James504 12 Posted March 18, 2011 Just have a few questions about 205 brake pedal boxes. I want to run a pedal box set up to alleviate my master cylinder issues. However I have no experience with them. 1. What are they like to drive with? Everyday? How much extra effort is required to bring the vehicle to a stop? 2. Anyone have experience with compbrake's one? Its nice and cheap, and no exchange required. 3. What m/c sizes should I be looking at? Im currently running 266mm 206gti front rotors and calipers, rear bx16v callipers and 266mm rotors. Ultimately I want to upgrade the front to willwood 4 pots, still with a 266mm rotor. Cheers, James. Share this post Link to post Share on other sites
Cameron 16 Posted March 18, 2011 The effort required depends on your master cylinder choice, smaller diameter = less effort but more pedal travel, larger diameter = more effort with less pedal travel. Not sure what size MC's people normally use though. You'll want to look into strengthening the bulkhead if you're fitting a servo-less pedal box, as the extra effort (which is quite a lot!) will flex the crap out of the bulkhead and make the pedal feel soft. Share this post Link to post Share on other sites
Rob Thomson 6 Posted March 18, 2011 Just have a few questions about 205 brake pedal boxes. I want to run a pedal box set up to alleviate my master cylinder issues. However I have no experience with them. 1. What are they like to drive with? Everyday? How much extra effort is required to bring the vehicle to a stop? 2. Anyone have experience with compbrake's one? Its nice and cheap, and no exchange required. 3. What m/c sizes should I be looking at? Im currently running 266mm 206gti front rotors and calipers, rear bx16v callipers and 266mm rotors. Ultimately I want to upgrade the front to willwood 4 pots, still with a 266mm rotor. Cheers, James. I fitted a TAS pedal box to my 309 years ago. I think TAS became Compbrake but could be wrong about that... The car had GTi-6 front brakes. 1. Bloody awful. It's fine on track but absolutely horrible on the road; the pedal effort needed are so high that you lose sensitivity and feel. 2. My TAS pedal box was poorly made. Instead of a proper spherical bearing it just had a tube welded to the pedal in which the bias bar slid. Horrible. It eventually failed too; some of the welds gave way under the force imposed by one of my weedy legs. Don't skimp on something like this. 3. I used 0.625" cylinders front and back, which were the smallest available at the time. I think you can get 0.5" cylinders now, and if I had my time again I'd definitely be interested in trying some. I don't think the Willwoods have an enormous piston area, so you're best off with the smallest cylinders you can find to reduce the forces involved. Having said all of that, I've driven a few Mk2 Escorts with bias boxes that have been totally different; much nicer to use with much less effort needed. I suspect a decent (PTS) box in a 205/309 would be much better than the one I had, so definitely avoid cheap. I didn't find bulkhead flex to be a problem; there really wasn't very much movement. Share this post Link to post Share on other sites
rallyeash 155 Posted March 18, 2011 (edited) don't bother, there generally s*it! ive sat in a few 205's with pedal box's and not liked it. never found anyone either who has stuck with them, me included. i bought a compbrake one, build quality is crap and didnt even fit in the hole without some trimming, that ended up on ebay. i then bought an opb floor mounted pedal box which is aparantly a bolt in job which uses the floor angle to mount, it doesn't, nothing works properly, clutch cable is a nightmare (unless you convert to hydraulic), pedals are way to close and at the wron angle and the steering wheel (even with a 90mm dish) and gear stick is to far away. im now running a standard gti pedal box with a 406 m/c with hispec 4 pots on 266mm calipers and a disc rear beam. much better if you do want to fit one generally the m/c choice seems to be .625 front and .7 rear Edited March 18, 2011 by rallyeash Share this post Link to post Share on other sites
Tom Fenton 1,536 Posted March 18, 2011 I have pedal boxes in both my 205's. I like them but they are an acquired taste. I am suprised at the comments regards no feel as I like the fact that the harder I press the harder I stop. Each to their own though, and I recommend having a drive in a car so fitted before you buy one for your own. Share this post Link to post Share on other sites
Cameron 16 Posted March 18, 2011 Yeah I don't get that either, it kinda follows that the harder you need to press the better the modulation and feel will be.. I can see it takes some getting used to though. Share this post Link to post Share on other sites
James504 12 Posted March 19, 2011 Thanks for the informative replies! I will steer clear of the compbrake item. I can understand the comment about being an acquired taste, I absolutely hate non pas 205's, while others swear by it. How I imagine a pedal box to feel like, is abit like a car with the vacuum hose to the booster disconnected. It may turn out to be my only solution. Share this post Link to post Share on other sites
