Proportional Valves

[autogyro]

It will if you make the bar driver adjustable.
How?

[Tim.Wright]

Use a threaded bias bar and one of these style remote adjusters.

Tim

[autogyro]

Use a threaded bias bar and one of these style remote adjusters.

Tim

Thanks Tim point taken.
However I do not see a problem with adjusting the brake 'pressure' bias.

[Tim.Wright]

This is where my brakes knowledge starts to run dry. Theoretically an adjustable proportioning valve will do the same as a bias bar but I've "heard" from a few people that they are not really the ideal way to go. Not being a brakes guy, I never bothered to investigate why so I cant really back up that statement.

I'm against the use of a static (non-adjustable) valve being used on a FStudent racecar because there are so many other parameters you can adjust to get the balance, why put in an extra component?

But in Gerard's case, since a lot of the system has been decided for him, he might need a valve AND a bias bar.

Tim

[gator]

I'll try my best to explain a little better without making this too complicated and having to post a bunch of graphs, plus I wouldn't want to take away too much from the O.P.'s design process fun.

If you were to consider a whole list of braking "things" you could plot a graph with two lines. The first of which would be curved, lets call it the "ideal" brake pressure during decel. But in racing, the presure input of the driver into the system results in a linear line. Your job as an engineer is to make these lines intersect near the traction limit of the intended tires that will be used with the intended driver input needed to lock up the system.

When you use a prop. valve in the system, you are effectively putting a bend in the linear line on the graph. The linear line will start out normal, as if the vavle isn't there, then at the set pressure the pressure will increase less from the same driver input as before. This could potentially fix an "over biased" side of the car. Generally this will be the rear of the car if the same components are run on both ends of the car.

Let's examine racing in the rain or in the later stages of a tire stint during a race to compare the two methods of bias adjustment. These two cenarios have a traction difference in what is available to the tires under ideal conditions. At the start of the stint, or in the dry, the driver will want more than 50% of the bias dialed into the front of the car. The car trasnfers more weight, the tires are better, etc. As the traction goes away due to tire degredation, or rain, the car cannot brake as well and cannot transfer as much weight and the front will begin to lock up first, therefore the rear needs more bias.

If you were to adjust the ratio with a bias bar adjuster, you are able to better match the "new" tire traction limit with your linear line on your graph as you are taking away input from one end and putting it into the other end's system. If you were to use a prop. valve things would be different because the system without the prop. valve stays the same and you are only manipulating the other system. So while this might seem like an okay thing to do, you are actually losing performance at the intended driver input force to create lockup. So basically, if you have a stong leg then the prop. valve could work if you don't mind the lockup point moving around on you. (Which I personally don't have and I would mind.)

In the end, it comes down to good tire data and a good engineer with the proper components to make it all work well. For what SAE does, an in-cockpit adjustable bias bar/valve isn't needed, but a normal threaded/adjustable bias bar for fine tuning is a must. It might look good to judges to have the in-cockpit one, but the sessions are too short to do any real good other than during the endurance event, maybe.

[autogyro]

Thanks Gator, that was very informative.
The road Mini had over 70% of its weight on the front wheels and was fitted with a set brake pressure bias valve on the rear subframe.
This was to prevent rear wheel lock up, with the large weight transfer to the front under hard braking.
When we raced mini's with 9 inch wide slicks and far more power, braking
(and cornering) became an art.
Making the brake pressure bias valve adjustable helped a little but if I think back it was mainly to reduce the rear braking to almost zero because almost all the braking was done on the front.
What we should have done was to fit very small disks on the rear with front and rear seperate systems and a brake bias bar as you say.
The Escorts did but we still cornered faster with the Mini's.
It was the 30 or so horsepower less that let them pass on the straits.

[gator]

Thanks Gator, that was very informative.
The road Mini had over 70% of its weight on the front wheels and was fitted with a set brake pressure bias valve on the rear subframe.
This was to prevent rear wheel lock up, with the large weight transfer to the front under hard braking.
When we raced mini's with 9 inch wide slicks and far more power, braking
(and cornering) became an art.
Making the brake pressure bias valve adjustable helped a little but if I think back it was mainly to reduce the rear braking to almost zero because almost all the braking was done on the front.
What we should have done was to fit very small disks on the rear with front and rear seperate systems and a brake bias bar as you say.
The Escorts did but we still cornered faster with the Mini's.
It was the 30 or so horsepower less that let them pass on the straits.

Happy to help. I am not familiar with Minis, did yours have a brake booster?

And I am SHOCKED that you were racing with only one braking system. What would have/ could have happened if there was a leak and the pedal went soft? One leak and you would have lost ALL stopping power on ALL corners. One of the main reasons there are two brake systems on the car (Front/ Rear) is for safety. Lose one and you still have the other to help you safely pull off and get the car to a stop. Glad to hear you never had any issues with that.

[marcush.]

having no balance bar is not saying you run only one circuit.
In fact Minis had had a brake booster only in the front hydrailic circuit in the early days and had a tandem Mastercylinder available from 1970 as an option for MKIII (windup window internal hinges doors)..
What is really an issue with the mini was the matter that it had drums in the rear ..so these performed a lot different to discs ,and could be made to lock very easy but also would fade under heavy use .
The real issue with the brakes locked at the rears was more of a too sudden increase of pressure not the force itself.I experimented with some US made valves -delaying the pressure buildup wich let me use a lot more bias to the rear without scaring the --- out of my driver...

