Article [13] ST1300 - Brake System Simulator

Thanks for the mention @mjc506 . Nice animation. I must have a play with that on a big screen.

Good to see someone else uses the technique of really getting engrossed in a subject to fully understand it.

mjc506 said:
Also, even if the bike's not moving, pressure in the SMC inlet line (same line as the front centre pistons) will bypass the seals in the SMC and operate the rear outer pistons too!! It seems that, when the bike's moving, the pressure generated by the front caliper being pressed forward exceeds the pressure bypassing the seals, but this will depend on the pads used (I've chosen fairly grabby 'EBC HH equivalent' pads for this, not sure of the friction coefficient of the stock pads.
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As I understand it, it takes very little pressure to get fluid to flow the wrong way past the primary seal. Any primary seal - not just the SMC. Two relevant bits of info:
  • Bleeding the rear outer pistons - fluid flows out of the rear bleed valve with very little resistance felt at the pedal. The seal just yields to the slightly greater pressure behind. It's tapered / flared shape enables this to happen.
  • The same happens at all master cylinders when the pedal/lever is released. The piston in the master cylinder moves back through the fluid, the seal yielding to allow that to happen. This is most noticeable when pumping new fluid into an empty system. If it didn't yield, it would suck fluid back each time the lever/pedal is released.

    This behaviour also ensures that new fluid is available in front of the seal as the pistons move out towards the disk and as the pads wear down. The compensation port allows pressure to be equalised when the lever is not in operation.

To imagine this, think if how a bicycle pump refills its tube with air on the upstroke. It is basically the same behaviour.
 
sirbike said:
Squeezing the ST1300 front lever by itself can lock up the front wheel, proving that maximum front braking has been achieved (prior to full lock up).
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Good point! I should, I suppose, add that I've ignored many physical limitations :)

Good point also @jfheath regarding fluid passing seals. Indeed there's nothing particularly special about the SMC itself in this regard, just a slightly unusual way of plumbing in what is essentially 'just' another master cylinder.
 
mjc506 said:
I was surprised how much the braking balance changes - for fairly light braking with the front lever only, there is more braking force provided by the rear wheel than the front (despite twice as many pistons acting on twice as many disks)! As the lever is squeezed harder, more front pressure is generated of course, but full front braking is only achieved with the addition of pressure on the rear pedal.
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Getting back from my ride last night I immediately felt each of the disc temperatures; both fronts were only slightly warm while the rear I'd say might have been around 140F [just too hot to hold onto]. I've been a bit more conscience of trying to use the forward brake more frequently and where possible and staying off the rear [sort of] and I was suspecting I may need to check the brakes further which I will anyway.
From what is explained in your above paragraph, I would expect that isolated light front lever braking would produce this [temperature] difference anyway due to there being less surface area contact on the rear compared to the two in front. It seems to me from this that the rear caliper is always going to run hottest, just hard to know how much is normal.
Once again just a great job and much appreciated.
 
Something that could probably be worked out and/or added to the model, but would need more data and another layer of complexity...

At a high level, heat going in to each disc will be (roughly) proportional to the pressure applied to each pad, and heat 'leaving' each disc will be roughly proportional to the temperature difference between the disc and ambient, and the speed of the airflow around the disc.

So with 'moderate' lever-only braking, lets say roughly equal braking force is being provided by each wheel. So half of the total heat is going into that rear disc, and the other half is being (roughly) split between the front discs. So the rear disc is receiving twice as much heat as each front disc. In addition, the fronts get nice clear cool airflow, whereas the rear is tucked up behind the mass of the bike and between two exhaust cans (not particularly hot themselves, but reflective) so will have a much harder time shedding heat.

So probably not surprising that the rear disc will usually be hotter than the front. Difficult of course to say how much difference indicates a problem - probably the 'best' check remains checking brake drag 'manually' before/after a ride?
 
mjc506 said:
I think I've captured the intended behaviour... Comments/corrections/etc more than welcome!
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Incredible!

Was looking at the clever G display and wondering.

Full front being applied without the rear results in 1 G.

Then adding full rear to it gets us up to 1.5 G, as you would expect after the 1 G result for front only.

But full rear without front getting all the way up to 0.8 G ? Cannot quite picture how this could happen and also doesn't quite match what it feels like when stepping on the rear only.

