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knowlep
Member
| Joined: | Sun May 25th, 2008 |
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| Posts: | 24 |
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Posted: Wed Oct 8th, 2008 07:57 am |
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Unless we try it in practice who knows the REAL answer compared to the THEORETICAL answer and on what kind of caliper / twin shoe arangement etc they were talking about in the exam and what effect a larger piston diameter would have.
It's an interesting debate and I'd love to ask the person who set the question for the NTT exam themselves on their opinion.
One other final take on this is that the smaller piston has less volume of fluid behind it in the caliper compared to the larger piston so, as the master cylinder travels down its stroke, the smaller piston would contact the disc earlier than the larger piston but would not produce a braking effort until the pressure equalised throughout the system by which time the larger piston is also contacting its own disc and based on the simple "Hydraulics" theory (i.e. small piston movement produces a larger force in a larger diameter piston albeit on a much reduced stroke which is why you have to pump the handle on a bottle jack for instance) then the larger piston would still create a greater stopping power.
Flippin eck - did I just write all that in one long sentence???
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Wesley
Trade Member
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Posted: Tue Oct 7th, 2008 11:42 pm |
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David wrote:
"Now when the service brake pedal is depressed, the force applied to the brake fluid creates a pressure which is constant throughout, undiminished".
When the service brake pedal is depressed, It Actually creates Flow, and very limited in its travel.
Its the resistance to flow that will create a pressure!
I`m getting bored now, laters, Wes.
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Wesley
Trade Member
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Posted: Tue Oct 7th, 2008 11:12 pm |
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David wrote: Hi Dubsy
That's the problem with VOSA, they have not passed the old age Mini with twin leading front brake shoes and drums, they still think in the sixties and seventies, like most of us now who only see brake discs and calipers at the front, so naturally I misread the post skimping over it and assumed brake discs and calipers, but the physics is right, which clearly illustrated which way the brakes would pull over. I used larger diameters for a reason. Anybody can disagree with it, but its right and VOSA would agree. and the next part from that I could use the results gained to prove how much effort a tester applies to the brake pedal to get your roller brake test readings, not just assuming, and don't need to be there?
So just imagine what VOSA senior Engineers could do with your brake test readings entered into computerisation, you say three wheels locked out, they/I could work out the readings you have given would not justify the forces involved, therefore could say that the minimum brake efficiency had not been met, allowing for an average additional 140Kg?
David
LOL!
I think this was put to bed a long time ago? Why didn`t you join in when current?
Good Fishing?
Laters, Wes..........
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knowlep
Member
| Joined: | Sun May 25th, 2008 |
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| Posts: | 24 |
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Posted: Tue Oct 7th, 2008 07:03 pm |
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Final Extract from Davids formula says :-
Now we can do the same for the offside front, but this time we will change the diameter, let’s say 05mm less.
A = d/4 = 3.142•34.3/4 = 26.94 4sf.
0.25•26.94 = 6.73N or 67.3 Bar. So as can be seen the offside front brake efficiency is less when the diameter and the area are less, therefore the brakes would pull to the nearside when applied.
Nearside front working pressure = 98.3 Bar
Offside front working pressure = 67.3 Bar
Hope this clears up any misunderstanding
David
WES
If you can,t see from the above extract in RED that the brakes pull towards the NEAR SIDE WITH THE LARGER piston then I give up.
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knowlep
Member
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Posted: Tue Oct 7th, 2008 07:55 am |
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Not at all - I believe Davids formula to be quite correct. There are no contradictions in his formula. It quite clearly states larger caliper piston therefore larger braking force.
I hope that David can comment on this topic and for then I think we should agree to disagree.
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Wesley
Trade Member
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Posted: Mon Oct 6th, 2008 11:18 pm |
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| So Your real issue is with Davids contradictions?
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knowlep
Member
| Joined: | Sun May 25th, 2008 |
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| Posts: | 24 |
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Posted: Mon Oct 6th, 2008 07:12 am |
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Final Extract from Davids formula says :-
Now we can do the same for the offside front, but this time we will change the diameter, let’s say 05mm less.
