Brake System

[vasily]

hey
I'm trying to choose the correct brake system for our car. I got all the equations down (I think), and an excel sheet with some results. One thing thats bothering me is should I consider braking loads in the turn. (I looked throught the forums but couldn't find anyhing related)
If we do, the load is a lot higher per wheel, and the pressures inside the lines are ridiculous.
Any thoughts?

[J. Vinella]

The question you need to ask yourself is:

Will the driver be using the brakes in turns?

Drivers sometimes trail-brake (ie "light"� braking during the turn). The real question is how much force is the driver putting into the pedal during trail-braking and is this condition important to analyze?

But what about extreme cases? Where braking, bump and cornering occur.

Answer: Yes I would analyze this case.

[vreihen]

Originally posted by vasily:

If we do, the load is a lot higher per wheel, and the pressures inside the lines are ridiculous.
Any thoughts?

Did you take the classic "friction circle" of the tire(s) into account when you came up with your ridiculous pressure calculations??? Just wondering.....

[terra_dactile]

Hi Vasily,

First of all what is your classification of a ridiculous pressure ? 700-900 psi at max deceleration is not that bad for the front of the car!

Is it possible that your line pressure is high because
A, your friction coefficient of the pads is very low,
B, The effective radius of the disks are to small or
C, the piston diameter on the calipers is very small I.E under 1.0 inche diameter.
D, This is a long shot, but that your only counting the force created by one of the two pads on the calipers, there fore pressure would be divided by two (sorry I had to ask when I first did the cacls that happened to me)

For the whole braking hard in the corner topic, i have trouble seeing how this would be usefull since, if you brake that hard on one wheel and you have no left right bias, then the other wheel inside of corner would be unloaded and with the same pressure each side would most likley lock and you would lose control of the car possibly,

anyways the comment about the friction circle is also valid,

my biggest concern would be insureing that the bias can be set so that the rear does not lock before the front and cause for unstable vehcile dynamics ( this really discourages drivers to brake hard)

hope some of my comment have helped to see the bigger picture,

Jude Berthault
ETS FSAE 2003- Current
Vehicle Dynamics Leader

[Ian McMurdo]

Can someone explain to me what the 'friction circle' refers to?

[vreihen]

Originally posted by Ian McMurdo:

Can someone explain to me what the 'friction circle' refers to?

Sure. The tire can only generate so much grip. Braking in a straight line, you get 100% of that grip. As soon as you start to turn, some of that grip is being used to turn the vehicle, leaving less grip for braking. If you plot the whole relationship on a graph, it comes out looking like an ellipse or circle, hence the name.

If you are doing a "worst case" calculation with 100% of the vehicle's mass riding on and being braked by one wheel, the friction circle is the limiting factor preventing you from ever hitting that worst case while turning. You'll lose traction before you ever get to that worst case.....

[Tim.Wright]

I assume your line of thinking is; during a turn, the outside wheel is more heavily loaded and therefore can take more braking force?

Since you (typically) don't alter left right brake bias on the fly, wouldn't this extra pressure lock the inside wheel?

I agree with the friction circle idea but additionally, tyre load sensitivity would result in a net LOSS of braking force. Basically this is where the inside wheel loses more tractive grip than the outside gains due to the shift in weight distribution left-right, and the non-linearity of the coefficient of friction of your tyre. I'm not explaining this very well but Claude illustrates this very well in his seminars.

I know this (tyre load sensitivity) occurs for lateral forces on a tyre, but I only assume it works with tractive/braking forces as well. Can anyone confirm this?

Tim

[D J Yates]

Originally posted by Timo:
I agree with the friction circle idea but additionally, tyre load sensitivity would result in a net LOSS of braking force. Basically this is where the inside wheel loses more tractive grip than the outside gains due to the shift in weight distribution left-right, and the non-linearity of the coefficient of friction of your tyre. I'm not explaining this very well but Claude illustrates this very well in his seminars.

I know this (tyre load sensitivity) occurs for lateral forces on a tyre, but I only assume it works with tractive/braking forces as well. Can anyone confirm this?
Tim

I think that load sensitivity is non-linear, and for a lightly loaded car is less significant than a heavier car. Consider that FSAE cars with 20" tyres see perhaps 1/3 of the load of the same tyres on a 'proper' Formula car with downforce, i expect that you'll not notice load sensitiviy too much.