I suppose there is a friction factor somewhere in the model you can play with?

Because first, no result should go over 1G , as over that we flip over the front wheel.

Maybe could try to "calibrate" the friction factor for a total (front + rear) of .75 G (your model would then probably give out .5 G for front and .75 front + rear?).

Then, more realistically, the output for the rear alone should be around .4 G?

Enjoyed your work!
 
Whythankyou :)

I noticed some comments elsewhere regarding PCV and delay valve pressures. I've managed to get hold of some old ones so will pull them apart at some point to measure pistons/springs/etc and get some theoretical numbers.

@MidLife
I should note that the 'sim' doesn't worry about various physical limits such as the grip between tyre and the road, or the rider and machine :)

However... 1G of deceleration with the front brake only is probably about right, looking at published 60-0mph times. I regret to admit that I've chickened out of verifying those numbers... I can however confirm that rear pedal only braking is easily enough to lock both wheels on a loose surface. It takes a good stamp, but 0.8G probably isn't massively far off (but is likely optimistic - actual rear braking force will be reduced with weight transfer)

Note that >1G of acceleration (or deceleration) is entirely possible - it depends on the wheelbase and location of the CoG. The ST carries its weight reasonably low (especially with a nearly empty fuel tank) and is of course quite long, so is capable of impressive numbers. Regrettably, again, I haven't measured CG location.. but suspect an aggressive rider with a loaded ST (the panniers are low and rearward) could exceed 1G braking without too much difficulty.

Hopefully, I can get some better numbers for the PCV and delay valves, then it would be worthwhile having actual numbers for front and rear lever effort (rather than the existing unitless sliders) and a crude weight transfer and tyre friction sim to get better output.
 
mjc506 said:
Whythankyou :)

I noticed some comments elsewhere regarding PCV and delay valve pressures. I've managed to get hold of some old ones so will pull them apart at some point to measure pistons/springs/etc and get some theoretical numbers.
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Lots of work it seems!

Because you'll also have to figure out preload and piston travel before shut off. Not easy because it is an unusual sealing arrangement and hard to see exactly when it would start to seal.

And from some of Larry's pictures, it even looks like the big spring that governs the pressure drop after the plateau may be a progressive one.

Of course, an approximation may be enough to feed your sim with.

But if somebody had the time and skills, another way would be what Chris09 was thinking about doing at one time (until he realized the brakes work very well even if you don't know all about the set up!): install pressure gauges (and bleed port....) on the PCV and pressure it up.

I suppose the pressurizing could be done with a brake lever and MC.

If it is something you'd ever consider doing, there are probably a few of us here who would chip in to cover your costs...even if in the end it doesn't work. Still worth a try.
 
Good point, a couple of gauges might be easier than disassembling... Unfortunately, either way, it's an "at some point" project :)

Ah, see, that CBR has got extra loading in the rear to keep the tail down during hard braking :) hahah
 
That is a really, really good display and very valuable for visualizing what is happening during brake applications. One thing that has to be clear is that the braking system is only designed to stop the wheel from turning. Which is a point made by several other in this thread.
 
mjc506 said:
Good point, a couple of gauges might be easier than disassembling... Unfortunately, either way, it's an "at some point" project :)
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If you ever get to this "at some point" point and do the gauges, it would help to have enough oil volume after the outlet (like a long line for instance) to match what we actually have on the ST, as this volume is what is going to govern the slope of the drop.


mjc506 said:
Ah, see, that CBR has got extra loading in the rear to keep the tail down during hard braking :) hahah
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Not much more than a comb and a water bottle in that tail bag!
 
Just wanted to stop in and say the animation was good timing when I found it. I introduced air to a piston while trying to advance them enough to clean them before replacing new pads, I know the piston came way out, but didn't think air was able to get in.
Not knowing yet when I put everything back together I had this issue until the front wheel spin test after placing new pads was a quarter turn until stopped, not good.
5 hours later over 4 or 5 days, trying to bleed the brakes I was finally successful, it was stubborn to remove the air from the right caliper, the offending air pocket, and this animation proved squeezing the front lever while not in motion was only limited to needing to fix the top bleeder on the front calipers and nothing else.
 
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