924Board.org

[UK924]

What is the preferred wheel size for optimum handling and comfort on the 924? Are the std wheels ok? I was looking to change the wheels for aftermarket ones but don’t want to upset the handling of the car just for the sake of adding new wheels.

[Khal]

I reckon the 15-inch wheels I've got on mine are good (I think the very original ones were 14-inch?).

I think you can go to 16-inch or higher but you might need lower profile tyres. From what I've heard, lower profile tyres generally improve handling but ride quality suffers.

Mine's got 55-series tyres on the front and 50-series on the rear (standard for the Turbo?). I reckon these give a good compromise between handling and comfort. Mine are slightly wider than standard, I think, being 205's on the front and 225's on the rear (were the originals 185's all 'round?).

Numbers are:

205/55/R15 front

225/50/R15 rear

I'm not 100% sure what size wheels/tyres fit each model year. I suspect there are different limitations with each model i.e. '77 924 probably can't have as big a wheel/tyre as an '81 Turbo.

What year/model do you have?
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'80 924 Turbo

[Khal]

Sorry, 'Net was a bit slow and I double posted.

So points about tyres as I understand them;

The "profile" or "series" is the height of the tyre's sidewall, expressed as a percentage of the tread width i.e. a 50-series/profile tyre has a sidewall height 50% of the tread width.

The tread width is expressed in millimetres i.e. 205/55/R15 tyre has a tread width of 205mm and a sidewall height of ~113mm.

Generally speaking, the wider the tyre, the more rubber you've got on the ground at any given time ergo, the more grip you have (of course, grip is affected by many variables, such as suspension geometry, too). But extremely wide tyres can actually be detrimental to wet weather handling. Racers probably don't care much but in the "real world" this is pretty important. This may be overcome to some extent with modern compounds and tread patterns, though.

Generall speaking (again), the larger the wheel diameter, the better the handling and ride -although I don't know why exactly.

Others here, especially the racers, can probably give you a good indication of what wheel/tyre combo will work well for you.

I reckon 15-inch wheels with 205/50 tyres are a good compromise...

Although I must add the all-important proviso, "within reason", because we've got Yanks on this 'board and no doubt they'll tell you that if you just modify your suspension mounting points, swap the axles and roll the guards, you can put 20-inch wheels with 30-series tyres on it! Oh yeah, and then the carbon-fibre brakes off the Carrera GT will fit, too!
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'80 924 Turbo

[Mikri184]

Well I can say from experience and prefference. I don't think too much will upset the handling. I have run 16X 8 and 7 with 205/55/16 on my 924 with race tires and they handle great my street tires are are 205/45/16 on a set of 16X6 rims and the handling seems the same. I have a friend who has alittle wider set up on his car and says he loves the handling. I think you can probally run up to a 17" rim as long as you don't go too wide. on the rim or the tire. 8 inch rims seem to stick out a bit, Hope this helps
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[marky522]

Actually, there has been a lot of discussion on this topic over on Rennlist and the consensous is tha the track guys are quickest with the 15 inch rims, this was from a couple people who would switch from a 15" rim to a 16" and see better times with the 15".

Mark

[Mikri184]

I got better times with my 16's but I didn't have rce rubber on the 15's so that may have alot to do with it. I think I may try and score of set of 15's and try them this season, Thanks
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[Neil924]

As far as wheels go, lighter is better. Increase to 16' and you add weight where weight is soo important. Go to 14's... maybe quicker but can ruin the way your car looks. Check out the weight of wheels that are cast and forged, the forged wheels are lighter.

"While all cars use wheels, their design and construction can vary greatly, from inexpensive stamped steel to high-tech forged alloy examples. Most people base their wheel selection on looks and price, but the process of picking the right wheels for your car should go much deeper.

For the most part, wheels are made of either steel or aluminum alloy. Steel wheels are cheap and easy to produce, which explains why many passenger cars come with them as original equipment. They may look a little plain, but a set of inexpensive hubcaps can quickly change that.

Aluminum alloy wheels, sometimes referred to as just "alloy" or even "mag" wheels, have some serious advantages over basic steel ones. For starters, aluminum alloy wheels, by nature, better dissipate heat and provide more rigidity under high-load cornering maneuvers.

