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  1. #11
    Join Date
    Apr 2012
    Location
    Wrexham
    Age
    47
    Posts
    532
    Hi,
    I do find mine quite shit compared to my daily driver focus estate.
    And the brakes were supposed to stop a 2 tonne jag?? Though the jag never had a rover 200 servo or a ak pedal box(no offense to ak but i dont know what the ratio of the original jag was compared to..?) Most say theirs are ok though or maybe i am going faster in the cobra....??
    Regards
    Gav
    AK Ls1/t56
    Body on April 2014
    Passed IVA March 2017
    Registered April 2017.

  2. #12
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    814
    Odd you should post today Gav as I have further info. It makes all the calculations and speculations I made so far… sort of irrelevant.

    Turns out GM made their D52 calipers with quite a variety of piston diameters. For example, the ones used on a 1974 Malibu are 2,15/16" in diameter compared to my current 2,3/4" pistons from a 75 Camaro. A cheap and easy mod although I really was trying to talk myself into those pretty aluminium 2-piston Wilwoods with their low un-sprung weight and SS rust-free pistons…

    I’d like to say that’s the end of it for now, but I couldn’t help but investigate more. The first thing I started to realize is any system can lock the tires (or more technically, be capable of providing threshold braking) if the pedal is pressed hard enough. I will admit to getting the feeling I’m about to break the pedal when I try this, but what am I really dealing with?

    So I decided to calculate the foot pedal force for threshold breaking. For the first assumption, I’m going to assume that 0.9 Gs is around where the tires would lock up. The manufacturer of my car quotes a test value of 135 ft for 60-0 braking. This works out to 0.896 Gs so that helps support the 0.9 assumption. Also, they have achieved 0.9 Gs on the skid-pad, so although lateral skidding maybe not be the same as longitudinal skidding, they are probably close and once again the 0.9 G figure is supported.

    So I went through some long (although not very complicated) calculations to calculate the pedal force required to get 0.9 Gs of braking. (I’m not going to bother showing these calculations here unless asked. Don’t want everyone falling asleep….)

    With my current set-up, it takes 128 lbs of force on the pedal to achieve 0.9 Gs.

    If I upgrade to the larger front calliper, it will take 117 lbs of force.

    Fred Puhn, in his Brake Handbook recommends a maximum of 75 lbs on the pedal for threshold braking, and definitely no more than 100 lbs. Oh-oh…

    Although I found nothing so far for the 427 version, I did find two different specs on brake pedal pressure vs. stopping rate for the early 289 Cobras. One states 120 lbs for 0.93 Gs and the other yields 116 lbs for 0.93 Gs, the latter being an interpolated value from the table presented.

    I suspect that my intended caliper choice will bring a braking improvement in line with the original cars. If it doesn’t, then there’s something currently wrong with my brakes I have never discovered.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  3. #13
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    814
    First of all, I must apologize to Kiel and the once-upon-a-time lad. I didn’t take their post seriously enough. As mentioned in my last post, I can get the same GM caliper with a larger piston area from a 74 Malibu. Sorry guys.

    Now… if anybody is still reading this,,, I did some experimenting and some more research. First the experimenting:

    I got out by old analog bathroom scales and put one foot on it and the other on the floor. Getting 128 lbs was easy. Hmmm…. I thought. So next I put the scales against the wall and with my back against the tub, pushed with one foot. Even though I have rather strong legs (I used to do a lot of running when I was younger), getting anything over 100 lbs was serious business.

    Next the research:

    I managed to find a US government document entitled “Horizontal Static Forces Exerted by Men Standing in Common Working Positions on Surfaces of Various Tractions”, and “Yes” it included a position where the back was to a wall and the feet pushed against a block (not so obvious in the title). The test report came from the US Aerospace Medical Research Laboratory, January 1971.

    For the Backwards Push, their mean average from a number of test subjects was 1380 Newtons, or approximately 310 pounds. That was with two feet. One might think that with one foot you’d get half of that, i.e. 160 lbs, but it’s probably less as it’s not a balanced position.

    (I will admit their results with the feet against a wall were quite a bit higher 531 lbs or 265 per foot), but the diagram against a footrest looked more apropos for comparison purposes.)