Batfink 201 Posted March 19, 2011 don't bother, there generally s*it! ive sat in a few 205's with pedal box's and not liked it. never found anyone either who has stuck with them, me included. i bought a compbrake one, build quality is crap and didnt even fit in the hole without some trimming, that ended up on ebay. i then bought an opb floor mounted pedal box which is aparantly a bolt in job which uses the floor angle to mount, it doesn't, nothing works properly, clutch cable is a nightmare (unless you convert to hydraulic), pedals are way to close and at the wron angle and the steering wheel (even with a 90mm dish) and gear stick is to far away. im now running a standard gti pedal box with a 406 m/c with hispec 4 pots on 266mm calipers and a disc rear beam. much better if you do want to fit one generally the m/c choice seems to be .625 front and .7 rear Do you think the tilton pedal set would be better then for angle and foot size? I have to get an aftermarket pedal box as I have a hydraulic clutch, but my seat is also bolted to the floor so position might be a little superior?? I was planning on mounting the OBP pedal box on a moveable base so I can adust the pedal position for the driver. Kev Share this post Link to post Share on other sites
welshpug 1,646 Posted March 19, 2011 I've had a play in a rally car that had a TAS pedal box (going back a few years! it was built pre-compbrake) I really liked the feel of the pedal once I got used to it, as Tom said, lean harder and you just stop harder! Share this post Link to post Share on other sites
Rob Thomson 6 Posted March 21, 2011 I am suprised at the comments regards no feel as I like the fact that the harder I press the harder I stop. Yeah I don't get that either, it kinda follows that the harder you need to press the better the modulation and feel will be.. That's true up to a point, but when the effort required is extreme you lose the feel and ability to modulate. Don't get me wrong, I hate over-assisted brakes, and the other rally cars I've driven with bias boxes have been great... but not my 309 with TAS bias box! Share this post Link to post Share on other sites
Tom Fenton 1,536 Posted March 21, 2011 I'm wondering if you got supplied the wrong master cyl sizes Rob. When Phil and I first did our track car the pedal was like literally standing on a brick, rock solid and didn't budge. We swapped the master cyls round which made a massive improvement and it is still the same 6 years later. Share this post Link to post Share on other sites
Cameron 16 Posted March 21, 2011 What size MC's and caliper pistons are you running out of interest Tom? Share this post Link to post Share on other sites
Rob Thomson 6 Posted March 21, 2011 (edited) I'm wondering if you got supplied the wrong master cyl sizes Rob. When Phil and I first did our track car the pedal was like literally standing on a brick, rock solid and didn't budge. We swapped the master cyls round which made a massive improvement and it is still the same 6 years later. Hi Tom, It was delivered with a .625" front and a .750" rear. I like lots of rear bias so it was installed with 0.625" cylinders front and back. These were the smallest commonly available at the time. Even then the only way to lock the fronts was by winding the bias to the front. I checked and double checked it was installed properly, and set-up as per all advice I could find but couldn't find anything wrong. I suspect - given the benefit of 20/20 heinsight - that the TAS/Compbrake box is a pile of s*it and binds under load, neatly transfering a lot of the pedal effort into the steelwork rather than the hydraulics. As an aside, it's not hard to calculate the theoretical pedal force required (to lock the brakes) all being well. I can't be bothered to do it again now, but from memory (which is f*cked) it was about 400N. Doesn't sound too bad but to put it into context that's about twice what the standard setup needs. On the other hand, F1 drivers need to push their brake pedal with about ~750N (enough to lift their own body weight) so I figured 400N would be ok. Cheers, Rob. Edited March 21, 2011 by Rob Thomson Share this post Link to post Share on other sites
Cameron 16 Posted March 21, 2011 (edited) 400-500N is perfectly normal and shouldn't feel too heavy, F1 drivers apply almost twice that as you said, but that's only when they first apply the brakes at 200mph and have max downforce, they apply way less at lower speeds. It's actually quite complicated to calculate how much force it takes to lock the brakes, you need to know they tyre traction coefficients, GC height and longitudinal location, on top of all your piston diameters and pad effective radii etc; tyre coefficients being the most difficult of those values to obtain! What you can do (and what I've done to spec my braking system) is to use the back-front weight transfer under braking at a certain decel g value to calculate the brake force front and rear. Then you can use the easy values - cylinder diameters, pad effective radii, CG height / location etc to work out the pedal force required to meet that brake force. This then ignores the incredibly complex and unpredictable (at least for us amateurs) tyre behaviour! Edited March 21, 2011 by Cameron Share this post Link to post Share on other sites
Rob Thomson 6 Posted March 21, 2011 (edited) 400-500N is perfectly normal and shouldn't feel too heavy, F1 drivers apply almost twice that as you said, but that's only when they first apply the brakes at 200mph and have max downforce, they apply way less at lower speeds. I've always thought that's one of the most impressive aspect of an F1 driver's craft - being able to maximise braking with very different grip levels at the beginning and end of a very short braking period. I guess, as with most things, it gets easier with practise but it's bloody impressive even so! It's actually quite complicated to calculate how much force it takes to lock the brakes, you need to know they tyre traction coefficients, GC height and longitudinal location, on top of all your piston diameters and pad effective radii etc; tyre coefficients being the most difficult of those values to obtain! It's complicated to get an