[autogyro]

No brake servo Gator lost to much feel on the pedal.
Cooper S hubs, disks and callipers.
The disks were very small.
Thanks marcush, I remember the options now, I think the dual cylinder had one reservoir. On one car I did away with rear brakes completely.
British Hot Rod racing on short stadium ovals mainly, although some grass track rally cross and circuits.
I had worse problems than just running out of brakes.
I lost the front left hand, outside wheel on Peterborough raceway 550 yard oval one weekend. I still got a third though having done two laps with no front wheel and no brakes driving the other wheel through a roller lock diff.
Rolled six times at Arena Essex on one occasion and finished with a track fence post next to me in the car after hitting the fence backwards at well over 80mph.
That loosened my teeth.
Dual braking systems? We old boys used to think they were for whimps. Just joking.
In budget saloons the regs prevented upgraded brakes and the Capri 3 liters we used ran out of brakes completely after three laps (they were only escort van brakes anyway). The capri drivers used to lean on the Rover 3500 cars that had sandwitch discs, to slow down going into corners. Caused lots of agro in the paddock. Great days.

[autogyro]

http://tinypic.com/usermedia.php?uo=OVJ ... mxoQ%3D%3D

Might interest you.
The yellow mini shown eventualy had 180 bhp.
The rear axle was interesting. I used the weight transfer to the front under braking to open up the suspension on a beam axle. The geometry rolled the axle through 90% giving rear toe out to increase turn in oversteer.
As the power was applied to exit corner, the weight went back, closed the rear suspension rolling the axle back 90& changing toe out into negative camber helping to straiten up the car and apply power earlier.
Might be interesting for some to try drawing it.
Worked a treat on ovals. Still would give very low lap times with the clutch flight AP clutchless automatic gearbox fitted. 5 gears on a short oval 5.3 ratio drops and diff, gave 0 to 90 in 4.7 seconds. Good enough? 1977 remember and BMW think they know about Minis. hahaha

[marcush.]

No brake servo Gator lost to much feel on the pedal.
Cooper S hubs, disks and callipers.
The disks were very small.
Thanks marcush, I remember the options now, I think the dual cylinder had one reservoir. On one car I did away with rear brakes completely.
British Hot Rod racing on short stadium ovals mainly, although some grass track rally cross and circuits.
I had worse problems than just running out of brakes.
I lost the front left hand, outside wheel on Peterborough raceway 550 yard oval one weekend. I still got a third though having done two laps with no front wheel and no brakes driving the other wheel through a roller lock diff.
Rolled six times at Arena Essex on one occasion and finished with a track fence post next to me in the car after hitting the fence backwards at well over 80mph.
That loosened my teeth.
Dual braking systems? We old boys used to think they were for whimps. Just joking.
In budget saloons the regs prevented upgraded brakes and the Capri 3 liters we used ran out of brakes completely after three laps (they were only escort van brakes anyway). The capri drivers used to lean on the Rover 3500 cars that had sandwitch discs, to slow down going into corners. Caused lots of agro in the paddock. Great days.

funny enough ,I know what you are writing about.Did a Smith inspired vent for the
front mini discs ,sort of aluminium can directing air from inside of the disc to the outside ,quite a tight fit in those 10inch rims ..but it worked quite well.
I did not bohter with the brakebooster ,but did stiffen the brakepedal/plus stretched metal to avoid slipping from the pedal,fitted ball bearings instead of the plastic bushes and build a proper pedalbox around it ,driver had to provide force and modulation then .
you really had to step on this ...

[autogyro]

Not Leon Smith?
I used to help David Vizard with the gearbox chapters for his books and some of the mini tech stuff. Did a lot for BL as well, we had a Leyland ST dealership.

Never had 10inch standard mini rims on our cars, 10inch racing slicks were very difficult to get hold of.
13 inch with 50 or 52mm side walls, Avon mostly. G54 compound and very low tyre pressures 15 to 18 ibs. I did have a demon set of Michelin wets made for us when I was testing michelin tyres across the Sahara on a turbocharged range rover. the desert michelins were XXYs.
Narrow sand tyres with side walls like steel.
I think Barry Lee used a similar tyre on the Dakar Metro XR4.

I would always laugh at minis with tint steering wheels. To drive a powerful mini you needed a big wheel with loads of leverage and a strong right arm to hold it into corners with power on. Just as you say about strong right leg for braking with no servo and little disks, although in my case left leg, because we used left foot braking as no clutch was needed after leaving the line.

[autogyro]

So sorry I am going way off thread again.
Proportional valves. Jury is still out I think.

[xxChrisxx]

How would you design a method of adjusting the front to rear braking bias?

Bias bar on the pedal and two master cylinders.

The sickening thing is, all the braking system components have been bought from previous years, over eager lecturers biting off more than they can chew, without understanding anything about braking. Our budget doesn't cover any new purchases in Braking System.

Very frustrating... seems like they've taken all of the actual engineering out of the project already.

Tim

Sorry Tim but a master cylinder bias bar will not give a method of adjusting brake bias on the move.

Twist cable connection to a threaded bar works a treat. It's the solution I use.
EDIT: Should have read the second page, Tom already said it (with a pic too)

However if the OP doesnt want to change the bias on the fly, a split system from a single master cylinder works, not ideal but it's cheap and simple.

[marcush.]

So sorry I am going way off thread again.
Proportional valves. Jury is still out I think.

+1 back on topic