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MG Midget and Sprite Technical - tire improvement?

I am doing some research on the history of car tires. Tires now are much better than 60 years ago. The stopping power has not improved on the Sprite. The Motor road test 15/58 (May 1958, drum brakes, Dunlop 5.20 13) comes to 31 ft. stopping distance at 30 mph. That is almost I G. That is more G than I can achieve now with new Vredestein tires, Lockeed discs. How did they measure. How was their reaction time? I tried to google a comparison between old and new tires, but could not find one. Do you have any suggestions?
Flip Brühl

You do need to know the surface that the test was performed on as it will effect the results. My neighbour used to work for ARC/Hanson and they own the quarry which was used to supply high mu (coeffiecient of friction) surface material for things like surface near pedestrian crossings and it was exported to surface the Abu Dhabi grand prix track http://en.wikipedia.org/wiki/Yas_Marina_Circuit .
David Billington

Flip,

Any chance that the test was performed on an airport using airport transfers tonbridge runway? Along the lines of what David is saying, I once did an autocross that was partly on an airport runway. I was amazed at the grip. On a tight corner I started out with light throttle in 2nd gear, and on later runs was to the floorboard in third. It seemed I just could not get it to skid.

Charley
C R Huff

Thank you, so I can not compare at all...Unless I find tarmac on a airport runay.
Flip Brühl

I have been calculating with the motor road 1958 test input. 31ft at 30 m.p.h. :

30 mph= 14 meter per second
Reaction time 0.3 sec ( this is the reaction time of L Hamilton or M Vestappen in complex situations, my reaction time will be about 1 second)

No downforce in the sprite

No drag braking power

0,3 sec reaction time at 14m/s = 0,3x14= 4,2 m

the sprite stopped in 31 ft.= 9.5 m

so 9.5-4.2 m= 4.3 m for braking. This is much better than F1 braking.

How did they test without reaction time?
If we do not calculate the reaction time we come to 9,7 G that is possible, a Porche 911 can archive 11,3 G from 200 km/h (with drag and downforce)



Flip Brühl

I don't know the relevance of this but my MOST tester always comments how good the brake readings are for my car, even compared to modern service assisted ones. He says it is because the car is so much lighter than most moderns. So does the weight of the car have to be included in the G force calculation?
GuyW

For the MoT test the tester has to have the kerb weight of the car. If it is a common model the VOSA web site comes up with the weight, but for my MGB and Elan, I had to tell the tester the weights. So, yes, the vehicle weight has an effect on how efficient the brakes appear to be. I had the same comments from my tester with the Elan - he said because of the very low weight (about 700 kg) the brakes were among the most efficient he had ever seen.
Mike Howlett

I would imagine that road tests use some sort of decelerometer, then the stopping distance is calculated from the g reading.

I have a book of old MG road tests. Amazingly, it has stopping distances as low as 27 ft (1932 midget) and 26 ft (1931 Montlhery Midget)!

Also, 30 ft ("Wet surface", 1935 Magnette saloon).

I'm not sure that this tells you much about tyre technology.

This is the book (cheaper copies may be available) http://www.ebay.co.uk/itm/MG-SPORTS-CARS-PETER-GARNIER-CAR-BOOK-/360447865757
Geoff MG-TF

A good idea this decelerometer. I also have a book where old road test of the Spridget are in reprint. The G forces are more detailed than the stopping distance.
The reason that I am so interested in G forces than and now is the brake balance. The rear tires (passenger side) locks first on dry tarmac in the original configurations. The brake balance is more to the rear than I want now. So I changed to smaller cylinders in the rear. I have read a lot original factory papers about the development of Sprite in the Healey museum in Breukelen, but not a word about brake balance. I supposed that the brake balance was more to the rear because of the tire quality. (les stopping power in 1958) Geoff Healey was a keen (racing) engineer an did not make mistakes with the brake balance.
Flip Brühl

Can you download an accelerometer app to a smartphone and fix the phone rigidly in the car? That would give you an indication of the actual deceleration. Presumably orientation of the phone would be an issue, unless the app can resolve the accelerations (decelerations) into one.
Jonathan Severn

If I remember correctly in the 50s and 60s the motor mags when doing brake tests had a device that fired a chalk pellet on to the road when the brakes were applied. Measure distance from there to stopped point gives you stopping distance. Reaction time is so variable it should not be included.I remember seeing pictures in Autocar of a device towed on the car to give accurate speed etc to determine acceleration times. Now no doubt all done by GPS. SHowing my age now

Mike Pearson
M J Pearson

There's a very nice android app called Physics Toolbox Accelerometer, which is a free download. (I have never paid for an app yet).
You need a fairly high end smartphone or tablet to use it and there is a bit of work to do on the csv file that it saves to filter the noise because it is very sensitive. But it would definitely tell you your acceleration (or deceleration, which is the same thing in physics terms) in metres per second squared or in G values.