exact answer, but - as with most things in engineering - you can make a pretty good approximation much more easily. What you can do (and what I've done to spec my braking system) is to use the back-front weight transfer under braking at a certain decel g value to calculate the brake force front and rear. Then you can use the easy values - cylinder diameters, pad effective radii, CG height / location etc to work out the pedal force required to meet that brake force. This then ignores the incredibly complex and unpredictable (at least for us amateurs) tyre behaviour! Yeah that's absolutely right; the hydraulic and mechanical calculations are easy, it's the weight transfer and tyres that make life complicated. The calcs I did were for my old 309 GTi which conveniently was fitted with a brake bias valve (this was before the bias pedal box). Through a neat fluke of physics, on a dry surface and with the bias valve 'open' the 309 would lock all four wheels more-or-less simulateously. From the respective front and rear brake specs I worked backwards to calculate the weight distribution at maximum retardation, and from that everything else fell into place. You have to make an assumption about the tyre CoF, but you can get close enough. Edited March 21, 2011 by Rob Thomson Share this post Link to post Share on other sites
Cameron 16 Posted March 21, 2011 Ah that's pretty cunning, so what did your weight distribution come out at at max braking force? Did you also manage to get a decel g value out of it? Share this post Link to post Share on other sites
robnic 0 Posted March 21, 2011 Does anyone use a remote servo with a pedal box would this provide a solution? Share this post Link to post Share on other sites
rallyeash 155 Posted March 21, 2011 i run a remote servo with a 406 m/c as the cage comes across where the res pot would be originally. i had some take-offs made which sit in where the res pot would slot into the m/c then using 8mm rubber braided brake pipe to the res pot which i bought from merlin motorsport which also has 8mm take offs. res mot is mounted up in the scuttle panel. Share this post Link to post Share on other sites
rallyeash 155 Posted March 21, 2011 Do you think the tilton pedal set would be better then for angle and foot size? I have to get an aftermarket pedal box as I have a hydraulic clutch, but my seat is also bolted to the floor so position might be a little superior?? I was planning on mounting the OBP pedal box on a moveable base so I can adust the pedal position for the driver. Kev i see, not done that much into researching hyd clutch but i believe you can use a be4 box which is halfway there then just get a thrust bearing sorted from ap racing then sort the pedal end out. colin welded in my seat mounts so there fairly low down. from memory aren't yours welded onto the floor with modified exhaust tunnel? worth knowing that wilwood do a m/c which have built in resoviors which saves trying to mount them somwhere inside the car. merlin motorsport also do extension pots for the top of the wilwood m/c's to allow abit more fluid ML5T box pic i found for this clutch Share this post Link to post Share on other sites
robnic 0 Posted March 21, 2011 Hi, A remote brake servo is not actuated by the pedal it sits anywhere in the car with a vacuum pipe to it and brake hydraulic line in and out usually for single circuit brakes but dual circuit ones are available just wondered if anyone ran one who has the advantage of clearance etc with the bias pedal box especially with Mi/GTi6 engine but didn't like the extra leg force required, If I'm understanding your set up you have the conventional servo and master cylinder set up operated via the pedal ie direct linked but you are using a remote fluid resevoir. Rob Share this post Link to post Share on other sites
rallyeash 155 Posted March 21, 2011 (edited) Hi, A remote brake servo is not actuated by the pedal it sits anywhere in the car with a vacuum pipe to it and brake hydraulic line in and out usually for single circuit brakes but dual circuit ones are available just wondered if anyone ran one who has the advantage of clearance etc with the bias pedal box especially with Mi/GTi6 engine but didn't like the extra leg force required, If I'm understanding your set up you have the conventional servo and master cylinder set up operated via the pedal ie direct linked but you are using a remote fluid resevoir. Rob sorry didnt notice it was the servo you were asking about, i believe MG B had remote servo's though edit, search mgb remote servo on ebay Edited March 21, 2011 by rallyeash Share this post Link to post Share on other sites
Rob Thomson 6 Posted March 22, 2011 Ah that's pretty cunning, so what did your weight distribution come out at at max braking force? Did you also manage to get a decel g value out of it? It was years and years ago so memory lets me down, but using the assumptions above the weight distribution is the same as the front/rear braking distribution. Front and rear discs were both 247mm although I think the effective radii were fractionally different in favour of the rears. Front pistons are 42mm IIRC (1385mm^2) and the rears are 30mm (706mm^2). All works out at about 65/35 front/rear distribution which seems a bit tail heavy with the benefit of a few years heinsight... You can only calcuate decel g if you have a good value for tyre/road CoF, which I didn't. Think I assumed 0.9, probably a bit high but I wanted my pedal effort estimate to err on the high side as a worst case. Anyway, I think the result was in about the right ball park but looking back perhaps it wasn't quite as simple as I assumed! Cheers, Rob. Share this post Link to post Share on other sites
