The matter of Unsprung Weight was recently mentioned in a Discussion thread about the De Dion suspension and it brought to mind my own introduction to the subject. The following story will eventually get to the point!
I bought a Ford Special (kit car) as an uncompleted project in about 1965. It had a lightweight fibreglass body mounted on a 1930's Ford 8 chassis and suspension. The suspension comprised two transverse leaf springs, one front and one rear, and it had feeble lever-type dampers.
The springs each had seven leaves which made them too stiff for this now-lightweight car so I took out three leaves from the front and four from the rear. This gave fairly good roadholding and an acceptable ride.
The tyres were very thin, 17" crossplies I think, so I went looking for wider replacements. I found that VW beetle wheels would fit but, even better, so would the fat 15" wheels/tyres from a Porsche that I found in a breaker's yard -- and these came with beautiful chromed-dome hub caps. I couldn't resist!
Unfortunately the wide 15" wheels with their massive tyres ruined the car's roadholding on all except very smooth bends. The car would go sideways in a series of small jumps at the slightest provocation, below the speed at which it would have skidded on the original skinny wheels.
What I think happened was that a wheel would bounce when it hit even a small bump on a bend and its weight had a momentum that couldn't be handled by the weak leaf springs and lever-arm dampers, so it would continue bouncing. The problem was, I think, caused by the so-called "unsprung weight" that the suspension wasn't able to handle.
Reverting to lightweight wheels and tyres reduced the unsprung weight and brought back the original level of roadholding and comfort. I have never been tempted since to think about fitting wider wheels to a car.
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I presume you were getting the opposite of the effect in large old-style American tubs with a relatively heavy body shell and light wheels, which bounce a lot especially when the dampers are shot. The result is 'washboarding' on dirt roads.
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Dizzy
Many issues here. Solid axles produce camber changes on both wheels when one bounces. Very wide tyres don't respond well to this, especially if the pressures are high. I reckon you'd need very low tyre pressures to make this work on a light car. I believe F1 cars actually use pressures well below those of road cars.
The sring rate and damper performance need to be matched, and that's tricky to achieve. I'm guessing you're right the soft spring/soft damper combination didn't help. Plus, as you say the wide tyres produced far more grip than the system was ever designed for.
Regards
John S
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Ever since learning in school Physics that the frictional force = the coefficient of friction X the normal reaction (ie the weight), I've been puzzled why wider tyres have more grip then narrow ones.
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I think you'll find the compound is a lot softer on wider tyres:
more grip, similar life to narrower harder tyres, if things are kept in proportion.
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It's just occurred to me that, a few years ago when cars like the small Fords were running on 17" tyres, it seemed very desirable to change to 15". It's now just the opposite, with 15" being typical original fitment and larger sizes being optional at extra cost.
The Rover 75 has 15" tyres with 17" offered as an option. The 17" wheels/tyres seem popular with members of the Rover75/MGZT Yahoo forum because, typically, "they fill the wheel arches better". However, changes in tyre profile between the two sizes means that the 15" and 17" end up with the same overall diameter! Am I missing something?
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Dizzy
Yes, the overall diameter will be the same or the gearing would be changed by the whel and tyre change. The larger wheels appear to 'fill the arches' better because the the very visible silver coloured wheel itself is larger. The black tyres merge into the shadows of the arch. It's totally a visual thing.
The modern trend to lower profile tyres is enabled by better suspension design. The modern low profile tyres provide reduced tyre wall distortion under load, and so improve steering response. However, they are sensitive to camber changes, so wouldn't work well on the older cars. The stiffer sidewalls are a key reason for the usual complaint that they don't ride as well as the 'normal' tyres.
How times change though. I ran a Cortina in the 60's on 80 profile 175 x 13's (using the 5 1/2 inch Lotus rims). These would now look like baloons compared to the 50 profile 205 x 16's on our Astra.
Regards
John S
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Cliff
True for 'classic' friction measurement, where two flat surfaces are in contact. Tyres don't operate in that way. The difference is that tyres are large deformable surfaces. They conform to the uneven road surface, so resistance to motion is a combination of friction and interference between the two surfaces. With the right pressure, wider tyres provide a greater area for interference, hence a greater effort needed to move them for a given load (weight).
Regards
John S
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Thanks for that explanation John, I see now.
So the smoothness of the road would be crucial in determining how much grip the tyres would have? Roads vary a lot in smoothness, and there appear to be no national standards. Some are very rough, even noisy, sometimes one comes upon an exceptionally smooth section with a kind of deathly quiet. They appear to vary quite arbitrarily, with no signs warning that the road surface is about to change. Indeed repairs are often done with a different surface from the surroundings.
Dizzy, if like me you are over 40 you probably think "proper" wheels are large with lots of rubber showing. Modern wheels look as if they come out of a Lego set. I believe the very low profiles on your 17" wheels give better sideways grip, but at the expense of a bumpier ride. Also the tyres and wheels are more easily damaged by ruts and kerb-bumping.
Interesting discussion about unsprung weight. Are springs purely for comfort, or necessary for roadholding?
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Over 40, Cliff? I'm over 60 but still feel as though my 40th birthday is somewhere in the distant future!
In fact, my Rover has the standard 15" wheels, not 17", but they are still 'low profile' compared with the big 175 x 14 tyres on my Triumph.
I agree about the increased possibility of damage with low profile tyres. My Rover recently ran over the end of a length of RSJ that had been left in the grass adjacent to a car park entrance road and it split the sidewall. Probably wouldn't have hurt the "proper" tyres fitted to the Triumph!
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In fact, my Rover has the standard 15" wheels, not 17", but they are still 'low profile' compared with the big 175 x 14 tyres on my Triumph.
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175 x 14 is what I consider small! My Triumph Roadster of 1947 had 6.00 x 16 tyres, and the spare took up a considerable proportion of the space in the 'dickey' boot.
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Interesting discussion about unsprung weight.
Very interesting thread!
Are springs purely for comfort or necessary for roadholding?
Definitely needed for roadholding. Just imagine the effect of hitting a mid-corner pothole without them!
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Back in the 90's F1 racing teams were looking at suspending the driver rather than the car as road holding would be better without suspension.
Anyone who's ever driven a go-cart will tell you that road-holding isn't at all impaired by having no suspension.
(Comfort does suffer a lot though!!!)
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But F1 don't have many potholes on their courses.
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No suspension = good roadholding is not entirely true.
In fact all things being equal (which they never are) softer suspension = better roadholding. This is because it allows up and down movement which keeps the wheel in contact with a bumpy roadsurface. All surfaces are bumpy - even race tracks.
The right amount of suspension for best grip is a compromise between this and the need for hard suspension/firm damping to minimise movement between the wheels and car body, to give good handling. For most road cars the suspension is softer than this, to give a good ride.
A Kart does not have good roadholding beacuse it has no suspension. It has good roadholding from slick tyres, very low weight and very low centre of gravity.
F1 cars have more variables - the changes in downforce at different speeds makes an enormous variation in the "apprarent" weight of the car, so the right suspension setting varies with speed. Despite this they need to keep the ground clearnace and angle of attack of the aerodynamics under control.
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I seem to remember that one car builder experimented with a dual-chassis design in the late 70s or thereabouts, with one chassis connecting the unsprung weight components, and the other having everything else hung off it. Not sure of the details, but I think it may have been banned before it even raced.
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