Alloy wheels are usually lighter than steel wheels, which reduces unsprung weight (all weight that is not supported by the suspension). Any reduction in wheel weight reduces the amount of inertial weight at the rotational axis. The less weight there is spinning around, the easier it is for the engine to turn the wheel and the easier it is for the suspension to control wheel and tire movements.

Even small weight reductions of two to three pounds can make a significant difference. As an example, imagine spinning a two-pound weight on a string over your head. Now imagine the same thing with a 10-pound weight. The 10-pound weight will require much more effort to spin, and will be harder to control. The same principle holds true for road wheels and unsprung weight.

Another benefit of reduced wheel weight is that steering response will improve as there is a lighter load on the steering rack. And lighter wheels reduce the rotating mass of the vehicle, providing quicker acceleration and shorter stopping distances.

If you look at the wheels used in our plus-sizing tire test, the 14x6-inch original-equipment steel wheels weigh more than the significantly larger 16x7-inch TRMotorsports Typhoon alloy wheels.

While it may not make your car faster through your favorite series of corners, alloy wheels also tend to look pretty cool."

http://www.grmotorsports.com/wheels.html

[Neil924]

Cast

Casting is a relatively inexpensive way to produce a high-quality, fairly strong alloy wheel; many aftermarket alloy wheels designed for street use are made this way.

In common gravity casting, the wheel maker begins with a prototype "plug" that is used as the positive to produce the mold. This plug is usually made by machining a piece of material (often plastic or other phenolic material) on CNC machining equipment to produce a highly precise model.

The "negative" is then made from the positive by pressing casting sand around it. The sand is actually a composite slurry that, when compressed under high pressure, becomes quite hard. Think of what happens when you walk down the beach: Your foot compacts the sand and makes a very accurate, very stable negative impression. It's the same principle, just with high-tech sand.

Next, molten aluminum alloy is poured into the sand mold and allowed to cool. When the sand is broken away, you're left with a wheel that only needs minor finishing (like drilling and possibly trimming of some excess metal) to be considering complete.

Negative pressure casting is a similar process, but instead of pouring the molten material into the mold, the molten alloy is drawn up into the mold using a high-pressure vacuum. This eliminates much of the trapped gas found in the gravity casting process, producing a stronger wheel that is much less porous than a gravity-cast one.

Billet

Billet wheels are machined from a solid chunk, or "billet," of material. First, a telephone pole-sized piece of aluminum alloy is produced (or bought from a vendor). Since this piece of stock is generally extruded, the grain runs through the stock, much like the fibers within a single strand of wire. The stock aluminum is then sliced up into sections which are machined down into either complete wheels or just wheel centers.

Since they retain the grain structure of the extruded stock material, billet wheels are extremely strong. This grain structure, which is not present in a cast wheel, gives the final product a backbone--makes the wheel even stronger without adding weight.

Of course, billet wheels are also extremely expensive to produce because much of the original material is wasted. A lot of time is also spent machining the original stock down to a finished wheel, which only adds to the cost of the final product.

Actually, most "billet" wheels are actually billet centers bolted into stamped or spun rim halves. Entire wheels forged from a single billet are so rare as to be almost nonexistent, and are usually seen only on show cars. Billet centers on multi-piece wheels, however, are common.

[Neil924]

Forged

Unlike casting or machining (billet), forging uses intense heat and pressure to transform a slug of alloy material into the final shape of a wheel. Under this heat and pressure, the original grain structure of the stock material is forced from the center of the wheel towards the outer edge. This grain structure is even stronger than the one found in a billet wheel because it runs along the spokes and serves to further strengthen the forged wheel's spokes, while the grain in a billet wheel simply runs through the spokes. Thanks to this process, a forged wheel can be up to 300 percent stronger than a cast wheel. Additionally, since forged aluminum is stronger than cast aluminum, less material is needed to produce the wheel, resulting in a lighter product.

When shopping for a forged wheel, you may want to ask how close to net the forging is--the closer the forging is to the final product, generally the stronger the wheel.

Because of the basic limitations inherent in forging, most forged wheels are two- or three-piece units. In two-piece construction, a center is forged and then welded or bolted into a spun or stamped outer rim. In a three-piece wheel, the center is bolted to an inner and an outer rim half. Three-piece wheels have the advantage of being easily customizable for a variety of widths and offsets. Crash damage in the form of bent rim outers can also be repaired.

