24-07-2021, 10:54 PM
(This post was last modified: 25-07-2021, 09:26 AM by Bob Culver.
Edit Reason: Asleep when original done. sorry.
)
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)
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)