Stopping distances !!

[morrisminor]

As a Driving Instructor I'm interested to see how quickly my car (34Rp) would stop from 30mph.
The answer was ?  25 1/2 metres.

How does that compare with you guys who have bog standard brakes ?

The current Highway code states the overall stopping distance at 30mph is 23 metres,  so my car didn't do too badly,  but just how out of date is the Highway code with certain things !!

[Reckless Rat]

Overall stopping distance in the Highway Code is thinking distance plus braking distance, hence the 23m at 30mph.

The thinking distance is based on distance travelled in 0.7 secs relative to the speed. The speed in mph being the thinking distance in feet, eg, 20mph, 20 ft, 30mph, 30 ft, etc.

This means that the 23 metres quoted should have the thinking distance deducted to give the braking distance, which is 45 feet at 30mph, or 13.7 metres.

---

Braking distance is calculated as speed squared divided by 20, so in miles per hour and feet it is:
20mph 20ft
30mph 45 ft
40mph 80 ft
50mph 125 ft
60mph 180 ft

[andrew34ruby]

As a Driving Instructor I'm interested to see how quickly my car (34Rp) would stop from 30mph.
The answer was ?  25 1/2 metres.

How does that compare with you guys who have bog standard brakes ?

The current Highway code states the overall stopping distance at 30mph is 23 metres,  so my car didn't do too badly,  but just how out of date is the Highway code with certain things !!

Are we working in Metric or Imperial? Surely it should be 25.5 metres?

And does the Highway Code really give a stopping distance in Metres (metric) at a speed in miles per hour, (imperial)???

The world has gone mad!

[Reckless Rat]

It matters not which system you use the formula for calculating overall stopping distance is the same, whether you use mph and feet or metres per second and metres.

Overall stopping distance = thinking distance PLUS Braking distance

Thinking distance = v = speed ( either in feet or metres)
Braking distance = v squared divided by 20

It's just easier to do it in feet and mph. No idea what the current Highway Code says but the maths doesn't change.

Please note these are averages. Reaction times vary and so do cars braking abilities.

[Graham Barker]

It would be fun to see some video clips of Austin Sevens braking from 30 mph. Also perhaps this could be an event to be staged at the Austin Seven Centenary.

[Bob Culver]

Those who remember form 5/6 Physics read no further.
For ordinary tyres on say smooth concrete the nominal max achievable deceleration is 1 G (32 ft per sec per sec), regarded as 100% braking and achieved at lowish speeds by near all cars for last 50 years and more. Assuming half the weight is maintained on the braked wheels throughout 2 wheel brakes are limited to 50%. For driver only, open tourer cars were not promising
Expectations have risen A 1950s book regards 60% as brakes in very good condition. It was considered very undesirable for buses to exceed 50% , especially with standing passengers.
The legal limits here (which would follow international) seem very modest (to moderns, not Sevenists). About 50% fully laden from very low speed.  Commercial vehicles also have an allowance for system delay so the results for those 52 tonne outfits tailgating your modern at 60 mph are disturbing.
It is convenient  to remember 60 mph is 88 ft per sec. All other speeds can be proportioned from.
[The following distances exclude reaction distance, typically 1 sec, 88 ft at 60mph, 44 at 30 mph etc]

from 30 mph 30% 101 ft, 50% 60 ft, 70% 43ft, 100% 30 ft (I initially copied the figs from a Brit book. Although I know the 100% figure I did not spot that the 22ft reaction time had been added! Now calculated as I usually do l and corrected!!)

25.5 m, 84 ft is 11.5 f/s/s, only 36%....... Model T territory. It is hard to judge just where on road applied.

Stopping distances for Sevens in the Companion are very variable.  Contemporary road tests were generally about 50% or less, with no details of pedal pressure or whether assistance was provided on the handbrake, or of load. The few common published tests in the 1930s did not reach 50%. Often tactfully refrained from mentioning brakes. The semi Girling car about 70% not untypical of the time .

On a not too smooth dry road the friction of a locked wheel is not much less than maximum. Some now claim to be able to lock the rears which should give 40% or so alone.

(note editted)

[Colin Reed]

HI All,
In NZ if the vehicle is built before 1952 and the braking systems is entirely from that period rods or cables,
the stopping distance is 23feet at 18 mph 
these are Exclusions that are built in to the regs to allow for older cars

Colin
NZ

[Bob Culver]

That corresponds about 50%. There is a clause in the wof rules which requires vehicles to be substantially as made so a modern with soaked linings should not pass. The basic formula is v squared=2 x accel x distance which gives accel= v sqd / 2 distance. A decel of 32 f/s/s is 100% so the accel as a fraction of 32 gives the "efficency"

I recall decades ago a workmate taking his prewar Morris 12 for a wof when scrutiny was token at best.. The "skilled" tester told him if it had hydraulic brakes he would have fail it. As with most Morris it did have hydraulics! Coupled Sevens do not show well on rollers as the front axle does not move back to allow the rears to function.
With full Girling and other systems there was in the late 30s a real improvemnt in car braking generally. From cycling experience the huge contribution of a 2nd brake is very evident. On narrow metal roads with blind corners must have been exciting with 2 wheel brakes.. Local 1920s Guides state that sounding the horn was a legal requirment.
The feel of braking is very deceptive. With a heavy pedal and no dive often better than seems. Passenger in my Girling RP used to brace to the dashboard but to me wih 2 cwt on the pedal nothing seemed to be happening.
(Note my original above is editted)

[Tony Griffiths]

The Highway code figures for the mechanical part of braking are a ludicrous 45 years out of date. For what a modern can can achieve, see: https://www.youtube.com/watch?v=0zbZweqlZPw

[jansens]

If you assume the concrete surface is at 90 degrees the stopping distance is basically zero!



Simon