Eight inch front brake test.

hadronuk

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My Spares Co 8” front brake seems good now it’s well bedded in, but I thought I would try and get a more objective measurement. I installed an “app” on my Android phone so I could use the phones accelerometers to record the G forces.

8%20Inch%20brake%20test_zpstngsstyl.jpg



This was recorded as I braked increasingly hard from about 70 mph. The horizontal scale is seconds since the beginning of the recording, the vertical scale is G. I was slightly surprised and pleased that it is fairly easy to exceed 1G deceleration. I was still not braking as hard as I could, I didn’t dare until I was going slowly. At which point it felt as if the back wheel was trying to overtake the front. I have no plans to see if a “stoppy” is actually possible. All tests were solo, but with quite heavily loaded rear panniers. (Spares, tools, oil, petrol, electric tyre pump, wet weather gear, water, stand plank, tea bags, sink……)

I did about 7 hard stops in 4 miles with no noticeable brake fade. When I arrived home, the front drums were both at 65 degrees C. One rear drum was at 55 degrees C and the other was at 120 degrees C. Some adjustment to the rear brake balance clearly needed!
Most contemporary road tests of Vincents record the stopping distance from 30mph as less than 30 feet. As 30 feet requires 1G deceleration and one test even recorded a distance of 22.5 feet (1.3G!) I was very sceptical, but given that asbestos brake linings were apparently a lot better, perhaps they were correct.
I have used both the standard balance beam and rear brake arms on the front to get the maximum leverage. With any less leverage the 8 inch brake must feel rather wooden unless you have a Gorilla grip. I think my 8” brakes also have the latest improved lining material.
I have also filed down the last 1/4 of the lining of the trailing end of the trailing shoes. It is claimed that this can make a SLS brake nearly as good as a TLS brake by removing the “anti-self-servo” effect. (Apparently this was recommended by Titch Allen in the 1960s.) As I did the mod before my 8” brake was fully bedded in, I can’t really judge its effectiveness.
As the 8 inch brake gives as much braking as I feel safe using, I would think with the above mods the Spares Co. 7 inch SLS Alloy brake plates with Shadow drums would be "good enough" and with less weight and cost.

One other thing about the forks and their behaviour under braking.
Only after finishing these tests did I recall David Dunfreys description of his Girdraulics becoming locked at full extension under heavy braking. For a long time now, for comfort I have used 55 lb-inch front springs with internal “pull down” springs to minimise the forks tendency to extend too readily. On one of these brake tests, I hit a large bump under heavy braking. The front tyre squealed, but nothing else happened. I am fairly sure that as I came to rest on each test the forks were heavily compressed, but I didn’t think to pay close attention to this. They certainly didn't seem as if they were trying to extend.
I have twice now experienced the Girdraulic “Fearful Shudder”. The first was with the standard fork springs and damper. Hitting a pothole just as a grabbed the front brake a bit too sharply caused a loud machine-gun vertical oscillation of the forks. The second time, with my soft springs and AVO damper, was about two weeks ago in similar circumstances. This time though the vertical oscillation was much less pronounced, less violent and only 3 or 4 oscillations. I was going slower this time though.
So the pull-down springs may not have cured this vulnerability, but I think I can at least claim they appear to have minimised the probable negative effects of soft front springs with the associated high pre-load.
 

Albervin

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I think that if I considered fitting the 8" brakes I would also go to an alloy rim as the weight gain is quite a lot. Too much unsprung weight is a bad thing and the Vincent wheel is already a bit of a lump.
 

greg brillus

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Strangely enough alloy rims are actually not as light as you might think, with their main advantage being, they are stronger. When I built the racer I did comparisons of the weight difference between normal steel verses alloy rims, and the two were not that different. I have installed the new 8 inch brakes, and I would say they are a good improvement over the Vincent stock brakes, though they do take some "Bedding In" and they are quite heavy. I would be very careful how they are used with the bike rolling backward if installed on a Series "B" with original Brampton's..........Those brake plate anchor lugs on the lower fork ends are only so strong............o_O
 

vibrac

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Strangely enough alloy rims are actually not as light as you might think, with their main advantage being, they are stronger. When I built the racer I did comparisons of the weight difference between normal steel verses alloy rims, and the two were not that different. I have installed the new 8 inch brakes, and I would say they are a good improvement over the Vincent stock brakes, though they do take some "Bedding In" and they are quite heavy. I would be very careful how they are used with the bike rolling backward if installed on a Series "B" with original Brampton's..........Those brake plate anchor lugs on the lower fork ends are only so strong............o_O

