Which set of wheels

[Pastry]

ORIGINAL: 944 man

If anyone sees me sprinting over the next year or two with 15" Fuchs and spray-on tyres, then youll know why.....

it would look like a roller-skate though ;D

ive probably said before but i inherited a lowered bmw e34 with 17s on it. it handled well overall but the poor thing hit the bump stops a number of times becasue of potholes; despite this it almost took off once (during a spirited drive on a yompy country road) and landed fine, so the suspension was correctly weighted, but it was a pothole that eventially broke the front suspension.

I had read that 15s were the best for handling but as it has the 17s on and it looked cool i kept em.

With the 944 im keeping the 15" teledials for the moment, i'd consider 16" cups for the look. 17 upwards without revised suspension is asking for trouble (imho). As others have said its what the car was designed to run on.

 

[TTM]

ORIGINAL: 944Turbo
But you need to take the tyre size into account and look at the total diameter and therfore its effect. For road use at least most people with bigger wheels use smaller tyres so overall the diameter remains the same. In motorsport the wheel size is regulated and I am sure that in F1 they would rather have less rubber so they could control the movement with dampers rather than having all that undamped air springing.
Tony

I believe the density of materials used to make wheels remain to this day far superior to the density of any rubber compound, so the mass of the wheel is still predominent. and taking account of the mass of the tyre won't make much of a difference.

 

[944 man]

ORIGINAL: 944Turbo

But you need to take the tyre size into account and look at the total diameter and therfore its effect. For road use at least most people with bigger wheels use smaller tyres so overall the diameter remains the same. In motorsport the wheel size is regulated and I am sure that in F1 they would rather have less rubber so they could control the movement with dampers rather than having all that undamped air springing.
Tony

Youll note that I have referred to the 'rolling radius' of the wheels: this is around the outside of the tyre. The size of the road wheel alone is not the issue.

Simon

 

[944 man]

ORIGINAL: Pastry
With the 944 im keeping the 15" teledials for the moment, i'd consider 16" cups for the look. 17 upwards without revised suspension is asking for trouble (imho). As others have said its what the car was designed to run on.

For series two cars the original 16" Cup wheels are definitely the best option.

 

[944 man]

Where we are talking about braking, mass isnt really the issue (although the weight of the roatating wheel/tyre combo will be a factor): really its all about moment/leverage.

 

[Neil Haughey]

ORIGINAL: 944 man

Similarly; you can increase your braking effort be reducing the rolling radius of your front wheels.  If you do this all round then it can usefully shorten your overall driven ratio too, which can be very useful; and its far cheaper then sourcing a different final drive. []

If anyone sees me sprinting over the next year or two with 15" Fuchs and spray-on tyres, then youll know why.....

Only problem will be with getting tyres with a suitable load rating for the car. I asked one of the tyre manufacturers about this a few years ago (Avon) and was advised to stick to the load rating of the OEM tyre spec, however with a car that is much lighter I would imagine you could get away with a lower number. Would be nice to know what the load ratings relate to directly to pick a suitable load rating.

 

[John Sims]

ORIGINAL: 944 man

...... With regards to the brakes: get someone to hold the wheel in a similar fashion and spin it as quicky as you can. Now stick your finger in the spokes ........

Comments on this forum are entirely the personal opinion of the poster and Porsche Club GB does not recommend sticking your fingers (or any other part of your, or anybody else's, anatomy) in the spokes of spinning wheels. [8|] [] []

 

[TTM]

Is it ok to throw a Hamster in the spokes?

 

[sawood12]

ORIGINAL: 944 man

Another inescapable physical fact, is that increasing the rolling radius of your front wheels will reduce the effectiveness of your brakes...

It isnt understeer, its the gyroscopic force which resists your attempt to turn it away from centre. You can use a bicycle heel to demonstrate both problems. Hold a cycle wheel on its spindle in your hands and it will be easy to turn, as though you were steering the cycle: now get someone to spin the wheel quickly and youll see exactly what gyroscopic force is and realise how great an effect it has. With regards to the brakes: get someone to hold the wheel in a similar fashion and spin it as quicky as you can. Now stick your finger in the spokes at their far end by the rim, in order to slow down the wheel: easy. Now spin the wheel again and attempt to slow down the wheel by sticking your finger in between the spokes, but for the sake of the example do it in the middle of the weel, simulating a constant brake disc size but an increased roadwheel diameter..... Thats why larger road wheels decrease your braking effort, because the braking force remains constant whilst the leverage that the wheel has over the brakes is increased with diameter.