A = d/4 = 3.142•34.3/4 = 26.94 4sf.
0.25•26.94 = 6.73N or 67.3 Bar. So as can be seen the offside front brake efficiency is less when the diameter and the area are less, therefore the brakes would pull to the nearside when applied.
Nearside front working pressure = 98.3 Bar
Offside front working pressure = 67.3 Bar
Hope this clears up any misunderstanding
David
Therefore if the off side diameter is 0.5mm LESS that's-- LESS-- or SMALLER and the final applied pressure is LESS or 67.3Bar compared to 98.3 on the nearside (which has a LARGER piston) car will pull towards the greater force or GREATER DIAMETER PISTON.
No insults from me BTW - too professional.
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Rebel
Trade Member
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Posted: Sun Oct 5th, 2008 11:48 pm |
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"No wonder there are some many unecessary new parts fitted to customers"
Snigger
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Wesley
Trade Member
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Posted: Sun Oct 5th, 2008 11:42 pm |
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David wrote: knowlep wrote: I believe that interestingly enough Dubsys thoroughly written explanation / calculation is correct however he has based it on the OSF piston diameter being SMALLER than the NSF where the actual question in the exam is the opposite way round .........eh??
So the car would pull to the OFF SIDE or RIGHT i.e. towards the larger diameter piston.
Hi
I think avocco originally said;
3. A car is accidentally fitted with O/S/F wheel cylinders that have pistons 2.0mm GREATER in DIAMETER than the N/S/F wheel cylinders pistons. This is likely to cause ?
C = the brakes to pull to the N/S
Please read my previous post explaining hydraulics operation with reference to a braking system, please remember when reading that I made up the example to explain how brakes would pull to the nearside, so my example used is not identical but illustrate the correct theory to calculate hydraulic forces. I used calipers instead of wheel cylinders, but theory is the same.
Thanks
David
Look No Further!
I`m pretty sure that in the English Language, The Words, Greater and Larger, might just equate to the same?
nite nite Richard!
oh! Me thinks his friends might just call him "DICK"!............LOL!
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Wesley
Trade Member
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Posted: Sun Oct 5th, 2008 11:33 pm |
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knowlep wrote: Yep - read Davids postings and the car pull towards the LARGER piston. Agreed?
Sorry, No! NOT Agreed!
David, Will return soon, and hopefully correct your misunderstandings
Regards, Wes.
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knowlep
Member
| Joined: | Sun May 25th, 2008 |
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| Posts: | 24 |
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Posted: Sun Oct 5th, 2008 10:58 pm |
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| Yep - read Davids postings and the car pull towards the LARGER piston. Agreed?
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RFR
Member
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Posted: Sun Oct 5th, 2008 06:31 pm |
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David wrote: Hi Kev
No Brake test data is not a refusal you do a decelerometer test and issue VT32
David
Hi David,
just a quicky , why do a decelerometer test and what would I say on the vt32 if I did?
rfr
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Wesley
Trade Member
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Posted: Fri Oct 3rd, 2008 10:20 pm |
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knowlep wrote: Attend college - don't think so - try reading it again.
Dubsys detailed explanation calculates the OSF piston as SMALLER than the NSF so in his example the car pulls to the left or Near Side i.e. towards the larger piston.
Summary = car pulls towards LARGER PISTON.
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In the exam question the OSF piston is LARGER than the NSF piston so the car would pull to the RIGHT or Off Side i.e. the larger piston again.
Got it ? 
Yeah I have Got It!
Summary = car pulls towards the smaller diameter piston!
Read David`s postings.
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knowlep
Member
| Joined: | Sun May 25th, 2008 |
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| Posts: | 24 |
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Posted: Fri Oct 3rd, 2008 05:54 pm |
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Attend college - don't think so - try reading it again.
Dubsys detailed explanation calculates the OSF piston as SMALLER than the NSF so in his example the car pulls to the left or Near Side i.e. towards the larger piston.
Summary = car pulls towards LARGER PISTON.
