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MG ZR ZS ZT Technical - BIG WHEELS, small wheels

performance cars seem to come with bigger wheel options. Can anyone explain the difference in the ride? For instance compare the smaller wheels of the R25 offering 14" to 15" wheels, to the Zed R offering 16" to 17" wheels for the 160.
Gary

As simplistic as i can be:

Small wheels and big rubber = more flex in the side walls = more comfortable ride = less precise handling.

Big wheels and small rubber = less flex in the side walls = less comfortable ride = more precise handling.

So, using your R25 14", 15" and ZR 16", 17" analogy:

R25 14" = Very comfortable, reasonable handling
R25 15" = Comfortable, slightly better handling
ZR 16" = Reasonably comfortable, good handling
ZR 17" = Firm ride, excellent handling

It's a balancing act and comes down to personal preferences in the ride/handling/aesthetics arenas.

SF
Scarlet Fever

It might also be worth metioning that a wheel/tyre size may effectively alter the gearing, with some alteration to 0-60 and top speeds! Anyone got a calculator?

Kelv
Kelvin

hey Scarlet, that's just such a good explanation, but what about complete wheel size, meaning the rims and the rubber combined?
Gary

Oh god there's no pleasing everyone.........
Eric

I don't think SF was talking about running around on the rims Eric. If the tires all had the same rolling radius, for which one strives in order to hacve an accurate speedometer, then the results are as he stated.
GWB

Sorry Eric, I meant Gary.
GWB

Phew!
Eric

my original interest was in the 'rolling radius'

(so THAT'S how you say that.....)

thanks guys
Gary

Can anyone explain the difference in the ride?

To answer Gary's original question, I went from a 75 on 15 205/65 to a ZT on 18 225/40.

As SF points out, it went from superb, 'best-in-class' ride to superb, 'best-in-class' handling! Downside is that the ride quality suffers and even fag ends can send shock waves through the car body.

However, ride and handling are also dependent on the suspension design and settings, and to be fair, the ZT now has a slightly softer factory setting with little compromise on handling.

The Rover 25 range also benefitted from tightening up on the suspension compared to the 200. I believe this came from the design of the 200Vi.

Martin.
Martin

Kelvin,

Common misconception. The size of the alloy does not effect the overall size of the wheel. Thus (say) 4,000 revs on a 15" wheel will give exactly the same speed as 4,000 revs on a 17". You just get less rubber between rim of the alloy and the road - hence less pleasant ride in return for better handling.

Martin the 75 also has more complaint mounts between the suspension and the chassis. We went for the 17" on the 75 and the hadling is great while still very comfortable! Love it!

Cheers

Patrick
Patrick Beet

Because of the way the speedo is calibrated, you need to try to keep the rolling radius of the overall wheel and tyre the same. You can change this figure, but you will then need to get your speedo re-calibrated as it will be incorrect.

Off the top of my head i don't know what the rolling radius of the wheel+tyre combination on the ZR is, but the following should help you work it out.

The way it works is this:

On a tyre size, for example 205x45 16 (this is what i have on my MGF), the first figure (205) is in mm and is the width of the tyre tread. The second figure (45) is a percentage of the first one and relates to the height of the side wall (or profile, also known as aspect ratio). The last figure is the diameter of the wheel the tyre will fit on and is in inches (16").

So, you have 'width in mm' x 'percentage of the width = profile' x 'wheel size in inches'.

So, in my example:

45% of 205mm = a 92.25mm profile

16" wheel diameter, expressed in metric = 16 x 25.4mm = 406.4mm, divide this by two, to get the radius = 203.2mm

Now add both final figures together to get the rolling radius, thus:

203.2mm (wheel radius) + 92.25mm (profile of tyre) = 295.45mm

With this figure it is possible to calculate other important dimensions such as:

Rolling diameter = 2 x rolling radius

Rolling circumference = Pi x rolling radius squared.