I wrote before on powerful brakes and Bramptons
http://www.vincentownersclub.co.uk/index.php?threads/2-x-2ls-brakes.6634/#post-50060
half way down the page
 

timetraveller

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A very nice test Rob. So good to see some science coming into the discussion. Could you let us know the name of the app you used and how you transferred the graph from the phone to the Forum.
 

hadronuk

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A very nice test Rob. So good to see some science coming into the discussion. Could you let us know the name of the app you used and how you transferred the graph from the phone to the Forum.

Thanks. The app was more or less the first I came across. Others may be a better choice. It’s called Car G-Force Meter. Here’s a link: https://play.google.com/store/apps/details?id=com.haofu.gforcemeter
Swipe from left for settings, instructions etc. To exit settings etc, swipe from left again and choose G-Force Meter. Took me a while to find this, I’m new to smart phones. I did lower the sensitivity setting in the hope of reducing signal noise due to road shock. Swipe from right for record control and calibration.
The intended operating position of the phone in a car is vertical only. The reason for this is not made clear, I think it may be because not all phones can measure G in all directions. My phone apparently can, I did some simple tests and decided it was OK to place it horizontally on top of my tank bag.
This could probably stand more scrutiny in the quest for accuracy, but my results seem sensible.
I did the test ride, stopping and starting the recordings for each test. Viewing the results on the phone screen is interesting, but not very informative, so I connected the phone to my PC and browsed around to find the .dat files within the G-Force app folder. Copied these to PC, opened with Excel. There are 6 columns of data, A-F in Excel.

A column is sideways G.
B is brake/accelerate G.
C is timestamp in milliseconds. For some bizarre reason, this started at a very high number.
D is date and time.
E is latitude
F is longitude.
(If you copy Lat and Long into Google maps, you can see where you were when each measurement was taken. Vibrac will know this spot 52.43822 -1.03454 ).

Obviously columns B and C are what we are interested in.
After the first look at the results, it was clear there was some calibration error. Stationary should show zero G. Perhaps the act of pressing “calibrate” changed the phone angle.
To correct this, I created a new column. I assumed the average G over the whole test must be zero. So I averaged column B, then subtracted this error from each value to create the new column of data. For the second new column, I subtracted the first Column C timestamp value from all timestamp values so each test started at zero time. I also divided by 1000 to get the time in seconds. Generated a chart from these two new columns. Selected the area of interest, created second chart showing only the braking event. With this chart displayed, I selected “copy as picture” (how to do this depends on Excel version), pasted into my Image Manipulation Program, (Gimp2) saved picture to hard drive. Dragged and pasted this image into my photobucket online image repository, copied and pasted link for this image into forum.
There are obliviously many potential sources of inaccuracy, including the bikes change of attitude under braking tilting the phone. But if I have made some fundamental error -please message me privately!
 

timetraveller

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Thanks for all that Rob. I guessed that it would not be a straight forward sort of job. Very good of you to find your way through it all and to let the rest of us know what is involved.
 

champion

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Really impressive work. Well done and thank you for sharing.

It would be really interesting to see a back to back test done between an old standard 7" and a top spec 8" setup. Due to the significantly increased surface area of the new shoes, I bet the improvements would be instantly obvious.
However, I am aware of a couple of people who hate the 8" brake, I believe the linings make an enormous difference. (The VERY HIGH cost of getting bad or worn linings replaced is a conversation which Timetraveller can start...) It might also therefore be interesting to record data for the popular 8" linings vs the hated 8" ones.

As for pulling 1.3g, if I am honest, I was cynical to begin with. Road cars (and old motorbikes) can not handle forces much above 1.1g without significantly improved aerodynamics. Therefore our road tyres are only designed to have a proportional coefficient of grip. I wonder about the frequency / resolution / accuracy of the logger,
Whilst writing my dissertation on motorcycle aerodynamics, I used my iphone at 100 Hz to record deceleration during a coast down test in order to establish CoD. I found a supporting document to say that mobile phone data was +/- 5 to 10% of a reasonably good data logger, however when I looked at the results, they were so noisy it was hard to be confident of the reliability of the data. I wonder about an easy method of calibrating a phone's accelerometers.