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In the exam question the OSF piston is LARGER than the NSF piston so the car would pull to the RIGHT or Off Side i.e. the larger piston again.
Got it ?
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Wesley
Trade Member
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Posted: Thu Oct 2nd, 2008 11:07 pm |
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knowlep wrote: I believe that interestingly enough Dubsys thoroughly written explanation / calculation is correct however he has based it on the OSF piston diameter being SMALLER than the NSF where the actual question in the exam is the opposite way round .........eh??
So the car would pull to the OFF SIDE or RIGHT i.e. towards the larger diameter piston.
There seems to be a Contradiction in your statment?
Quote,"O/SF piston diameter being SMALLER than the NSF".
And then, "So the car would pull to the OFF SIDE or RIGHT ie; towards the LARGER diameter piston"?
Based upon the fact that, "O/SF" and OFF SIDE RIGHT are the same thing? and one of them is either Larger or Smaller than the other?
Me Thinks, YOU need to either Interperet what has been posted or Attend College!
Last edited on Thu Oct 2nd, 2008 11:21 pm by Wesley
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David
Banned
| Joined: | Mon May 26th, 2008 |
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| Posts: | 376 |
| Mana: |  |
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Posted: Thu Oct 2nd, 2008 07:32 pm |
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knowlep wrote: I believe that interestingly enough Dubsys thoroughly written explanation / calculation is correct however he has based it on the OSF piston diameter being SMALLER than the NSF where the actual question in the exam is the opposite way round .........eh??
So the car would pull to the OFF SIDE or RIGHT i.e. towards the larger diameter piston.
Hi
I think avocco originally said;
3. A car is accidentally fitted with O/S/F wheel cylinders that have pistons 2.0mm GREATER in DIAMETER than the N/S/F wheel cylinders pistons. This is likely to cause ?
C = the brakes to pull to the N/S
Please read my previous post explaining hydraulics operation with reference to a braking system, please remember when reading that I made up the example to explain how brakes would pull to the nearside, so my example used is not identical but illustrate the correct theory to calculate hydraulic forces. I used calipers instead of wheel cylinders, but theory is the same.
Thanks
David
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knowlep
Member
| Joined: | Sun May 25th, 2008 |
| Location: | |
| Posts: | 24 |
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Posted: Thu Oct 2nd, 2008 06:24 pm |
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I believe that interestingly enough Dubsys thoroughly written explanation / calculation is correct however he has based it on the OSF piston diameter being SMALLER than the NSF where the actual question in the exam is the opposite way round .........eh??
So the car would pull to the OFF SIDE or RIGHT i.e. towards the larger diameter piston.
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KevG
Super Moderator
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Posted: Sat Aug 16th, 2008 04:21 pm |
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David wrote: Hi Kev
Not sure I agree with you regards the brake test weight being the same as the vehicle weight, + driver and fuel, when you go back to work, look at your car brake test weight on the wall chart, then compare it to your vehicle gross weight on the VIN plate, you will find a difference. There is a method and formula for working out the vehicles brake test weight.
David
Its the Kerb weight we want not the gross weight. (Class 123 &4 are all i'm familiar with.)
eg. the gross weight for my bike is 450Kg. its kerb weight is 225 kg.
I agree there is a method, one would be to weigh it , like we do on the class one and two tests.
Kev
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David
Banned
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Posted: Sat Aug 16th, 2008 02:52 pm |
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Hi Kev
Not sure I agree with you regards the brake test weight being the same as the vehicle weight, + driver and fuel, when you go back to work, look at your car brake test weight on the wall chart, then compare it to your vehicle gross weight on the VIN plate, you will find a difference. There is a method and formula for working out the vehicles brake test weight.
David
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KevG
Super Moderator
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Posted: Sat Aug 16th, 2008 01:39 pm |
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Wesley wrote: KevG wrote: Thanks Rebel, I didnt know that. I've always put the weight in, I only do 1 or 2 decelerometer tests a year though. Ive got one of those new fangled digital ones!
Kev
and theres a "loophole" if you require one.
Loophole? Tell me more. Theoretically of course!
Kev
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