What this all boils down to is this, if you know the rolling radius of the standard wheel/tyre combination and the wheel/tyre combination you want is within 6mm (below) of it (or the normal wear on a tyre tread), you shouldn't have any problems with your speedo calibration. :-)

SF
Scarlet Fever

So, for 15" wheels you would need:

15" x 25.4mm = 381mm / 2 = 190.5mm

295.4mm - 190.5mm = 104.9mm profile

104.9mm expressed as a percentage of 205mm tread width = 51.7%, say 50%

Therefore, the tyres needed on 15" wheels to maintain the rolling radius (assuming a 205mm tread width) is:

205x50 15

-------------

16" wheels have been covered.

205x45 16

-------------

17" wheels work out thus:

17" x 25.4mm = 431.8 / 2 = 215.9mm

295.4mm - 215.9mm = 79.5mm profile

79.5mm expressed as a percentage of 205mm tread width = 38.78%, say 40%

Therefore, the tyres needed on 17" wheels to maintain the rolling radius (assuming a 205mm tread width) is:

205x40 17

-------------

Get the idea? Remeber, the above figures apply to an MGF, so what you need to do is find out the tyre size on your ZR and apply the above calculations to find out the corresponding tyre sizes for the different wheel options.

Good luck

SF
Scarlet Fever

Scarlet
Look mate you're wasted in SVP you should have been a maths teacher

LoL
Dave
dave stonehouse

But if for argument's sake we accept that a smaller wheel is a lighter wheel then isn't it better to go with as small a wheel as possible, given that a smaller wheel will have less unsprung weight?

I'm no engineer but wasn't it a side effect of the Morris Minor's 14" wheel (then considered daringly small) that the car was endowed with a lower unsprung weight which contributed to that car's excellent road manners.

And when the Mini came alng with its 10" wheels handling and ride (yes I kid you not) were better than if a larger wheel had been used (had that been possible). Smaller wheels also had the desirable effect of lowering the car's centre of gravity.

I suppose the smaller is best philosophy is best demonstrated by Minis with 13" Sportpack alloys. These are atrocious ride/handling wise when compared to an early 10" wheeled Mini.

Or have I missed something?
Frank Peabody

>> Or have I missed something? <<

Not sure on this one Frank as i have never weighed a modern alloy wheel and tyre and comapared the two, but i will say that I don't ever recall the Morris Minor having alloy wheels (i always thought they were steel rims) and certainly the later Minis were fitted with larger alloys.

There is an argument about unsprung weight and it certainly is a factor in the handling capabilities of the car, but (i assume) that modern alloys, although varying in weights, are in a similar region to the weight of tyres (and the compressed air contained within).

The issue here is flex in the side wall of the tyre. During hard cornering the centre of gravity of the car is transferred from the straight line position of the central longitudinal axis, to an off centre position by the centrifugal force of travelling round the bend, this loads up the suspension on the outside of the curve and increases the load on the tyres laterally as well as vertically causing the side walls of the tyres to compress and flex. The flex in the sidewall of the tyre has a number of consequences: Part of the sidewall of the tyre is now in contact with the road, this is not a wearing surface and has less grip than the tread. The mass of the car is closer to this point than the other side of the tyre, meaning that the part of the tread opposite the sidewall has less mass on it and therefore the grip available here isn't being fully exploited. Because of the offset in the mass of the car, the centre of the wheel is outside the line you are steering affecting how true your line through the bend is, this impacts on driver feedback and direction of travel.

In order to maintain as true a line as possible, flex in the sidewall should be kept to a minimum. This can be achieved in a couple of ways: You can stiffen up the side walls (radial to crossply tyres), our you can reduce the profile (aspect ratio). This is the current 'trend' in handling fine tuning, low profile tyres have been around for a good 20 or so years but these days, especially in European and Far Eastern cars, the current trend is large alloys and low profile tyres on even the most average of saloons.

The down side to all of this is that a large proportion of smaller road surface imperfections are absorbed by the sidewall of the tyres. Switching to lower profile tyres means there is less flex in the sidewall to absorb these imperfections and the shock waves from them are therefore transmitted to the suspension and bushes where they are not as easily dealt with - especially these days where polyeurethane or even solid bushes are becoming more popular on performance cars along with stiffer suspension packages to reduce body roll.

