+ Reply to Thread
Page 2 of 4 FirstFirst 1 2 3 4 LastLast
Results 11 to 20 of 32

Thread: Braking System Design questions

  1. #11
    MCoach,

    The only issue with this caliper design, is that the only pad available for the PS-1 is Sintered Metallic (SM), which the temperature-mu curve falls under the TBD category. The only information available is :
    • Formulation for Power Sports and Industrial applications. High friction
    • excellent cold torque response, medium temperature range
    And then they say :
    • Medium friction compound with very little change in friction throughout its temperature range
    • Good wear and friction properties with a moderate tempearature range.
    It seems it is "High" and "Medium" friction at the same time? Anyway, at least it seems the mu is pretty constant with temperature, which should simplify things not having the data. Then the only question will be by how much your pedal force / ratio is off in your design.

    It is a design pad for steel rotors, so I would guess it more in the upper temperature range compared to the pads designed for aluminum. They say Medium temperature range, so probably 500F to 1000F. In this range, their Medium-high friction pads fall into the .55 to 0.65 CoF. But all those pads are Race Only, the SM is street use ok. The guess would be somewhere in between 0.45 and 0.55 CoF. But this is all pure speculation! So in all the cases, the design should work to lock up the wheels with the lowest Mu at your highest «operating» driver force.

    If you absolutely want data for the pads, you will need to machine other pads to fit the caliper, bit of an hassle for nothing I think.

    Make sure that your brake balance adjustement range at your operating conditions covers your incertitude range on the pads CoF, and then test, and retest!
    :::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::
    2007-2012 - Suspension, chassis, and stuff (mostly stuff)
    Université de Sherbrooke

  2. #12
    Bastards. That data would be useful!

    We used to run the purple pads coupled with our infamous aluminum brake rotors. There isn't any data for those but our mu estimate was .47. Yeah, that could be a slight problem. Maybe someone on here who's run the Wilwood PS-1 can chime in for us.

    In relation to the bias bar comment, what type of master cylinders do you run? What I've done recently is use the push type bearing supported master cylinders that allow me to design variable pedal ratios into it's mounting bracket. I can estimate a range of the overall gain I'm aiming for and then dial it in that way.

  3. #13
    About the PS-1 pads; I have an email from a rep at Willwood stating "The Sintered Metallic pad has a 0.60 CF at 200 degrees Fahrenheit" after asking about friction properties. I took this to assume that they have some pretty good bite at low temps, but are susceptible to fade when they're worked hard. Similar to typical "road car" pads, which is not surprising given that they are labeled as industrial and not race pads.

    Regarding caliper/mc pressure, all the components will be rated and tested at a higher pressure than stated on the spec sheet (safety, quality, etc). But, the component in question (calipers especially) were DESIGNED to operate within a pressure range. If you run a caliper rated for 800 psi at 1200, it might not fail right away or ever, but how much unwanted flex do you think might show up and give you a spongy pedal anyways? I'd be willing to bet it is significant.
    Owen Thomas
    University of Calgary FSAE, Schulich Racing

  4. #14
    Owen, are you a user of the PS-1 calipers? Any concerns about using them relating to OP?

  5. #15
    Quote Originally Posted by MCoach View Post
    1500psi is on the high end of what we've seen for normal braking. Are you exceeding that on normal braking calcs or the panic braking calcs?

    We've assumed a tire mu, but I curious what you are using for a brake pad mu?
    It's nearly impossible to come up with that value unless you have a reference temperature-pad mu curve, or you can estimate an operating value.
    I'm also curious what pad material and rotor size you've chosen. It'll tell me a lot about the rest of the system. If your hydraulic (MC:Caliper) and mechanical (pedal ratio) ratios are really low, there may be an error in your calcs.

    Seals are a big thing, bore integrity is another one, but I feel more comfortable pushing the safety factor on parts that I know are replaceable. The caliper housing itself will take literally thousands of psi, but the seals and pistons may be the weak point. Good thing those parts are replaceable.
    MCoach,

    I did make a mistake in my calc. My equations were in fact over predicting the torque the brake system would have to provide in order to have the car brake at 1.5 g's. With the mistake corrected, the line pressures are just as you said earlier not close to the 1200 psi limits.
    I wish I had seen this mistake much earlier this semester.

    I was using a brake pad mu of 0.6.
    Last edited by pereira1; 09-30-2013 at 04:35 PM.

  6. #16
    Excellent. Glad you found that.
    I'm currently chasing a similar thing on my end. Trying to figure out why my calcs predict some value and it takes 3/4 of that to actually lock up the tires....
    The hunt continues.

    0.6 seems reasonable considering the input from Owen.

  7. #17
    Senior Member
    Join Date
    Nov 2010
    Location
    NSW, Australia
    Posts
    352
    MCoach, just taking a stab in the dark but have you tried to physically lock the tyres from high speed with gradual brake application? Typically the lockup test is done with a very quickly applied stab, and I imagine this would be quite different from the tyres perspective than a gradual application (someone can probably explain why a lot better than I can. In fact I hope someone does; would certainly explain why the correct braking technique works better). Other possible causes could be the interaction between pad and rotor, especially if you have drilled/grooved rotors and non-tapered pads, but once again I'm just guessing.
    Jay

    UoW FSAE '07-'09

  8. #18
    @Jay

    The difference in needed locking force between a very fast or slower brake application reside in part in the dynamic behavior on the tire. Look at this paper: http://soliton.ae.gatech.edu/labs/pt...pers/vsd02.pdf they model the tire longitudinally considering the tire relaxation and then make a ABS model with it. I do believe that FSAE tires exhibits similar behavior, excepts that the relaxation length would be shorter, I have no data on this whatsoever.

    My simplified interpretation is that, when applying the brake as a dirac, the slip ratio will augment rapidly since the tire forces are smaller than in its steady state behavior. By the time the tire reaches it's steady force, the slip ratio is already higher than the peak slip ratio of the tire. Therefore, the needed torque to overgo the peak is lower. There will also be temperature related effects of both your tire and your brake rotor/pad that will affect there CoF, in this case I have no data on this, so I have no idea how much of role it plays in this.

    Interesting to note that in the LuGre model (which seems to have a good correlation with dynamic experimentation from what I have read), the peak friction diminishes with speed. The faster to go, the less brake torque you need to lock. It is also another reason why the acceleration event is won in the first few meters!


    @Mcoach

    How did you measured your locking force?
    :::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::
    2007-2012 - Suspension, chassis, and stuff (mostly stuff)
    Université de Sherbrooke

  9. #19
    Yeah, we were finally getting data from that a few weeks ago. Rotors have a smooth surface, no slots or holes. Max decel testing from ~60mph with some runs of gradual lock up. Everything seems to line up except line pressure. My guess is underestimating efficiency of the system. We used to use really, really poor MCs. I apply an efficiency factor (F_w, Force_wasted) for compliance in the system because it used to be that big of a deal. The other culprit could be underestimating the brake pad friction. There could also be a bubble in the line where I my sensor is located...

    We're running more testing this week, so I won't have time to chase it for a bit. Need to look over the data as we go to keep up.

  10. #20
    Senior Member
    Join Date
    Nov 2010
    Location
    NSW, Australia
    Posts
    352
    Francis,

    Thanks very much for that, I was thinking along those lines also. The other thing I considered was that perhaps a step input into the system would cause the dampers to operate in a possibly less controlled zone (somewhere between high speed damping and low speed damping) thus reducing contact patch control.
    Jay

    UoW FSAE '07-'09

+ Reply to Thread
Page 2 of 4 FirstFirst 1 2 3 4 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts