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May 12

Understanding the Wheels

It's easy to get the concept of rear-wheel drives - using the rears to transfer engine's motion to overall movement while keeping the front ones free to be steered as needed. But as far as front-wheel drive is concerned, along with passing serious motion of the engine, the critically precise steering movements needed to be translated as importantly. Without letting the two to interfere with each other. How it's done, please ? 210.56.127.224 (talk) 05:46, 12 May 2017 (UTC)[reply]

Here is somewhere to start, and something similar it seems to be facilitated by the use of universal joints. Richard Avery (talk) 07:18, 12 May 2017 (UTC)[reply]

Also note that the rear-wheel drive is not as simple as it seems - to avoid slippage during turns, it needs a differential, to distribute power to the wheels. The outer wheel in a curve has to travel a longer distance than the inner wheel, so if both would turn at the same speed, at least one of them would need to slip against the ground, with corresponding lack of control. --Stephan Schulz (talk) 07:54, 12 May 2017 (UTC)[reply]

Normally a Constant-velocity joint for front wheel drive vehicles.Phil Holmes (talk) 08:34, 12 May 2017 (UTC)[reply]

The "Without letting the two to interfere with each other" is not completely straight-forward, nor is it always completely achieved. See Torque steering. In my "boy racer" days I was well aware of the sudden tug huge you could get on the steering wheel if the tyre on the inside of a bend lost traction, which used to happen often when I pulled out of bends on full power in a front-wheel-drive car. -- Q Chris (talk) 08:45, 12 May 2017 (UTC)[reply]

Four-wheel drive is older than front wheel drive and has some of the same problems to solve. Such vehicles began as slow off-road vehicles, so could cope with many drawbacks. The first used heavy, complicated front axle drives that required a lot of maintenance. The 1924 Thornycroft Hathi had a system of vertical shafts through the steering pivots, with lots of bevel gears to the axle. Most early four wheel drives (Land Rovers and Jeeps) used a rigid front beam axle with a Hookes joint each side to allow steering.

The difficulty with the common Hookes joint is that when bent and rotated at a constant input speed, the speed of the output shaft varies up and down through the rotation cycle. This is fine on a slow off-road vehicle, but makes handling poor for a faster vehicle on a tarmac road.

The solution is the constant velocity joint (CV joint). This is more complicated to make than a Hookes joint, but rotates at a constant speed, even when bent. The first of these was the Belgian Tracta joint, used to make their front wheel drive cars in the 1920s, and remaining on military vehicles into the 1960s. Most modern cars now use the Rzeppa joint instead, which resembles a sphere with grooves in the surface and balls rolling in these grooves. A simple idea, but requiring very precise machining to manufacture it.

A modern FWD car has a transverse engine and gearbox alongside it, with a jointed shaft out to each wheel. These shafts may be the same length, or one may be longer than the other, in order to position the gearbox more efficiently inside the engine compartment. As the outer joint of the shaft needs to bend more with the suspension, and also permit steering rotation, that is a sophisticated CV joint with a lot of articulation. The inner joint doesn't move as much, so can either be an identical joint, or a simpler and cheaper type. Sometimes the shafts are of fixed length, and may form part of the suspension linkage too (like a Chapman strut, although these are rarely used at the front), otherwise suspension links control the position of the wheel and the drive shaft may be slightly telescopic to allow its length to change with suspension movement.

As four wheel drive began to be replaced by permanent all wheel drive, and the cars became faster and also usable on roads, vehicles like the Range Rover also switched to using CV joints rather than Hookes joints. Andy Dingley (talk) 08:47, 12 May 2017 (UTC)[reply]

From the point of view of the tire the traction forces are longitudinal and the steering forces are lateral. Therefore an important part of the suspension designer's job is to minimise the influence of traction forces on steering by optimising the geometry. Essentially this means aligning the centre of the contact patch with the steering axis of the suspension, so tractive forces cannot generate a torque around the steering axis. That is called the scrub radius. There is a separate torque created by the halfshaft torque, because the halfshaft is not truly perpendicular to the steering axis. Various elaborate solutions such as Revoknuckle attempt to minimise that. Greglocock (talk) 08:15, 14 May 2017 (UTC)[reply]

Also relevant: Torsen traction (which should redirect → Torsen) & Limited-slip differential — 2606:A000:4C0C:E200:0:0:0:1 (talk) 22:35, 17 May 2017 (UTC)[reply]