There are loads of other apps that would do it, but I liked this one particularly.

NB: I don't think it works on iOs, but there might be a version in the Apple app store for iPhone addicts.
Greybeard

My Nextbase in-car camera has a decelerometer (is that the right term??) built in. It records sideways, and fore and aft G forces.
Mike Howlett

I downloaded the Physics Toolbox Accelerometer (in IOS) and I am going to give it a try in the sprite. Thank you for the information on the way they measured with the calk device.


Flip Brühl

Yes I can archive about 1 G! on dry tarmac. (see figure.) The rear wheels lock first. So I am going to look for a rear break bias valve as I use the smallest 11/16 cylinders I can get for the rear slave. And I use very hard unknown linings in the rear brakes. Is there any other solution without changing front rotors or calipers to get the brake balance right?



With this Physics Toolbox Accelerometer I have my own rolling road!

Blue = bumps
Red = curves
Green = decelleration/acceleration


Flip Brühl

Back to A-level physics now. The late great sir Isaac Newton told us that force equals mass times acceleration
or in our case, decceleration(which is just negative acceleration) equals force divided by mass. So for a given set of conditions, the lower the weight of the car, the quicker it will stop.Take a measurement with a passenger then repeat the test but remove the passenger, the spare wheel,the passenger seat, drain the tank and the radiator and it will stop a lot quicker.
Also as Flip noted, the rear passenger side wheel is the first to lock up - not because the brakes work the best on that particular wheel but because it is the least heavily loaded, as the weight of the engine and gearbox is carried more by the front wheels than the rear wheels i.e. the apparent center of gravity of the car is more towards the front than the rear.
M J Chapman

Rubber will give more resistance when more loaded. Mass does not influence the stopping distance when the brake balance is just right and the car is not in a curve. In formula 1 it is always the inner front wheel that blocks first and smokes.
Flip Brühl

For the rear brake balance isn't it bigger bore of pipe/cylinder there will give less less pressure there(?).
Nigel Atkins

A bigger surface on the slave pistons gives more force on the brake pads or brake shoes. Smaller master cylinders give more pressure.
Flip Brühl

Nice job of filtering Flip! I liked the app but none of my experiments looked that clean.
But I was using my pushbike which has no suspension.
I had my phone oriented vertically - what did you use?
Greybeard

Well, my friend Sir Isaac would have noted a few minor differences between a Midget and a F1 car, one which is that in a F1 car the engine and gearbox are mounted towards the rear wheels which together with the rear aerofoil puts more weight at the rear than the front.
Now, the only time a F1 car uses its brakes is to slow down for a corner and in starting to turn in to the corner, the apparent centre of gravity is shifted to the outside, hence the reason the inside tyre loses grip first. I guess that if you braked a F1 car in a dead straight line, both front wheels would start smoking at the same time.
M J Chapman

I find it hard to believe that mass of the vehicle isn't relevant in achieving rapid deceleration. I am pretty sure that my Sprite would outbrake a 40 ton artic, even allowing for the fact it has many more wheels and far more rubber in contact with the road
GuyW

"I find it hard to believe that mass of the vehicle isn't relevant in achieving rapid deceleration"

Quite right. Strap in a 15 stone passenger, load up the rear with 200Kg of sandbags and you might find it takes a bit longer to stop!
M J Chapman

If the brakes ar the limiting factor more mass gives a longer stopping distance. With a Sprite all wheels can lock. So the tires and the surface, that the test was performed on, are the limiting factor for a normal loaded sprite.
Rubber brakes best if it has the right temperature and if it has about 10% slip.

ForNewton's Law and Equation of Motion see Wikipedia.
Flip Brühl

Flip, those printed tables with the words "equivalent to 31ft" imply this is a calculated and not a measured distance. They would have been using a brake test decellerometer as commonly used for MOT testing before the roller type testers were installed. The test machine was placed on the floor in the passenger foot well and used suspended weights inside which moved and registered​ the brake "effort" on a dial.

So the quoted theoretical braking distance didn't include any thinking distance and was calculated on a peak reading which probably wasn't sustained throughout​ the whole of the real life braking distance.
GuyW

Something similar to this Tapley brake tester on eBay number 332171043779
GuyW

Thank you Guy


Flip Brühl

This thread was discussed between 08/04/2017 and 19/04/2017

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