Anyway, the more I think about it, the more I think it might be possible to pull more than 1.3g on an old motorbike with good drums as a motorcycle weighs less than a car and a good rider can prevent the front brake from locking up by keeping weight over the rear wheel. (easy in a straight line test).

I am now wondering if I can find some modern motorcycle braking data to compare disc to drum......

I totally agree with Albervin about the requirement to reduce unsprung weight, Vincent forks and brakes are very heavy, however Greg is correct, alloy wheels are heavy because light weight alloy wheels have a very limited fatigue life before they fail. (google images: alloy wheel fatigue failures.)

Before looking for performance improvements in alloy wheels, I would consider changing:
1. The fork geometry (another conversation for Timetraveller)
2. Lightening the girder forks with some milled slots.
3. Hughs' modification of the bearings in the headstock.
 

hadronuk

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It would be really interesting to see a back to back test done between an old standard 7" and a top spec 8" setup. Due to the significantly increased surface area of the new shoes, I bet the improvements would be instantly obvious.

However, I am aware of a couple of people who hate the 8" brake, I believe the linings make an enormous difference.

As for pulling 1.3g, if I am honest, I was cynical to begin with. Road cars (and old motorbikes) can not handle forces much above 1.1g without significantly improved aerodynamics. Therefore our road tyres are only designed to have a proportional coefficient of grip. I wonder about the frequency / resolution / accuracy of the logger.

Whilst writing my dissertation on motorcycle aerodynamics, I used my iphone at 100 Hz to record deceleration during a coast down test in order to establish CoD. I found a supporting document to say that mobile phone data was +/- 5 to 10% of a reasonably good data logger, however when I looked at the results, they were so noisy it was hard to be confident of the reliability of the data. I wonder about an easy method of calibrating a phone's accelerometers.

Thanks for your interesting comments.
A few slightly random thoughts on the points you raise.

As I recall, in my school physics frictional force was assumed to be independent of contact area. (In my engineering course I don't recall it being mentioned.) I know it is not entirely true in the real world, but I would have thought it was near enough so that a increase in lining area would not in itself produce a large benefit. But the greater area does result in cooler running, allowing "softer" higher friction coefficient linings to be used -if available!

Regarding some people hating the 8" brake, I have seen one bike set up with the longer rear brake arms, but then the standard balance arm was "replaced" by a centre-pull balance bar at the handlebar, thereby halving the cable force at wheel. No wonder the brake felt wooden. The 8" brake lining spec. has also been improved, and lastly, that large lining area takes a long time to bed in. (Demonstrating that increased contact area does have an effect! Or is there something else going on?)
The greater weight of the 8" brake is I agree not helpful, hence my interest in much more accurate and rigid 7" brakes that allow high leverage to be used safely.
There is though just one thing to said in favour of increased unsprung weight- although it is bad for roadholding it is good for comfort as the tyre absorbs more bumps!

I don't quite understand your point about road cars not being able to handle above 1.1g and therefore our road tyres are only designed to have a proportional coefficient of grip.
Aerodynamic downforce adds to the normal force created by the mass of the car, so that a higher cornering force can be generated, but if you were to calculate the tyres coefficient of friction taking into account the added down force, isn't still likely to be around the same 1.1G?
Might it not be that 1.1G is a sweet spot for the grip/mileage balance, and that above that a modest increase in grip results in a large drop in tyre and fuel mileage?
And aren't motorcycle tyres likely to be a fair bit stickier than car tyres as the customer expectations are different?

In your coast down test, were you using the phones accelerometers (which I can see would have a poor S/N ratio when decelerating gently), or were you using the satnav function to record speed and distance? I can see that the latter would have some complicated errors! My ultra basic accelerometer calibration test consisted of placing the phone at 45 degrees and noting that it read 0.5G !

Lastly, if you have access to accurate information on the exact position of the C of G of a modern bike, it should be easy to calculate what braking G is necessary to do a "stoppy"! Stoppys might be easier to calculate than wheelies, as the slow speed means aerodynamics can be ignored.
 

Chris Launders

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Before rolling roads didn't MOT stations used to have an instrument you could put in the car to measure the braking, surely they haven't all thrown them away, perhaps one could be borrowed.
I know cast iron drums are cheaper but what would would be the extra cost of alloy drums with liners and how much lighter would they be ?
 
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