Servo assisted steering and brakes - why?

[Randy V]

It just occured to me that I have seen servo assisted (power) steering and brakes on most of the cars built for use in the UK. Other than a possible requirement of European countries for registration - I have no idea why?
These cars are so light, they should not require either to maintain control easily.

So I have shown my ignorance here.. Please school me!

 

[Richard]

Slow speed handeling. parking. Fat tires, big contact patch equals higher friction only noticed at slow speeds.

 

[Tony Hunt]

It just occurred to me that I have seen servo assisted (power) steering and brakes on most of the cars built for use in the UK. Other than a possible requirement of European countries for registration - I have no idea why?
These cars are so light, they should not require either to maintain control easily.

So I have shown my ignorance here.. Please school me!

Randy,
Firstly no UK cars, to my knowledge, have ever run power steering. Yes, the steering is heavy at low speeds because of the wide section tyres, but it lightens up and at normal running speeds is light and responsive.
Some UK cars run servo-operated brakes. My GTD had them when I bought it, but when I uprated to 6-pot Wilwoods at the front, I did away with servos back and front as there was enough slave cylinder area not to need them.
The servos weren't very effective anyway, because of lack of suck from the Holley, and were unreliable and kept sticking on.
Tony

 

[Phil]

Randy:
I have had cars with and without boosters, personally I prefer no booster but I think with the advent of disc brakes here in the US it was more of a selling point and also it made possible a very "hard" pedal, by that I mean it was right at the top.
Most manual master cylinders are 3/4 to 1" and much more than that it is difficult to stop the vehicle.
Going from a 3/4 to a 7/8 master drops line pressure about 25% so imagine trying to stop a large vehicle such as some of these over bloated SUV's with a 1 1/8 or 1 1/4" master which is what most have in front of the booster.
Women in heels don't appreciate a pedal with travel, it feels right to have the brakes right there when you step and thats what the larger master does by virtue of much more fluid movement...it just needs a lot more input pressure to make the brakes work and that's where the booster comes in.
Try shutting off your car on a back road and stopping it after the vacuum bleeds off, its like standing on a solid block and there is minimal braking.
Most of the new master cylinders actually have a third piston right in front of the booster that only travels about 1/4" but takes up the play at the calipers on initial input and gives that solid feeling in the pedal.
A good matched setup without servos works just fine.
As far as the steering, I might go with the power assist, but not overly powerful, just enough to make the car manageable and reduce some of the input effort.
Cheers
Phil

 

[40 Bud]

I think that the GTD cars went to servo brakes because they had a very poor pedal ratio ( only about 3.5 to 1). Fix that and you probably will not need servo assist.

 

[Randy V]

Hi guys,

Okay - so then it's apparent that there is no requirement or either servo assisted steerin or brakes.. Good!

My cobra carried a lot of positive caster and weighed 2600# loaded with fuel and me. I had 275 series tires on it and did not need any sort of assistance in either category.
Of course, the pedal ratio and MC/Caliper piston sizes make a large difference as does the compound of the friction materials in the pads..

 

[Dylan]

I've been just doing some brake research today on this very subject, coincidentally. The Tornado pedal box gives ~4:1 ratio. There's a significant packaging problem if you want to get more ratio than that and still maintain a good pedal location.

 

[Cliff Beer]

Servo assist on the brakes is nice, and helpful - gives the pedal an easier "feel" which is good for proper modulation. It's also not complicated and easy to install.

Power steering, on the other hand, is cumbersome and complex to engineer and install...relatively speaking. It shouldn't be necessary at all on a GT40. The front end of a GT40 is relatively light. Even with wide tires it's not difficult to turn the wheel for parking.

 

[Jim Cowden]

Electric power assistance takes the mass of components out of the system.
Afew guys here in OZ have already done it.
I personally am considering it as I run 6.5 caster and it is a handfull, I have to grab a hand full of wheel through sweepers when I have to shift mid corner, It doesnt worry me but it can make your hand work a bit messy.

Brakes, I run no booster but I did find throwing the calculator away made it easier to size master cylinders.
I just kept going smaller till I got a pedal that was part way between a boosted and non boosted system.
I currently have 5/8 front and rear and I balanced of with the bar, pedal ratio is 5:1.
It stops well and it allows me to modulate.
I find a high hard pedal tires you out.
My 2c worth.

Jim

 

[Dylan]

Jim, 5:1 seems pretty small ratio for manual brakes. How bad is the pedal throw like that?

I'm in the middle of sketching up an adjustable pedal box for mine. None of the wilwood/tilton hanging assemblies were close enough...either put the masters through the top paneling or put the pedals way down towards the floor.

 

[Jim Cowden]

Dylan
If anything they are a tad long I found getting anf more than 5:1 was impractical, as you are finding getting more ratio into the design and space is difficult.

The throw is good the pedal moves down about 50mm and has a nice feel.
When I had a 3/4 rear cyl it felt like I had a brick under the pedal.
I have size 11 feet to match the size of my nose and hands but when my wife drives it I have a false floor that sits in front of the pedals to bring her feet up to the pads.

At the track not many can outbrake me, to be honest I stay off the back of other cars as I have got into trouble by getting under the rear bars.

I have made a few sets with about 4:1 and 2 sets at 3.8:1 due to space, I never got feed back but I think they would need to run small cyl bores to overcome large pedal pressure.
I have had a few race cars over the years and the worst thing is high pedal pressure required to slow the car, over time it just wears you down and It gives a lack of confidence.+

As we all know caliper piston size plays into this I have twin 41 mm pistons on each front caliper and twin 38 on each rear caliper if people like running numbers, I just changed cylinders till I got what felt right as I have a few in stock.
Always use tilton short bore masters because if one fails it will bottom out earlier and allow the other curcuit to work before you run out of pedal travel.

I appoligize if this is getting off the subject or hijacking

Attached a pic

Jim

 

[Cliff Beer]

Dylan

I have had a few race cars over the years and the worst thing is high pedal pressure required to slow the car, over time it just wears you down and It gives a lack of confidence.+

Attached a pic

Jim

Jim, I would think some boosting off the vacuum of the manifold would help you with this. That is, after all, the very point of power assist on the brakes.

As you rightly point out, maintaining the right ratio of bore sizing (MC v. caliper) is important, and a good way to get the right pedal feel for proper modulation...better than pedal ratio in my experience, although pedal ratio is important too. Just my $.02.

 

[Randy V]

Here are some good links that talk about pedal ratios. I ran a 6.5:1 ratio with my Cobra race car and it worked very well..

Wilwood High Performance Disc Brakes

While the link below looks like the link above (for some unknown reason) it is not.. The one below has some very good illustrations that are very worthy of your time..

Wilwood High Performance Disc Brakes

I have always relied on Wilwood and Tilton for the straight scoop on braking

 

[EGLITOM]

STeering on my RCR does not need any servo support in slow and fast speeds. One can park this car onehanded. It is not the quickest steering, but perfect in high speed runs and corners ( did a 290km/h run last sunday with one hand on the steering wheel and one hand on the shifter, it is as easy as running my Audi at 240km/h, just runs deadstraight, doesn´t get light at all)

i think 5:1 Ratio is what the RCR pedal ratio is and its good. I could go even smaller with the masters to get a more softer feel. Travel is 50 - 70 mm till full block currently.

I had once posted a excel sheet with all that calculations and resulting pedal pressure. This chart which i had adapted form someone else is quite exact on the spot.
TOM

 

[David]

Well Ford thought the GT40 needed servos as a road car.... Or at least one MkIII did anyway (there were so few and they vary in spec so I don't know if it was a one off request or not).