I think you are referring to inertia here and not gyroscopic effect. The wheel has inertia so when you stick your finger in the spokes close to the rim you need less force than if you stick your finger close to the hub - but this is not what is happening on a car as your brake is not acting on the wheel directly (like it is on a bicycle for example) and the torque generated by the brake acts through the wheel hub thereby negating the effect of the differing relative radii of the wheel and brake disc. Also braking force required by the caliper depends on a number of variables. If you have a constant radius brake disk your braking force will vary according to brake pad swept area as well - more area more friction therefore less braking force required for the same decellaration. That is why bigger brakes are generally better - less force means less temperature.

The gyroscopic effect works by creating a corresponding force at 90 degrees to the axis of rotation of the wheel. So you have a wheel spinning. Then you turn the steering wheel and as the actual wheel turns on it's steering axis you will get a force proportional to the inetia of the spinning road wheel that will act on the chasis of the car. However I don't think this force will be particularly detremental. This can be demonstrated by two experiments:-

1. Sit on a rotating stool and hold a spinning bike wheel in front of you by the hub axel ends. Now rotate the axel ends in front of you and the gyroscopic force will cause you to rotate on your stool. Now imagine you are doing this facing the ground to simulate a car wheel and the resulting force will be in the body roll plane of the car - so a heavier wheel will cause a larger gyroscopic force in the roll axis of the car.

2. Again take a spinning bike wheel and you can balance it on one finger on one end of the hub. This is because as the wheel tries to fall off your finger the gyroscopic effect lifts the wheel with a force that is equal and opposite to the force of gravity acting on the wheel. This will balance until the bike wheel rpm's have reduced such that adequate force can no longer be generated. Not sure how this experiment translates to a car but it is quite interesting in anycase.

I've always read and thought the primary reason for reducing unsprung weight is to improve suspension response performance and therefore grip. However i've got an open mind on this.

 

[sawood12]

Thought i'd doublecheck on Wikkipedia:-

Effects of Unsprung Weight The sprung weight of a wheel controls: a trade-off between a wheel's bump-following ability and its vibration isolation. Bumps and surface imperfections in the road cause tire compression--which induces a force on the unsprung weight. In time, the unsprung weight then responds to this force with movement of its own. The amount of movement is inversely proportional to the weight - a lighter wheel which readily moves in response to road bumps will have more grip when tracking over an imperfect road. For this reason, lighter wheels are often sought for high-performance applications. In contrast, a heavier wheel which moves less will not absorb as much vibration; the irregularities of the road surface will transfer to the cabin through the geometry of the suspension and hence ride quality is deteriorated. Pneumatic or elastic tires help by providing some springing for most of the (otherwise) unsprung mass, but the damping that can be included in the tires is limited by considerations of fuel economy and overheating. The shock absorbers, if any, damp the spring motion also and must be less stiff than would optimally damp the wheel bounce. So the wheels execute some vibrations after each bump before coming to rest. On dirt roads and perhaps on some softly paved roads, these motions form small bumps, known as washboarding or "corduroy" because they resemble smaller versions of the bumps in roads made of logs. These cause sustained wheel bounce in subsequent vehicles, enlarging the bumps. High unsprung weight also exacerbates wheel control under hard acceleration or braking. If the vehicle does not have adequate wheel location in the vertical plane (such as a rear-wheel drive car with Hotchkiss drive, a live axle supported by simple leaf springs), vertical forces exerted by acceleration or hard braking combined with high unsprung mass can lead to severe wheel hop, compromising traction and steering control. Though this is usually not considered important, at least in the popular literature, there is a positive effect. High frequency road irregularities, such as the gravel in an asphalt or concrete road surface, are isolated from the body more completely because the tires and springs act as separate filter stages, with the unsprung weight tending to uncouple them. This can improve overall safety. [edit] Unsprung Weight and Vehicle Design Unsprung weight is largely a function of the design of a vehicle's suspension and the materials used in the construction of suspension components. Beam axle suspensions, in which wheels on opposite sides are connected as a rigid unit, generally have greater unsprung weight than independent suspension systems, in which the wheels are suspended and allowed to move separately. Heavy components such as the differential can be made part of the sprung weight by connecting them directly to the body (as in a de Dion tube rear suspension). Lightweight materials, such as aluminum, plastic, carbon fiber, and/or hollow components can provide further weight reductions at the expense of greater cost and/or fragility. Inboard brakes make a big difference, but put more load on half axles and (constant velocity) universal joints and require space that may not be easily accommodated

 

[ikillcopiers]

I got my '44 Turbo over a year and a half ago, and it came with Boxster wheels on it.
They're even the right size (205/55R16 Front & 225/50R16 Rear), and I've really
grown to like them.

They modernise the look a bit without being a low profile, and lots people have commented
on them.

Best of all, they're not hard to come by at the scrappies!


How the frig do I add a piccie? << Solved my own problem!

 

[George Elliott]

I-K-C, you had the back torsion bars re-indexed?

Blimey, some of those posts would have been more appropriate 2 days earlier!!!!!

George
944T

 

[turbocabrio]

Thanks Nick for those specs.those 18 gt3 style look the part on the 944.cheers
Paul.