With regard to braking. The limiting case is more how hard you're driver will stamp on the pedal when he gets scared, the loads on the system will easily be greater than those required to lock the wheels. I'd also bet that in most cars, the pedals fail before the brakes do. I'm sure one driver managed to push the whole pedal assembly through the front of the car trying to pass the brake test at FStudent in 04.

[vasily]

thank you, for some dumb reason I didn't think about the car loosing grip while braking in a turn too much.
Perhaps if I exaplined a situation a little better it would be easier to understand. Since we decided to use 10" wheels (for the last time in this college), its a big packaging prob.
The max I can use for the front is a 7" rotor, and the only caliper that fits has a 1.12" piston. You will laugh at the pressure, but its close to 1500 psi. I did find a wilwood Billet Spot Caliper, that has a 1.75" piston and should the pressure down to 700psi, but it has a weird mounting.
Also, is it possible for anyone to take a look at my excel sheet? Perhaps I made a mistake while making it.
As for the friction circle, is that something that is usually provided by the manufacturers?

[vasily]

by weird mounting of the brake, I mean that it should be mounted with a craw foot type mount, going over the caliper and onto the other side. And I can't judge by the picture if I'll have enough space. The link for the caliper is below

http://www.wilwood.com/Products/001-alipers/023-SFM/index.asp (http://www.wilwood.com/Products/001-Calipers/023-SFM/index.asp)

[vreihen]

Originally posted by vasily:

thank you, for some dumb reason I didn't think about the car loosing grip while braking in a turn too much.

Hence the reason why ABS has become so popular. http://fsae.com/groupee_common/emoticons/icon_smile.gif


As for the friction circle, is that something that is usually provided by the manufacturers?

Since it really depends on surface conditions, tire/suspension geometry, loading, etc, this is more of a theoretical concept for your vehicle...as well as a lesson taught very early in most driving schools for why you should never brake in a turn.

Here's a link to a complete explanation with illustrations of the concept, followed by a real-life illustration captured by an accelerometer-based data logging program during an autocross run:
http://www.joe250.com/motorcycles/ride/friction.htm
http://www.napylon.com/short1099.htm

[rjwoods77]

Vasily,

I am so tired of people saying packaging a ten is hard. Its not. We are using wilwoods gp320 calipers with a 7.25" rotor in a ten inch rim and they have 4x1.25" pistons. I would have used a dynalight single but the mout ears hit the upright center section but that is becasue I am using a large diameter bearing. What I am saying is is that you can fit something with a 2x1.75" piston caliper with a 7" rotor in a ten inch rim without any comprimises in design and we have a 8" center to center on the ball joints with zero KPI. This will work out roughly to be 800psi max pressure on the front brakes to lock them up with .625 master cylinders which isnt that much.
If you need more brake than that then you are making your can way to heavy. There are monster advantages is packaging and weight by going to a 10 inch rim. Jumping up to a 13" because you cant package just means you are doing it right.

[flavorPacket]

ok rob, but what's your scrub radius?

[rjwoods77]

FlavorPacket,

1 inch right now. It all really depends on how much steering angle we will really need. We are gunna try it tight first(limit 25 degrees steering angle) and if it is too tight then make a new stub axle(live spindle) and bring it out. The arms are shaped for rim clearance on steering if that is what you are getting at but that is the tradeoff for it. I figured that was a decent tradeoff for a 10" rim versus having 25 bucks less a tire and all the extra weight,etc. Obviously everything is a tradeoff but i really think people have this "its pretty tight so I am not going to bother" attitude about it. If you look back about a year ago all you hear is that 10" rims arent worth it and there are too many comprimises. I guess that is why RMIT's car suck so much ass.

[Welfares]

Yeah, that RMIT car, what a heap of crap :P

We set up a track at oran park Yesterday as close in style to the Melbourne track as we could get, Our best kart driver was about a second quicker in a J over 40seconds than our best car driver in the car.

This seems to point the way towards those smaller cars.

[Homemade WRX]

Originally posted by Rob Woods:
Vasily,

I am so tired of people saying packaging a ten is hard.
agreed 100%, we've run bigger and smaller. The bigger being alum and this year are going with 6" rotors...should be interesting to see how they handle the heat.

