Article [13] ST1300 - Brake System Simulator

mjc506

mjc506

Matt
Joined
May 1, 2021
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314
Age
37
Location
Wales, UK
Bike
ST1300A2, CRF250L
A distinct lack of contributions to the forum recently (I blame "other projects"... :well1: ) so here's a dirty "simulation" of the ST13 braking system!!


I've tried to get it as accurate as possible, but don't have actual numbers for the PCV or delay valve threshold pressures. But the defaults seem to be vaguely sensible (but feel free to change them if you disagree - the pressures are in N/mm^2, the delay valve pressure is the pressure the valve opens and starts activating the front right centre piston, the PCV valve pressure is the pressure the valve closes and prevents extra pressure from reaching the rear caliper).

There are two sliders and a checkbox for 'controls' - one slider for front brake 'squeeze' and the other for rear 'stomp'. The checkbox can be 'unticked' to see how the system works when the bike's not moving (ie, SMC not activating, but still passing pressure from the rear pedal).

I think I've captured the intended behaviour... Comments/corrections/etc more than welcome!
 
Wow! that's a great simulation and shows the system working much better than anyone could explain. :cool:

I didn't realise the Rear brake pedal activated the front calipers - every day's a School day!
 
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Very cool.. but, the rear brake activates the 2 outer pistons on the rear caliper correct?
 
Although it only shows 2 light brown REAR pistons - fairly sure it represents all 4 pistons, similar to the front.
 
mjc506 said:
A distinct lack of contributions to the forum recently (I blame "other projects"... :well1: ) so here's a dirty "simulation" of the ST13 braking system!!


I've tried to get it as accurate as possible, but don't have actual numbers for the PCV or delay valve threshold pressures. But the defaults seem to be vaguely sensible (but feel free to change them if you disagree - the pressures are in N/mm^2, the delay valve pressure is the pressure the valve opens and starts activating the front right centre piston, the PCV valve pressure is the pressure the valve closes and prevents extra pressure from reaching the rear caliper).

There are two sliders and a checkbox for 'controls' - one slider for front brake 'squeeze' and the other for rear 'stomp'. The checkbox can be 'unticked' to see how the system works when the bike's not moving (ie, SMC not activating, but still passing pressure from the rear pedal).

I think I've captured the intended behaviour... Comments/corrections/etc more than welcome!
Click to expand...

Thanks for creating this.

So do I understand correctly that the front brake activates the 2 outer pads of both front calipers and the 2 outers pads of the rear?

And the rear brake activates the center pads of each caliper?

I didn't realize that much rear braking force was engaged by the front brake.
 
This is the clearest explanation that I have seen. Nice job. You definitely made up for your previous lack of contributions.
 
You can tell me how it works a hundred times but the “aha moment” for me only comes when you show me.

Nice job and helpful to visual learners.
 
This is really cool. This is a complete tool that shows all of us how the braking system works.

So you asked for comments and corrections. I can't comment on corrections so I am assuming the simulation is correct. Here are two additions based on things that I have read over the years in this forum that I think would be very useful to us shade tree mechanics.

Can a bad SMC be simulated and the affects on the braking system?

And can the brake bleed process be simulated.
 
This is the most understandable diagram and explanation I have ever seen. THANK YOU!
 
Thanks all :) It certainly helped me improve my understanding of the system, glad it's useful!

A few notes...

Front lever only directly acts on the outer pistons on the front caliper, but when the bike's moving, however, the SMC will also activate (the front left caliper is being pushed forward due to the 'squeeze' from the front left outer pistons) and operate the outer pistons on the rear caliper as well.

Rear pedal only directly acts on the centre piston of each caliper (via the delay valve for the front right), but as above, when the bike's moving, the SMC will activate (front left caliper being pushed forward due to the 'squeeze' from the front left centre piston) and operate the two rear outer pistons too. 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. HH is ~0.5u, some long life pads can be as low as ~0.2u) The PCV valve will prevent the braking force from the SMC exceeding a certain level (to help prevent rear locking during heavy braking)

The only way to activate the front outer pistons is by the front brake lever. The only way to activate any (all) of the centre pistons is by the rear brake pedal. The rear outer pistons will activate from only the rear pedal when the bike is stationary, or either control when the bike is moving.

Only three pistons in each caliper (one middle, two outer)

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. I suppose this makes sense from a vehicle-dynamics (and gumby rider) point of view, but does perhaps explain some of the faster wear rate of the rear pads compared to the fronts.

A bad SMC could be simulated to some extent... A 'locked' SMC (some riders try to diagnose/ignore problems by shoving a lump of wood between the SMC and fork to stop it activating) can already be simulated by unticking the 'moving' check box. A 'blocked' SMC (not releasing pressure) could be added in... I'll see what I can do.

Not so sure about simulating the bleed process. I would refer to the excellent articles by @jfheath, to be honest I'm not sure what I could add to those! https://www.st-owners.com/forums/threads/st1300-maintenance-brakes-avoiding-the-pitfalls.135125/ https://www.st-owners.com/forums/th...g-all-the-air-out-of-the-brake-system.156754/
 
mjc506 said:
ear pedal only directly acts on the centre piston of each caliper (via the delay valve for the front right)
Click to expand...

I noticed that as I was playing with the toy (saw that the right front braking force stayed at 0.0 until I mashed the "pedal" input hard enough to overcome the delay. Can you explain why Honda did that? I'd have thought activating both front calipers in equal amounts at all times when the brake system is active would be better but I'm no engineer so what do I know? (Very little, as a matter of fact, and even less about the intricacies of motorcycle brake system designs.)
 
I've read somewhere that it's to minimise diving during low speed manoeuvring (makes sense I guess? Lots of light pressing on the rear brake, and don't want too much front at low speeds while turning...)

Edit: I should note that the 'real world' system is a lot 'softer' around the threshold pressure - compressability in the lines/seals/fluid etc makes it less "on/off" than this model shows. But I didn't want to start trying to model squishy lines :)
 
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Grabcon said:
Can a bad SMC be simulated and the affects on the braking system?
Click to expand...
Done! Also did the front and rear master cylinders, because why not :)
And included 'seized caliper' faults - only simulating 'brake stuck on faults' as anything seized enough to not apply the brakes will have other, more important, issues to worry about...

Link again for convenience: https://mjc-506.github.io/st1300braketoy/

Code here if anyone wants to play: https://github.com/mjc-506/st1300braketoy
 
mjc506 said:
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.
Click to expand...

I’ll say everything you’ve said is spot on except this statement.
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).

Along the lines of your statement I’ll make a less studied supposition that maximum front braking can be achieved with less pressure on the front brake lever when the rear brake pedal has pressure on it.
 
sirbike said:
proving that maximum front braking has been achieved
Click to expand...
Minor correction, locking the front wheel doesn't imply maximum braking force. It implies brake force exceeds available friction from the road surface.
 
mjc506 said:
I've read somewhere that it's to minimise diving during low speed manoeuvring (makes sense I guess? Lots of light pressing on the rear brake, and don't want too much front at low speeds while turning...)

Edit: I should note that the 'real world' system is a lot 'softer' around the threshold pressure - compressability in the lines/seals/fluid etc makes it less "on/off" than this model shows. But I didn't want to start trying to model squishy lines :)
Click to expand...
I agree with the idea and my practical experience that light pedal pressure during slow speed maneuvers doesn’t apply much or any front brake. This allows the LBS to act like a non-linked system does when discrete rear brake is desired. There is also benefit in limiting pedal bias to rear brake only to settle the suspension when setting up corner entries and in the mid to late stages of trail braking as total brake pressure is tapered off.
 
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