 

[944 man]

Scott, the benefit of reducing unsprung weight is beyond discussion here surely? With regards to sticking your finger in the spokes: this demonstrates how increasing the rolling radius of the wheel reduces the effective braking effort. Perhaps I shouldnt have used the same bicycle wheel for both examples. The fact remains though; that the heavier the wheel the harder it will be to turn it away from centre and that is the issue.

 

[944 man]

ORIGINAL: ikillcopiers

I got my '44 Turbo over a year and a half ago, and it came with Boxster wheels on it.
They're even the right size (205/55R16 Front & 225/50R16 Rear)

Your wheels should be 7" & 8" or 7.5" & 9", depending on model year. Those 16" Boxster wheels are/have 6" fronts and 7" rears, so whilst you may have the correct sized tyres; youre seriously under-wheeled. Ive fitted a set to my 924S, although Id prefer a set of ABS offset Turbo 7"x16" ATS telephone dials; or equally as rare, four 7"x16" D90s...

Simon

 

[ikillcopiers]

There's a degree of tolerance in those wheel sizes, it's no biggie.

The guy who set up my car does rally cars over here, and we tried a couple of
combinations of rim and tyre sizes with a couple of different suspension setups,
and that setup was just about one of the best. There wasn't really any noticable
difference between the 6J/7J combo and the 7J/8J combo's we tried, but then
again the car is using coilovers all round with a midge of negative camber too.

My car's a road car, and our roads are a bumpy & twisty, the setup at the moment
works great, and it was still more than able to hold it's own at the track.

Might be more of a issue on a car with standard suspension setup?
Can't really say - we've fiddled to death with the suspension settings
and the car now performs flawlessly!

All I can say is they work for me!

 

[Hilux]

The compliance in the sidewall can be exploited to maximise the contact area. It's like everything - there will be an optimum. What that is on any given car will probably vary but it is not the case that the lower the profile the better.

Exactly my point earlier.

Depending on which calculation you believe 1 Kg on the wheel is worth up to 4 kg in performance terms

As a general rule it is considered that removing 1kg of UNSPRUNG weight is an approximate ratio increase x 4 of SPRUNG weight. ie: Lose 1kg off your wheels and calipers at each corner and its worth 16 ish kg off the car as a whole in performance terms.

The reduced gyroscopic force is even more important than the overall reduced weight and the reduction in un-sprung weight.

You do know what you are talking about [], most people do not realise this but this is absolutely true and increases turn in accuracy and reduces oversteer. Increasing disc size has a similar effect for in effect it is a flywheel providing momentum and mass

Absolutely.
Larger diameter = higher polar moment of inertia, and if I remember correctly, I=mr²/2

Not necessarily - its only when the gyroscopic effect is overcome that the momentum and inertia aid understeer, the initial tendency is to oversteer so on light cars the effect is very noticeable indeed.

I went to the Jordan F1 factory a few years ago and was shown a rear tyre and wheel and was asked to pick it up with my little finger, which I did (easily) it was like a beach ball, seriously (unbelievably) light.

On motorbikes unconsciously the way motorcyclists deal with this is when the bike leans into a (say a right hand) corner the rider actually pushes on the inside r/h bar and then turns once the wheel follows.

Similarly; you can increase your braking effort be reducing the rolling radius of your front wheels.

....or keep the same rolling radius so as not affect the gearing and instead vary the width so depending upon weight of car the tyre contact area is increased (or decreased)

 

[Hilux]

Unsprung weight does aid and assist the car to `settle` better with less violence from the mass of weight jumping up and down however it is the cornering that benefits from reduced oversteer, better braking (in part - see below) and better turn in................end of.

Scott, the benefit of reducing unsprung weight is beyond discussion here surely? With regards to sticking your finger in the spokes: this demonstrates how increasing the rolling radius of the wheel reduces the effective braking effort.

Not entirely as this can be offset by the increase in (or regaining back) the mechanical advantage by using larger discs BUT at the disadvantage of increased unsprung weight.

Mind you with all this weight loss in the cars, the unsprung weight becomes an ever bigger percentage (and problem) so its a tricky (and expensive) road.

 

[Hilux]

Comments on this forum are entirely the personal opinion of the poster and Porsche Club GB does not recommend sticking your fingers (or any other part of your, or anybody else's, anatomy) in the spokes of spinning wheels. [8|] [] []

'Nanny' and 'State' come to mind [&o][&o]

How long have I been a "guest" of the club [] ....................only just noticed [8|]

 

[944 man]

ORIGINAL: Hilux
Not entirely as this can be offset by the increase in (or regaining back) the mechanical advantage by using larger discs BUT at the disadvantage of increased unsprung weight.

Of course. My point is that increasing the rolling radius without increasing the size of the drake disc, reduces the braking effort felt.