    So there’s no doubt that my Cobra, as well as the original 289 would take quite an effort. An effort I have not personally done with mine.

    Certainly, from my testing, a decrease by about 11 lbs to 117 lbs from 128 seems significant and worth doing.

    However I’d like to maintain my brake balance, so I’m also looking at higher friction pads for my rear brakes. They are from a Jaguar XJ6 circa 1965-1983. Anyone know of anything that’s available? I must do more research. Fortunately the weather is bad here and I get to spend a little more time indoors.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  4. #14
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    416
    Quote Originally Posted by Eggbert View Post
    GDCobra: Odd you should mention this as I was just thinking something similar last night. The fact is, I'm used to servo assisted brakes in almost everything I drive. The Cobra's manual brakes take a lot of leg effort. I'm sure part of the problem is that I don't seem to be slowing down enough compared to the leg effort I'm putting out.

    I do have a little device (the original G-Tech meter) that measures G forces in a car as well as Hp and 1/4 mile times. I've never used it in the Cobra as it's designed for a cigarette light that I don't have. As it's a simple matter to run some wires back through the firewall temporarily, I could try it out. Another solution would be to mark stopping distances on a road using chalk and compare the Cobra to my Toyota.

    Thank-you all for your comments.
    Now there's a coincidence, when I did the testing on min, the car I used as a "control" was also a Toyota!
    Most mobile/cell phones (the so called smart ones) have accelerometers and gyros in them and there are apps available to measure performance, why not use that instead of wiring?


    IIRC I got 0.96G for the Toyota (ABS just kicking in) and 0.74G for the cobra (fronts locling up). I improved the rears by swapping my inboard discs to outboard and fitting some Wildwood four pots. I later fitted similar calipers to the front (slightly larger pistons) so I could use same pads all round, but it was the change to the rear system which upped my deceleration figure.

    One problem you may have is that any changes you do to the mechanical or hydraulic leverage ratios to give you higher pressure at the pad will require longer pedal travel which may not be practical in a cobra footwear. Other options are higher friction pads, which it sounds like you already did, or larger diameter discs. What diameter are your discs now?
    Another possibility is grooved discs which are supposed to stop the pads glazing. Probably give more dust though.

  5. #15
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    814
    Thank-you for your reply. It was starting to get lonely in this thread... LOL.

    Quote Originally Posted by GDCobra View Post
    Now there's a coincidence, when I did the testing on min, the car I used as a "control" was also a Toyota!
    Most mobile/cell phones (the so called smart ones) have accelerometers and gyros in them and there are apps available to measure performance, why not use that instead of wiring?
    That's a very good idea. I've never owned a cellphone, much less an iphone, but my son has one and I'm sure he'd be glad to help. He's always adding Apps, so that should be easy for him.

    Quote Originally Posted by GDCobra View Post
    IIRC I got 0.96G for the Toyota (ABS just kicking in) and 0.74G for the cobra (fronts locking up). I improved the rears by swapping my inboard discs to outboard and fitting some Wildwood four pots. I later fitted similar calipers to the front (slightly larger pistons) so I could use same pads all round, but it was the change to the rear system which upped my deceleration figure.
    I've been researching that too. Basically I have a narrowed XJ6 rear suspension mounted in a cage that bolts to the chassis. I'm not prepared to make the changes to outboard discs although the factory offers a new set-up with outboard brakes and larger front brakes. Very expensive, and since I feel close to success here... and am self-proclaimed as "frugal", it's a no go for now.

    However I am researching rear brake pads with a higher friction rating. I started doing that in order to help maintain the front/rear brake bias when I switch to larger calipers. At this point EBC Yellowstuff pads seem to be the best choice, but needs further confirmation. Brake pad manufactures don't often list the coefficient of friction for their pads, and even if they do, it changes with pad temperature. Goes higher as things heat up... to a point, and then drops off. One needs a graph.

    I'm currently using Hawk HPS front pads which did give a significant improvement, but they don't make pads for the Jag rear. Wilwood offers a really good pad and with data too, however once again, no go for Jag pads.

