle of one of the wheels using this trait)

2) camber: This is the angle of the wheels vertically with the ground if you are looking at the front/rear of the vehicle. 0 degrees means the bottom of the tire will be perpendicular with the ground and is the idea condition IF it can be maintained ALWAYS. In reality some negative camber (top of the tire pointed in more towards the center of the car) is used, as the cars suspension geometry will cause a build-up of positive camber when the body rolls over in a corner. So if you had 0 degrees going straight, when the car leans in the turn you will end up riding on the outside edge of the tire only. This causes there to be a smaller contact patch, and thus less traction.

3) caster: (aka, dynamic camber). by leaning the steering knuckly back, you can have camber change AS the wheels are turned. That is, when you steer straight your camber may be 0 degrees, but by turning the wheels full left you may get -2. This is useful in helping counter the positive camber build-up in corners, without poorly effecting acceleration/braking by using alot of static camber.

4) Scrub radius: this has to do with what part of the tire rotates around the steering knuckle (the point where the wheel actually steers). This is hard to describe without a diagram, but basically using wheels that stick out (and therefor sort of 'swing' instead of rotating about their centers when steered, causes a higher scrub radius. Usually less is better. In a sim there is no reason not to always have zero, as the only things that change this are physical room limitations of some front suspension designs.
5) toe angle: the angle the wheels point relative to each other. Parrallel is zero degrees. Pointing towards each other is 'toe-in' and generally promotes stability. 'toe-out' can help a car cut into a corner but may make it unstable under braking (and acceleration if the drive wheels are toed-out)

Quote:

What's the name for the bit which connects the drive axle to the output of the gearbox?

Driveshaft or propeller shaft.

Quote:

Finally, my cars all have locked differentials ie a solid axle with a wheel at each end. Is this a realistic model used in any cars at all or are limited-slip diffs used at all? If so what are some good types to search for and attempt to simulate?

The front end will typically be completely seperate (in a rwd car). The driving end will usually have an 'open' differential (the math for this I'm told is rather complicated, but I've never actually looked into it. Limited-slip diffs are an extension of the open diff in that they are usually torque or load-sensative. That is what causes them to 'lock up'. There are also differentials that can be locked by the driver (usually for 4WD offroad use, rockcrawling and such). Drag cars are the only place you'll usually find a solid drive axle.

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http://auto.howstuffworks.com/differential3.htm

Differentials are used so that two wheels connected to the same axle can rotate at different speeds. Theres a pretty animation showing exactly how it works on that site.

In a RWD car, the front wheels are not on a connected axle, so there is no need for a differential. The back wheels have one differential so that while going around turns, they can move at different speeds (otherwise, one wheel would scuff on the road surface).

In an AWD car, 3 differentials are needed: one in the front axle, one in the back axle, and one in the shaft connecting the front and back, so that they can all spin at different speeds.

Most differentials are open differentials. A Limited Slip Differential distributes power to the wheel with the most traction, so that, for example, if one wheel is on ice, and the other isn't, you can still move forward, and not waste all your torque spinning the wheel that is going nowhere. A LSD also makes doing donuts (burnout in a circle) much easier, since the lateral movement of the car will put more weight on one wheel. With an open differential, there is a chance that that wheel won't "lock up" (spin without traction), but with an LSD, more power will be applied to that wheel, letting it overcome the friction with the road.

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Wow, what excellent replies! Thanks very much for all the great information. Having different brake strengths for front/rear axle and not having a front axle are of particular use I think - I currently have the 2 front wheels and two rear wheels on solid axles. I'm not at all sure implementing another kind of differential will be easy, apart from maybe a single-axle open diff. For AWD I'd planned to effectively apply a torque to the two axles but not link the axles together at all...

Based on the fact that I have locked axles, would I be more able to drift in my current simulation if I unlocked the non-drive axle or is that due to other factors

[Edited by - d000hg on September 6, 2004 7:59:48 AM]

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Locked drive axles in the rear will usually cause an understeer condition, unless you manage to completely break traction, then you end up doing a donut.
An open diff may not be that hard to do, I'm not sure, I've never even tried it. I just remember my Kinematics Prof saying it was rather involved when we did planetary gears. Once you have an open diff it shouldnt be a huge jump to get an LSD. Also you could implement different 'locking factors' for power versus coasting as this can drastically effect the way the car handles into/out of corners.

There are different ways of doing AWD systems. Having a center open differential is one way. A locked center system (like one shaft in a transfer case) is another. A 3rd way is by using a 1 way clutch so that the system locks under power, but is free to rotate at different speeds while coasting/braking.
I beleive most 'modern' AWD systems will use 3 open diffs, and then have the ABS system control which wheels get most of the torque. So if the front-left wheel starts spinning a huge amount faster than any other wheel (say you are accelerating out of a left hand corner and it picks up that wheel) it will put on the brakes (for that wheel only), causing the 3 others to get more % of the torque.

edit: a note on using a solid center shaft for AWD setups: most full size offroad type trucks use this setup. It is also why it's not such a hot idea to leave 4wd active while driving around on pavement with them, as changes in the curvature of the road and minor changes in tire diameter will cause different front/rear wheel speeds. Because that is impossible with a solid center section it can cause a jittery hopping effect (as one set of the wheels is forced to slide instead of rolling) and damage transfercase/transmission internals. In the simulation it may cause slip-Ratio instability or other odd problems as you may end up with some kind of positive feedback loop.

edit2: an even further note on the use of ABS as 'traction control' or 'fake LSD'. Cars with open differentials and anti-lock brakes will often have the computer activate the rear brakes independantly. So if your RR tire starts to spin alot faster than the LR, it can put on the RR brake cause more torque to go to the LR. This is a problem on real cars, as it can cause all of your torque to go through ONE half-shaft instead of being distributed 50/50 between two of them. This can overload and snap the shaft.

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In the type of racing I drive (a cheap version of rallycross), some off the drivers uses a locked differential on there rear wheels. They get more grip, but when they go into a turn and uses the clutch it's like using the handbrake. They must have wheel spin when they turn. So it's very difficult to drive with a locked rear diff.

Not all cars uses the handbrake on the rear wheels, but the only one I know that uses the front wheels are SAAB.

The brakes is normaly 70 - 80% on the front wheels. The brakes works with a frictional force between the brake block and the brake disk. And between the wheels and the surface. The maximum friction between the brake block and the brake disk depends on the diameter of the disk, the area of the blocks and the brake temperature. The maximum force between the tyres and the surface depends on the "standard" friction between the surface and the tyre, the weight on the front wheels and how good job the suspension do with keeping the tyres on the surface.

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