[rjwoods77]

Vasily,

On a serious note I can help you by showing you a couple jpegs of how to fit a caliper into a rim without modelling. In solidworks you can import jpegs into a part file. The jpeg would be like wilwood drawing for a caliper for instance. You scale the jpeg until you can draw a circle or a line to overlay a known dimension on the jpeg. Line weights make this accurate to about plus/minus 1/32" which is more than accurate enough for initial size up purposes. Wilwood and others show where the top of the disc is supposed to be on the friction material which locates the center of your rim. Pop a couple circles on there to represent your rim ID(assume for smallest ID just before taper at rim half for clearance , i.e 9.5" ID) and you will see what contacts. You will find that the points that contact the rim are the outer most edges of the caliper or to you upright the underside of the caliper. Works real well. Jpeg scaling is a really neat method to determine if something is even in the ballpark. Youll find you can get up to about a 7.25 to 7.375 inch rotor with a dynalite single or gp320.

Homemade,

Let me know how that works because I was originally going to do an inboard disc(between a arms) with a 6" rotor but I didnt like the clamping torque for the calipers and pad compoundsI found to fit that size rotor and also the volume of material as well.

And I was thinking this would be a neat idea if a 6 inch worked well. With my 8" balljoint and zero KPI a 6 inch rotor would actually fit on the KPI line. So I thought it would be neat to do a split half upright(ala Western Washington) that bolts together with a bearing on each side of the rotor with a live spindle. You could then have a caliper(custom made and even integral with upright half ala who ever did that int he past) that bolts onto each upright half eliminating the bending moment of the caliper. The brake feel would be a solid as you could possibly do. I would suggest the same design on a 13" rim but using a 8" rotor with each wheel having 2x1.75" calipers. You would have an ultr rigid braking package and your live spindle you be brutally stiff because you would mount the wheel center directly against the upright since the brake pakage doesnt get in the way anymore. The idea was too late to incorporate in the car this year but I was gunna leave as a thing to do for the new team. Would be sick to machine the caliper into the upright and use the bridge bolts on the caliper to actually clamp the upright together as well.

[billywight]

So I thought it would be neat to do a split half upright(ala Western Washington)

Is there a picture of this upright anywhere?

[rjwoods77]

If the rotor can fit between the balljoints why not do it. Rigid caliper mounting if you made your own that is. Pretty neat idea I think.

http://fsae.com/eve/forums/a/tpc/f/125607348/m/87310573911/p/2

[kwancho]

OH. Those two halves bolt together? That's sweet! Does the caliper bolt into those two holes in the top? Where do the balljoints go?

[McGuyver]

Why not make the rims to accept buttons for mounting a rotor? The caliper would then sit inside the rotor.

[Travis Garrison]

http://dot.etec.wwu.edu/fsae/v28/image005.jpg

Like that?

It can and has been done, but what started as a way to fit larger rotors in a cluttered wheel, ended up making things even more crowded. If you go that route get some seriously deep wheels (massive offset) and push the rotor as far in as you can.

Other issues would be, getting the rotor to run true inside a wheel...and what I believe was Pat Clarke's pet peave keeping the heat out of your wheel...lastly don't bank on your wheels holding air ever again.

The uprights are bonded toghether (not the particular ones shown above). Works well enough and lets you make some trick hollow aluminum shapes.

[McGuyver]

That is similar. Check out Buell sportbike front brakes. The rotors float on the buttons. Matching Nissin motorcycle calipers appear to be more compact than the single piston caliper in the picture. Since front motorcycle brakes typically operate on hand pressure, the pedal ratio could be reduced to provide a very short travel with acceptable pressure.

[NetKev92]

As for the friction circle, is that something that is usually provided by the manufacturers?

It's more one of those intro things on tires in basic texts. You can look at it like the tire has a certain maximum amount of grip available with a certain weight on that wheel. You can point that available grip in any direction you want in order to change the car's speed and direction, but you can't get more grip than what the tire gives you.

If you imagine that you are using all the tire's grip for cornering force at the limit, you have zero grip left to apply to braking. If you touch the brakes at all, the momentum forces of the car will overcome the available grip of the tire and you'll slide. If you're only using 71% of your maximum grip in the lateral direction for cornering though, you can apply the same amount of force in braking and you will then be using 100% of your total grip in a vector direction to the rear but 45 degrees off of the car's nose. In otherwords the hypotenuse is your 100% of grip and you can use the components as you wish.