    It's a difficult thing to sort out as most of it is advertising hype about things like "great cold bite", "easy on the rotors", "low dust" etc., but without numbers these sort of things are fairly useless. So I take my best guess and try them.

    Quote Originally Posted by GDCobra View Post
    One problem you may have is that any changes you do to the mechanical or hydraulic leverage ratios to give you higher pressure at the pad will require longer pedal travel which may not be practical in a cobra footwear. Other options are higher friction pads, which it sounds like you already did, or larger diameter discs. What diameter are your discs now?
    Another possibility is grooved discs which are supposed to stop the pads glazing. Probably give more dust though.
    The difference in the front calipers and my current ones will only require about a 14% throw increase at the front master cylinder. Due to the balance bar feeding both front and rear cylinders I can expect only a 7% increase in pedal throw. This works out to be about 0.1 inches for my current 1.5" of throw. But your point is well taken. Too large a front change will require a lot of other changes.

    Current front discs are 11". I have room to 11.5" easily enough... maybe 12, but I don't have any idea of what caliper will work with my steering knuckle if I go to a larger disc. I think the rear discs are 10", but should probably confirm this.

    So... in a nutshell, if I go to the larger piston front calipers AND install pads with a co-efficient of friction around 0.55, I should be able to reduce my threshold braking pedal pressure from 128 lbs to 98 lbs. I know theory doesn't always work out in practise, but it's a good place to start.

    I'm almost ready to order all the parts, but am still investigating at my choices for high friction street pads that won't chew up my rotors (the front ones being precision re-drilled for mounting on my pin-drive hubs).
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  6. #16
    Join Date
    Jun 2004
    Location
    Northampton
    Posts
    1,585
    That’s what I like about your posts John. Every day’s a school day! What starts off as a simple post about brakes then enters the realms of physics etc. You obviously think very deeply about solutions to problems. Well done and keep educating us!
    Dek

  7. #17
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    56
    Posts
    31
    Hi to the comments on braking,
    This may not be much help as my sr v8 has an rv8 upfront so quite light.
    The setup that I have is all xj6 ,inboard disks ,four pot callipers up front and is connected to a Ford Fiesta servo/brake booster.
    Brake pads ,discs ,and brake fluid is all standard ,this gives good braking and will lock wheels without much effort.
    Also mention that the mot inspector has commented on how well the xj handbrake works,
    I would like to think that everyone on here be confident with how there brakes work !
    Regards,Chris.

  8. #18
    Join Date
    Jan 2012
    Location
    Perth
    Age
    53
    Posts
    1,676
    Quote Originally Posted by Eggbert View Post
    First of all, I must apologize to Kiel and the once-upon-a-time lad. I didn’t take their post seriously enough. As mentioned in my last post, I can get the same GM caliper with a larger piston area from a 74 Malibu. Sorry guys.

    Now… if anybody is still reading this,,, I did some experimenting and some more research. First the experimenting:

    I got out by old analog bathroom scales and put one foot on it and the other on the floor. Getting 128 lbs was easy. Hmmm…. I thought. So next I put the scales against the wall and with my back against the tub, pushed with one foot. Even though I have rather strong legs (I used to do a lot of running when I was younger), getting anything over 100 lbs was serious business.

    Next the research:

    I managed to find a US government document entitled “Horizontal Static Forces Exerted by Men Standing in Common Working Positions on Surfaces of Various Tractions”, and “Yes” it included a position where the back was to a wall and the feet pushed against a block (not so obvious in the title). The test report came from the US Aerospace Medical Research Laboratory, January 1971.

    For the Backwards Push, their mean average from a number of test subjects was 1380 Newtons, or approximately 310 pounds. That was with two feet. One might think that with one foot you’d get half of that, i.e. 160 lbs, but it’s probably less as it’s not a balanced position.

    (I will admit their results with the feet against a wall were quite a bit higher 531 lbs or 265 per foot), but the diagram against a footrest looked more apropos for comparison purposes.)

    So there’s no doubt that my Cobra, as well as the original 289 would take quite an effort. An effort I have not personally done with mine.