[Neil924]

What Should You Choose?

Honestly, most any type of wheel (except wire) is strong enough for most types of racing. A good quality, pressure cast wheel, if made with the right material (T-6 aluminum), is plenty strong enough for a road racing car, and certainly for an autocross car.

The payoff in forged wheels comes in weight and durability. These racing wheels certainly cost more, but are generally stronger and lighter than an equally-sized cast wheel. Plus, their multi-piece construction allows for custom offsets and the opportunity to be repaired as needed.

And while we're on the subject of repairs, we should mention wheel failure. Few things can be more catastrophic than a wheel coming apart on a race car. Wheels used for racing can be checked for cracks using inexpensive dye-penetrant kits available from racing suppliers or aircraft tool supply outlets. This treatment should be done seasonally at a minimum, and certainly after any incident that you feel could have damaged a wheel. Multi-piece wheels should also have their fasteners checked and re-torqued periodically.

All right, back to picking what's best for you. Let your type of racing and vehicle be the judge. If you have a heavy GT1 Camaro, cast wheels are probably out of the question. Likewise, if you race a Miata, you may not see the gain that you had hoped for with expensive forged wheels. A quality wheel, properly maintained, will provide you with years of competent service on your race car. What's that worth?

When you're talking wheels, some generalizations between the different types can be made:

[Neil924]

Steel wheels

pro
· cheap to build
· strong enough for most consumers

con
· some flex
· heavy
· not terribly attractive

One-piece cast alloy wheels

pro
· much lighter than steel wheels
· stronger than steel wheels
· dissipate heat from brakes
· good looks
· very wide selection of styles

con
· not as light or strong as forged wheels
· no custom fitments

Two- and three-piece forged and billet wheels
pro
· very light
· very strong
· almost unlimited fitments
· can be repaired

con
· can be more expensive than one-piece wheels
· usually need to be custom ordered

[Neil924]

Glossary:

bead seat: The area on the inside of the rim where the tire rests and seals.

centerline: The exact center of the rim width.

diameter: The measured distance across the bead seat where the tire mounts--not the overall diameter of the wheel itself.

hub centricity: The wheel hub is used as a common center for aligning the lug bolt holes on the center of the wheel to the axle or brake rotors. This ensures a vibration-free fit.

mounting pad: The flat surface at the rear of the hub that mounts against the hub of the vehicle.

offset: The lateral distance measured from the face of the wheel, where the wheel mounts, to the hub to the exact centerline of the wheel.

rear spacing: The distance between the mounting pad of the wheel and the innermost part of the rim, or inner edge of the flange.

rim diameter: The height measurement from beat seat to bead seat (not flange to flange).

rim flange: The outermost edge of the rim to wheel wheel weights are attached.

rim width: The width measurement from bead seat to bead seat (not wheel flange to wheel flange).

safety bead: The raised area circling the rim slightly inward from the bead seat. The safety bead is required on all street wheels to prevent the tire from slipping off in case of deflation

[Khal]

Um, re-read the post... I don't think he's going racing

"Handling and comfort" were the stated criteria.

BTW, the racing blokes might be faster in 15-inch'ers because in certain circumstances it'll effectively lower the gearing, meaning you can accelerate faster! (to the best of my knowledge, this only applies if the 16-inch wheel/tyre combo increases the overall diameter/circumference of the? And the effect may be minimal)
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'80 924 Turbo

[Neil924]

[Smoothie]

Too big of a change in offset can affect the handling... I have 2 sets of wheels, one set is the original 16x6 flat wheels with stock offset and the other are 16x6 Fuchs 5 spokes with a different offset that leaves them sitting approx 18mm further out than the stock versions. I have the same size tires - 205/55R16 on both sets. In theory (and in practice for that matter) the alignment should be changed for the Fuchs wheels since they sit further out - the toe-in should be increased and a change to the camber would be appropriate too (ideally). I'm not that fussy though and since the wheels get swapped twice a year, I live with just one alignment setting, but I can feel a difference, a slightly less sure feeling with the Fuchs. It's not enough of a difference to be dangerous at the speeds I normally drive, but it's there. -So you might want to consider that and not stray too far from the stock offset (or compensate for any difference in the alignment).
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox