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Re: Wheels and spokes by Little French
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Author Topic: Brakes  (Read 1016 times)

StefanN

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Re: Brakes
« Reply #15 on: 16 November 2019, 02:22:28 pm »
That’s interesting Rhys, I think I’ll try adding a return spring on my brake pedal.  Thanks for sharing

TheGiantTribble

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Re: Brakes
« Reply #16 on: 16 November 2019, 02:26:59 pm »
Coincidently one thing at Stretton I noticed was, with my new trainers on I couldn't feel the throttle pedal, previously
with my old ones (exactly the same make and style) I could, and I was wondering what to do to improve the situation.

Adrian

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Re: Brakes
« Reply #17 on: 16 November 2019, 06:36:26 pm »
Thinner socks?
 :)

Adrian

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Re: Brakes
« Reply #18 on: 16 November 2019, 10:30:41 pm »
Seriously folks, brakes are a very serious issue and as I understand it, Slack Alice was lucky to get away with a few days in hospital following his crash at Stretton. I hope your OK matey?
Now I don't know if I am right or I am wrong but I guess a sudden loss of brakes was the cause of the crash. It was certainly the cause of Rhys' crash and of course, he was driving the same car.
I remember looking at the braking arrangement following that and I thought the brakes were not capable of converting the kinetic energy of the cyclekart travelling at perhaps 15mph, into the equivalent heat energy, within the few yards the driver was perhaps hoping for. I'm sure Slack will rethink the brakes now but that's not what I am writing about. 
I am of the firm opinion that we need some sort of simple 'standard' that we, as builders and drivers, should adhere to and that the standard should include brakes and anything else directly related to the safety of the Cyclekart.
Now this will take some thinking about I know (and I don't expect anyone to retrofit anything) but for new builds its a necessity I think and I would personally welcome a meeting of members specifically to discuss the subject.

StefanN

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Re: Brakes
« Reply #19 on: 16 November 2019, 11:26:55 pm »
I think meeting and working on the question of braking is a good idea Adrian

Adrian

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Re: Brakes
« Reply #20 on: 17 November 2019, 10:59:01 am »
Thanks Stefan.
I would like to know if everyone feels the same?
We all know how important safety is but perhaps we don't always know how to 'build it in' as it were.
There are other things too, of course, all of which I am confident would come out were we to have such a meeting.
We have some good designers in our little group and we have people with 'senior' racing experience as well as good machinists, all of whom could contribute I'm sure.
The sort of thing I have in mind is a small A5 size booklet which is given to all new and potential members to help them on their way to building a safe and successful Cyclekart.
 

RhysN

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Re: Brakes
« Reply #21 on: 17 November 2019, 03:54:57 pm »
Sounds like a good idea to me Adrian.

TheGiantTribble

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Re: Brakes
« Reply #22 on: 19 November 2019, 03:39:54 pm »
Chris mentioned Silicone fluid, now everything I've read about it sounds excellent.
The one thing I can't seem to find a definitive answer to, can you use it in a braking system that has already had normal brake fluid in it? Is just draining the old fluid out enough or do you need to replace parts? Any ideas gentlemen?

Chris Brown

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Re: Brakes
« Reply #23 on: 19 November 2019, 03:46:37 pm »
You need to at least drain and flush the system through with the Silicone fluid. Ideally strip and clean everything thoroughly to get the full benefit.

TheGiantTribble

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Re: Brakes
« Reply #24 on: 19 November 2019, 04:15:56 pm »
Thanks for that info Chris.

Slack Alice

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Re: Brakes
« Reply #25 on: 20 November 2019, 04:00:00 pm »
Hi guys...when I started this thread I wasn’t thinking of my current situation!!
Hospital says rest and come back in 4 weeks. There won’t be much engineering done !
 
Having studied brake maths for the last few days, I believe the problem was lack of braking pressure in the system. It seems that a 6:1 ratio is ideal at the pedal, increasing say a 50lb foot pressure to 300lbs at the master cyl. Mine is only 2:1.
I am using a small bore M/C of 1/2” dia and this puts the effective pressure up to 500psi.
I believe this should be between 1000 and 1200 psi available for use.
Add to this the relatively small dia (200) of the disc and the width of 3mm, fade was probably the culprit.
Having said that, they did work for half the time before failure??
 
The basis was a matched pair, for the rear of a 650cc Suzuki bike.  A 1/2” bore M/C and a single cyl. Slave of 1.5” dia. The slave was unusable however because it needed a large aluminium bracket to support one end of the pads.
I changed it for a bike M/C with 2, 1” pistons. ( slightly smaller area overall).

My first change will be to the pedal ratio, to get psi higher.
Next post will be the formulae I believe are required and the application to Thunderbug.
See you later......Dave.

Slack Alice

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Re: Brakes
« Reply #26 on: 20 November 2019, 08:10:06 pm »
http://www.engineeringinspiration.co.uk/brakecalcs.html
This is the site I used for the formulae.
I first calculated the ck weight....W = ck mass + driver x9.81.
In my case.  130 + 85 x 9.81 = approx 2150N.
 
Calculate deceleration rate.  I assumed an initial rate of 33mph and a terminal speed of 7mph, stopping over 5 seconds. This is 15m/sec to 3m/sec.
Therefore 12/5 = 2.4m/sec/sec. x 2150 = 5160N.
 
The torque on the wheel...( I used 560mm for the wheel dia., 280 for half ).
T = F x Rad.  T = 5160 x 0.28m = 1445Nm.
 
Calculate clamping load on wheel. C = T/ eff rad of disc x coef of pad x no of pads
C = 1445/0.1. = 14450 N.
 
Calculate system pressure required P = C/ A ( slave piston area)
P = 14450/982 mm2.  = 14.7MPa. Megapascals
7 MPa = 1000psi.
 
Since I need 2000psi and only have less than 500, the brake doesn’t work!
Solution, more pressure and probably a bigger disc.
 
Please check my thinking/calcs.  Dave.

StefanN

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Re: Brakes
« Reply #27 on: 21 November 2019, 12:07:39 am »
Dave,
Nice work on the calculations.

I think that for the deceleration calculation, you're using force = mass x acceleration  where mass is in kg.  So that should be F = 215 x 2.5 = 537N.  However 5 secs feels like a very long time - (it's taking us less than that to do the whole length of the straight in front of the pits at Stretton) - so maybe 1.5-2.5sec?

Looks like you also needed to include the pad coefficient and number of pads in the clamping force calculation...

Plugging those back into your calculations:

Calculate deceleration force.  I assumed an initial rate of 33mph and a terminal speed of 7mph, stopping over 2 seconds. This is 15m/sec to 3m/sec.
Therefore 12/2 = 6m/sec/sec. x 215 = 1290N.
 
The torque on the wheel...( I used 560mm for the wheel dia., 280 for half ).
T = F x Rad.  T = 1290 x 0.28m = 361Nm.
 
Calculate clamping load on wheel. C = T/ eff rad of disc x coef of pad x no of pads
C = 361/(0.1*0.35*2) = 5157 N.
 
Calculate system pressure required P = C/ A ( slave piston area)
P = 5157/982 mm2.  = 5,251,527Pa = 762psi

So, it would seem that more pressure would be helpful to bring your performance up to what you're hoping for.

However, I think there's a very important distinction between brake performance and brake reliability.   If our brakes don't have great performance but do the same every time, then you can drive to suit that....but based on your description of what happened, what I think you experienced was a sudden change in your brakes' performance.  Sudden unpredictable changes are dangerous.

There's been talk about brake fade and "pad gassing" (lots of debate about whether this is real), pad glazing and boiling fluid.

Assuming a 215kg cyclekart incl driver, stopping from 15m/s/s to 0.

Kinetic energy = 0.5 x mass x velocity squared
                      = 0.5 x 215 x (15*15)
                      =24,188 Joules

Taking a brake disc of say 0.5 kg  (note the Gemini one weights about 1.5Kg so 0.5kg would be very light.)
The specific heat capacity of steel is about 420 J/kg.C.  Assuming all the kinetic energy is lost as heat in the brakes, then the temperature rise in that one braking event would be
24,188 / (420 * 0.5) = 115 C

This assumes all the energy goes into the brake disc, and that no heat is lost from the surface of the disc or brake calliper.  It also assumes that no heat goes into the brake hub or the rear axle. 

But, what temperature does brake fade happen at?  I've read and heard all sorts of temperatures, but the vast majority are around  600-700 C and higher.  So to me it seems unlikely that the brakes would get up to those sorts of temperatures except for on the flimsiest of disc rotors,  through a mechanical problem with the brakes, or the driver "riding" the brake pedal.

Boiling of brake fluid seems more likely than brake pad fade although normally, only a small proportion of the energy goes as heat into the calliper.  Also, we'd expect the brake pedal to go soft.

So, for me the jury is still out.  Any other thoughts?

TheGiantTribble

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Re: Brakes
« Reply #28 on: 21 November 2019, 09:36:07 am »
Impressive work with the numbers you two.

A couple of thoughts about the numbers...

Top speed 33 MPH, that's got to be higher, we know CK's have hit 50 in USA and I would personally add 10 percent for safety on top of that.
Time spent braking, like Stefan 5 seconds seems way too long, from my own driving experience it's more like 1 - 1.5 seconds.
Also should you not be using a terminal velocity of zero, since worse case scenario we would be 'red flag' situation, immediate stop on track? 

As Rhys and Stef have both eluded to, not so good braking but reliable/constant is safe, but brakes that start excellent but then degrade not good. I agree with this.

Also just a thought, with your driving position Dave, your knees are heavily bent, this physically push's the end of your foot downwards, are you riding the brakes without knowing it, even if only slightly? I know when we are sitting in the cars stationary it's easy to notice these things, but when we are being bounced around on track different story.
I think we have been looking for the one answer when maybe two factors are involved. Increase both pressure and disc size/mass. Pressure will give you braking force, mass will give you reserve of energy absorption...think that all makes sense???
 

ChrisS

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Re: Brakes
« Reply #29 on: 21 November 2019, 05:07:00 pm »
You two have lost me, I'm impressed!

I think if it was me, I would try to find some straight tarmac and run up and down giving the brakes real abuse until they fail again. Trying to narrow down the factors responsible.