[Travis Garrison]

Sorry about the thread hijacking...

McGuyver,

The lack of buttons is due to the lack of space, motorcycles have room for huge rotors and can afford to give up an inch for buttons...take a closer look at the picture the caliper (two piston not one) is nearly rubbing the inside of the rim. This is a case where adding buttons reduces your rotor diameter which in turn means you'll need more force pedal force and defeats the original purpose of the inside out brakes (max diameter in min space).

Look at it like this if the buttons are wider than the outer section of a typical brake caliper (pretty sure off the shelf ones are) then you haven't gained anything in terms of rotor diameter.

The buel is apples to oranges...for starters I'm guessing they did it mostly to be different. Also for them keeping the caliper closer to the center of the wheel should make for a stiffer mount...us, we can't push our calipers out far enough for that to be the primary concern, certainly not with 10's.

[PatClarke]

Quote Travis..."and what I believe was Pat Clarke's pet peave keeping the heat out of your wheel..."

My pet peeve?? Uh, the only time i ever mentioned this was in a list of criticisms of the WWU V8 car from a few years ago. One of the points I raised was welding the rotor to the inside of the rim, and the problems with doing so.

It would be extremely difficult to keep such a rotor true to the caliper. It would make changing the wheel a complicated job. It would make keeping the rim 'true' difficult (hence the comment about 'holding air')..and finally, I mentioned the complication of feeding brake temperature directly into the inflation air inside the tyre.

There are reasons why you would do this...It is a good argument for outboard brakes to get tyres up to temperature quickly after a period of yellow flag running in circuit racing.

The Buell system of carrying a large diameter rotor on buttons can work for two reasons. Firstly, the floating mounting can permit the rotor to align and insulates the rim from brake heat. The large open air rotor can shed heat to the airstream pretty easily, not the case with a small brake shrouded inside a small wheel.

I have noticed a trend lately to misinterpret my style of Design Judging. I play Devils Advocate on design areas of a car, asking the team why they didn't do it some other way. This gets interpreted as meaning 'I don't like the way you did that" When I really mean "Why did you do it that way"? Through the week I became aware of a team briefing paper that stated something like "Pat doesn't seem to like Torsens, and expresses good reasons why" When, in truth, I think the Torsen is a great device, as long as it is employed correctly.

You will find that more and more Design Judges are employing the same strategy as I do. This is because we are seeing more and more "legacy" cars, and we have difficulty disagreeing with most of the design decisions made. What we are trying to determine is that the team presenting the car actually know why these decisions were made, not some young engineers who graduated and left some time ago, leaving the design legacy.

Regards to all
Pat

[markocosic]

Originally posted by PatClarke:
Through the week I became aware of a team briefing paper

Not quite - though I reckon the author would take the 'collated thoughts to self whilst musing over the feasibility of starting an FSAE team' being misinterpreted as a 'team briefing' as a compliment http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Travis Garrison]

Ah..appears I/we misunderstood. That's what I get for name dropping eh? http://fsae.com/groupee_common/emoticons/icon_wink.gif

You're spot on with wheel change issues. It's not impossible but it's not as easy as it should be.

Getting them to run true requires some fancy lathe work. And getting them to seal properly, to the best of my knowledge has never been done.

I guess my point is they can work but they end up being a pain in the @$$ Buttons would be nice, but they end up being counter productive in this case, if you're going to cut your diameter down that much you might as well go back to conventional brakes.

[Charlie]

Originally posted by PatClarke:
I have noticed a trend lately to misinterpret my style of Design Judging. I play Devils Advocate on design areas of a car, asking the team why they didn't do it some other way. This gets interpreted as meaning 'I don't like the way you did that" When I really mean "Why did you do it that way"? Through the week I became aware of a team briefing paper that stated something like "Pat doesn't seem to like Torsens, and expresses good reasons why" When, in truth, I think the Torsen is a great device, as long as it is employed correctly.

Pat

Pat, by the way you responded to that post, I see that you don't like people posting on message boards. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[PatClarke]

Charlie, Charlie,
Please don't take away one of my last sources of amusement in my geriatric years =] http://fsae.com/groupee_common/emoticons/icon_biggrin.gif
Pat