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Ditching the brake servo
Jesus-Ninja - 29/9/08 at 10:26 PM

I'm building a spaceframe to take the running gear from my S14 (see avatar). I'v ecome to fit the pedals, which I'm also taking from the S14. As standard it uses a vacuum assisted servo, which I'd like to ditch.

I've looked in the haynes book for what they do. Although there's no mention of "get rid of the servo, you won't need this", I know that the locost has no servo, and I'm pretty sure that Sierras do.

The question is, will I be OK doing the same with my Nissan bits? I was going to reuse the pedals, but do I need to do some clever thinking about the leverage the pedal has on the BMC?

blakep82 - 29/9/08 at 11:24 PM

hmm, its a good question. i think the best thing to do is try it, and see how it is. is the S14 still drivable? can you disable the servo to see what thats like without it?

bear in mind the spaceframe will probably be a lot lighter.
i should imagine my pickup will weigh about the same (if not slightly more. think the racing ones are balasted to about 865kg) as your car, uses 0.70 master cylinder on the front, 0.75 for the rear,

not sure on the calipers, 1.75" front and rear i think (certainly wilwood dynalite on the rear, superlite on the front)
310mm discs on the front, 243mm rear.

i'll measure the pedal ratio tomorrow for you if you like.

they don't use servos

hope this helps a little in working things out

[Edited on 29/9/08 by blakep82]

JAG - 30/9/08 at 07:13 AM

You can disable the servo (for a quick trial) by simply pulling off the vacuum hose. Just push a suitable bolt in the end if you do as you don't want any dirt getting inside the servo.

I've built a Locost WITH a vacuum servo and it works great. Just choose the right servo for the job. Mine came from a Nissan Micra and I chose it because it has a low boost ratio (3:1) but a big m/cyl' bore.

Jesus-Ninja - 30/9/08 at 07:43 AM

Thanks guys. The S14 has been a shell for some time, and was removed, somewhat spectacularly, at the weekend, so I'm a little in the dark now!

I think maybe I'll try to use the servo. Trouble is, it's a bit of a squeeze getting it in.

The alternative, is I look iunto buying a prebuilt pedal box which includes all the brake and clutch master cylinders, but that kind of defeats my original objective of spaceframing my current car (although I kind of blew that out of the water when I decided to put the engine in the boot...)

loggyboy - 30/9/08 at 10:14 AM

Overal weight of the car would be the key consideration, cars like the elise dont have servos (and old minis), and allthough my Nova has a servo the vacuum hose is not connected. Ive never had any issues. But those cars are all sub 8-900KG.

Jesus-Ninja - 30/9/08 at 10:52 AM

The car will be (hopefully!) under 800kgs

JAG - 30/9/08 at 11:01 AM

My Locost was 565kg at SVA - it still has a full servo and a nice, big diameter, master cylinder.

My brakes work fantastically well and feel very good. Also pedal efforts are very similar to a modern saloon - so no nasty surprise when going from the everyday car to the Locost.

I have a short pedal travel and low pedal effort.

contaminated - 30/9/08 at 01:02 PM

My Six has a servo, which I've just replaced actually. I did toy with the idea of binning the whole setup and putting in a new pedal box with twin cylinders. Couldn't justify the cost when compared to just getting a clevis pin welded on a new sierra servo I exchanged on here for some stone guards!

By the way, in my case if the servo is present but either not woking properly (a leak) or the vacuum hose is off, the brakes are horrible. It's not that they are too hard to depress, it just that not a lot happens when you do.

Ninehigh - 30/9/08 at 06:52 PM

Isn't the servo the bit that transfers pedal pressure into brake pressure? Thus without a servo braking is more akin to pulling the handles on a pushbike rather than the brakes on my mondeo?

Sorry it's not sounding like a good idea to me :-( but then I really don't know, I'd like to find out though :-)

Jesus-Ninja - 30/9/08 at 09:15 PM

quote:
Originally posted by Ninehigh
Isn't the servo the bit that transfers pedal pressure into brake pressure? Thus without a servo braking is more akin to pulling the handles on a pushbike rather than the brakes on my mondeo?

Sorry it's not sounding like a good idea to me :-( but then I really don't know, I'd like to find out though :-)


Well, that's the question isn't it? Is it a good idea?

But then this is, to be fair, the locost builders forum, inspired largely by the build your own sports car on a budget book, which doesn't use the servo.

loggyboy - 1/10/08 at 09:30 AM

a servo simply helps you apply pressure to the brakes using the vacuum created by the engine.
Im sure everyones rolled a car forward and then tried to brake and realised they need to press the pedal harder to stop the car.
But although it seems like your brakes arent working as well, thats not true, its just that you need to apply more pressure to get the same effect as when you do have the servo. So as long as your used to the brakes then theres no problem.

Jesus-Ninja - 2/10/08 at 12:04 PM

Right I have decided... I think

I'm going to keep the servo. Ditching it is an unknown quantity given the calipers and BMC and their lack of previous use by anyone in a similar set up.

Space iss a bit of an issue, even though I'm mid engined.

I think I'm going to do one of two things - offset the Servo from the pdeal by 400mm to the left (basically the centre of the car in front of the tunnel) and use a crank. The crank will be under great torsion though, and may effect brake feel.

As a better option, I'm considering putting the servo in the centre, as above, but backward - ie with the BMC pointing toward the rear. A bellcrank can then be used to translate the pedal movement into the servo.