    Certainly, from my testing, a decrease by about 11 lbs to 117 lbs from 128 seems significant and worth doing.

    However I’d like to maintain my brake balance, so I’m also looking at higher friction pads for my rear brakes. They are from a Jaguar XJ6 circa 1965-1983. Anyone know of anything that’s available? I must do more research. Fortunately the weather is bad here and I get to spend a little more time indoors.
    Not an issue John....will always enjoy reading your posts.

    cheers
    Kiel and Duncan (dad and the lad errr young man now)

    AHP Crendon Chassis No 1
    Ford FE 434, dual quad, Toploader total old school all the way
    https://www.crendonreplicas.com/
    http://www.absolutehorsepower.co.uk/

  9. #19
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    814
    Before I get started; Awe... nice comments fellows. Thank-you. I should have mentioned this a while back, but things went a little awry at home and the missus wound up in the hospital for 8 days due to a heart attack. She's fine now, although they are monitoring her every week and may do more tests if required. Yesterday she had needles in both eyes due to macular degeneration (I find it hard to watch that being done). Last year this time she had an adrenal gland removed. She's not had an easy time.

    I'm also back and forth next door to tend to my 93 year old father who is starting to dementia. My time isn't always my own, but that's the way things are.

    Since she got home, I managed to get the first part of my brake upgrade done and went for a test drive. Basically all I did so far was to install new front brake calipers with larger pistons. Step 2 will be to install higher friction rear pads. The front pads I already upgraded to Hawk HPS pads some time back and it was a significant improvement. Unfortunately, Hawk doesn't make rear pads for my Jaguar rear brakes, so I'm going to try EBC Yellowstuff and managed to find a set quite cheap. But one thing at a time.

    The test drive went more or less OK. The braking was much stronger and I am satisfied with the results. I didn't manage to lock the front wheels, but that was due to the brakes feeling spongy.

    Working alone, I merely did a gravity bleed and I don't think I got all the air out. Even if I did, after the bleed, I went to tinker with the bias bar and noticed some brake fluid dripping from the rear brake master cylinder. The end of the hose from the remote reservoir had deteriorated. In order to fix that I had to remove the front brake remote reservoir hose for access to the rear one. Air may have entered at that time. Anyway, another bleed with a helper ought to fix it. Oh... I don't like the hose in jar method because I can't see what's happening down there when I'm sitting in the car alone.

    So there was no point in measuring stopping distances at this time. I need to bleed the brakes again first. Mind you, I can tell it's much shorter than before as I know when I had to slow down when approaching my driveway. I did purchase a cigarette lighter type socket which I can temporarily install for my G-meter. My son didn't seem terribly interested in getting a G App for his phone. I may need to push him on this, but as I have a G-meter, I bought the electrical receptacle.

    I may not get to installing the Yellowstuff rear pads for a while. They come with an abrasive coating on the outside of the pad material to help clean the disc in preparation for bedding. They say this can take up to 200 miles of driving. I don't mind the cold, but as we had a slight dusting of snow last night, it won't be long before conditions require salting the roads.

    I'm temporarily busy doing work to my Ford truck. Inner rear axle seal needs changing. Emergency brake needs adjustment, which you have to remove the rear axle to get at if the adjusters are stuck, or in my case, the parking brake shoes are saturated with differential fluid from the leaking seal. I also have a front lower ball joint to replace which is an awful job on my old rusty truck. Also a rear leaf spring hanger bracket needs replacing. So far I have the rear axle all apart on the side that's leaking and managed to get the parts yesterday. I might put the job of until tomorrow as it will only get to 3degC today, but tomorrow is calling for 10degC.

    The main issue is all the bolts have the corners/points of the hex heads worn away from rust. It's bad enough my local garage turned down the job.

    Oops... all this sounds like a blog instead of Cobra brake improvements. But then again, many of my posts do! LOL.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  10. #20
    Join Date
    Apr 2012
    Location
    Wrexham
    Age
    47
    Posts
    532
    Sorry to hear that and best wishes to yourselves. Sounds like you got your hands full though.
    Regards
    Gav
    AK Ls1/t56
    Body on April 2014
    Passed IVA March 2017
    Registered April 2017.

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