So, it tends to be a trade off. Sharper, more precise handling verses ride comfort on poor road surfaces - ironic really when you think about it as the roads where the sharper handling is likely to be utilised tend to be lanes and B roads where the surface is most uneven...

SF
Scarlet Fever

Good points, SF, and I wish some of the drivers I have to follow on some roads realised the capabilities of their car instead of driving at 40 and braking for every corner as if they were driving a 1930's car on cross-plies!

For the majority of drivers, I would imagine that comfort is more important than handling, and with the state of the UK roads, I wonder whether we will see less of the alloys/low profile tyres as owners discover the cost of bent alloys!
Martin

>> driving at 40 and braking for every corner <<

That is one of my pet hates at the moment, have you noticed that the same drivers tend to stick to 40 mph regardless? By that i mean, in a 60 zone, driving at 40, in a 50 zone driving at 40 (this is annoying to others as it shows a lack of courtesy to other road users, but is bearable, i mean who am i to judge the capabilities of the driver? Maybe they are not confident behind the wheel above this speed and therefore would be a hazard to themselves and others?).

In a 30 zone, driving at 40! - OK, now it becomes dangerous and is IMO unforgivable. Forget what i just said, they have lost the benefit of the doubt and have proved themselves unfit to be behind the wheel. They have no regard for speed limits or other road users and have shown that they are unaware of what is going on around them. At this point i look for an alternative route, or somewhere to pull over and let them get on with it - trying to overtake someone like this is IMO dangerous as they are unlikely to realise you are overtaking them!

Is it any wonder i get annoyed? :-)

SF
Scarlet Fever

Hey Scarlet. Great descriptions !! I had a reasonable understanding of the concept but wasn't fully aware of the physics of it all.
Steve
S Preston

Just doin' me bit for the BBS... ;-)

SF
Scarlet Fever

SF - re. the constant 40mph thing - my wife and I are convinced that these people belong to a special "42-mile-per-hour" club. Next time you follow one, look closely - you'll probably be doing almost spot on 42mph, regardless of the speed limit currently in force.

As for the ratio of metal wheel to rubber tyre affecting handling - don't forget the gyroscopic effect - twisting a spinning disk of metal exerts much more lateral force the heavier the disk.
P Hudson

Wonder if the members of the 42-mile-per-hour club are also fully paid up subscribers to the MLOC (Middle Lane Owners Club) :-)

Martin

Oooh, don't get me started on *them* !!
P Hudson

Hmmm, the gyroscopic effect isn't something i have come accross before, but it is sound science so it must make a difference. The question is how much of a difference and how it relates to the original question of the effects of upsizing the wheels and downsizing the profile of the tyres in order to keep the rolling radius in the same ball park. Anyone got any thoughts on this?

I suppose it all comes down to the ratio of weight between a modern alloy wheel and a low profile tyre (complete with compressed air).

Not having the time, the components or the inclination to try it out, i suppose the method would be as follows:

Weigh a modern 15" alloy on it's own.
Weigh a modern 16" alloy (of the same style) on it's own.

The difference between the two (should be) the extra inch. You now know how much extra weight is added to the alloy per inch of diameter.

Then,

Fit a tyre to the 15" wheel, pump up to pressure and weigh again.
Fit a tyre to the 16" wheel (of the same style, but sized using the above formulas to ensure the rolling radius is the same), pump up to pressure and weigh again.

By deducting the first set of figures you have the weights of the tyres and compressed air and can compare the two.

There will be a difference, but i would be surprised if it is very great. How this affects unsprung weight and the gyroscopic effect i don't know but it's a start.

SF
Scarlet Fever

Andy, the gyroscopic effect is not just affected by the weight of the wheel. It is also affected by the distribution of the mass.

For example, consider two wheels of identical radius and overall weight. One wheel is eliptic in cross section, so that the mass is evenly distributed across the radius. The other is a spoked wheel, where most of the mass is at the extremity of the radius. The latter wheel will have a much greater gyroscopic effect.

Ralph
Ralph

This thread was discussed between 23/04/2003 and 02/05/2003

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