I'd imagine the pivots have no extra force multiplying effect as they would have kept master cylinder sizes and pedal dimensions similar between LHD and RHD models. Going on that assumption I'd say you want to be getting your pedal ratio to >6.5. Smaller master cylinder would also help.
The Autocavan linkage, which replaces the OEM pivots linkage, runs at a ratio of 1:1, so that's a reference point against my old set up. Basically, with all the info now obtained, by installing the pedal box: Hydraulic ratios look pretty similar; I'm minus the OEM 16v servo I had; and I've gained 0.4 in leverage. I actually run a remote servo on the front circuit, supposedly at a 1.9 ratio. It helps, but it doesn't solve. Will do the maths on the whole set up now. What did you base the 6.5 suggestion on?
The suggested 6.2 from the earlier article but a little bit more due to the mk1 having a higher ratio of 5 to start with than there 4. Not the soundest of approaches I admit. Would it be possible to make your pedal ratio adjustable by having a few differing pivot points? If not doing the maths should be the safer option.
Summary so far: Brake spec|M/cyl dia|M/cyl area|Pressure (psi) at 40kgs/88lbs foot pressure|Pedal ratio|Total foot pressure (psi)|Servo effect (psi)|Total pressure @ caliper (psi)| Wilwood Dynalite 280s, Autocavan linkage, 16v 9" servo|22mm|0.589|149.4|5.0:1|747|417/595*|1,164/1,342 Wilwood Dynalite 280s, twin cyl pedal box, remote 1.9 x servo|0.625mm (x2)|0.614|143.4|5.4:1|775|349/558**|1,124/1,333 * @ 70% / 100% vacuum respectively ** 1.9 ratio servo, acting on 50% / 80% of front braking circuit respectively The first brake spec will outbrake the second by miles
I think you problem with these comparisons chris is front to rear ratios vs cylinder size. These are the thoughts I am having, might be well off track though. With your current setup you need to look at just one cylinder working on the front brakes not the pair. I think That would half your pressure on your front brakes. So your down to 666.5. Similarly if we consider the front/rear split on the original setup to be a ratio of the different piston areas we get. A ratio of 5.9:1 between front and rear areas a(ssuming 14mm cylinders). So your front Caliper pressure would then be 1147 with full assistance
I think it's the opposite - if I take out one of the 0.625s (got Excel running alongside this), it doubles the 40kg pressure on the other cyl and it then doubles the line pressure. Can you show where the 1147 comes from?
(1342/6.9)*5.9. = 1147. Thats working on the 5.9 for the ratios of area. Although your right about halfing the area it will double the pressure. But also you need to consider the balance bar splits the 40kg force between front and rear proportionally. So force into the master cylinder will be less than the 40kg.
Agree - I'll tweak the numbers and stick 80% on the front as an extreme, rather than 50:50. I'm still trying to follow your numbers. Where is the 6.9/5.9 from?
dont go too small on Front master cilinder , as it need to handle 8 pistons > volume > deep pedal > not good for confidence in most Pedalbox setups the rear master cilinder are 2 to 4 sizes bigger going on your pedal > both will go in in equal until the the bigger rear master cilinder dont moves any further then the front will go deeper and it will give solid brake feel and controled braking.... and confidence with the car off the ground , brake until the front wheels locks up , now you need just being able to turn the rear wheels by hand.... a bit more pressure and they to lock up too thats the starting point
Thats my opinion too, .625 is the smallest cylinder to use really. I don't know if there would be an effictive improvement in upping the rear cylinder size if it's not needed.
most braking is done on the front ( 80- 90% ) on FWD cars the nose dives and this makes the rear pretty light ( unload the wheels) with a small rear master cilinder you will see a pressure spike > rear lock up .. with a bigger rear master cilinder ( less pressure spike) its more balanced
The rears locking isn't a problem (drums + bias valve), though I appreciate I can optimise the rear set up further down the line, by removing the 0.625 cylinder from it. The main issue is the front pressure, or lack of. Will changing the rear to 0.7 or 0.75 move more pressure to the front?
I've just built a pedal box, its similar to an autocavan i measured up from Mr. Hillclimbers car. I've used .625 front and .70 rear, 280 front discs with girling 54 calipers and am soon to be installing it so will be able to report with the results. I predict the pedal travel to be quite small so perhaps fittings front calipers with larger pistons, yes increasing travel but also increasing force applied to the pads/discs. Other than increasing pedal ratio to 6.2:1 thats the only logical step i can think of. Heres some pics of the beasty like autocavan one but in 3mm plate and fits the bulkhead properly. (not that i haven't finished it and the pedal length and location are yet to be finalised)
Updates in blue - they both scale down at the same rate, except the servo becomes a bigger proportion of the total force, as that hasn't changed: Brake spec|M/cyl dia|M/cyl area|Pressure (psi) at 32kgs/70.4lbs** foot pressure|Pedal ratio|Total foot pressure (psi)|Servo effect (psi)|Total pressure @ caliper (psi)| Wilwood Dynalite 280s, Autocavan linkage, 16v 9" servo|22mm|0.589|120|5.0:1|597|417/595*|1,014/1,192 Wilwood Dynalite 280s, twin cyl pedal box, remote 1.9 x servo|0.625mm (x2)|0.614|115|5.4:1|620|349/558**|969/1,178 * @ 70% / 100% vacuum respectively ** 1.9 ratio servo, acting on 50% / 80% of front braking circuit respectively *** 80% of total force assumed at the front
Checked the spares the car came with: ~17.5mm ID, so the latter likely fitted (ex race car spares, why carry diffferent ones etc?). Out of interest, which ones were the ones which came as standard with the pressure reg?
17.4's with the pressure reg, according to vwspares. Also missed the 6.9/5.9 bit earlier. I roughly calculated the front brake piston area is 5.9 times larger than the rear piston area. The total system area for ratios is therefore 6.9. Then divide the pressure by 6.9 to get 1 unit pressure and multiply by 5.9 to get the pressure given to the fronts. I'm not 100% sure if this use of ratios really describes the split of braking force though as you have a pressure regulator in there which will be changing things.
I've just checked this, and it was a 20mm tandem OE-type cylinder operating on the OE pedal, with no servo! Nightmare thing! Added in green below! Brake spec|M/cyl dia|M/cyl area|Pressure (psi) at 32kgs/70.4lbs** foot pressure|Pedal ratio|Total foot pressure (psi)|Servo effect (psi)|Total pressure @ caliper (psi)| Wilwood Dynalite 280s, Autocavan linkage, 16v 9" servo|22mm|0.589|120|5.0:1|597|417/595*|1,014/1,192 Wilwood Dynalite 280s, twin cyl pedal box, remote 1.9 x servo|0.625mm (x2)|0.614|115|5.4:1|620|349/558**|969/1,178 Wilwood Dynalite 305s, tandem m/cyl|20mm|0.487|145|5.0:1|723|-|723 * @ 70% / 100% vacuum respectively ** 1.9 ratio servo, acting on 50% / 80% of front braking circuit respectively *** 80% of total force assumed at the front Need to come back to this thread. Snow before Xmas thwarted getting the pedal box modified, but I'm on the case!
Chris, I've copied mr. hillclimber's pedal box in his mk1, being an autocaven one im now worried i've the same rubbish leverage as yours, i'm going to take it out and adjust the pivot points to get that desired 6.2:1 pedal ratio. I think this is a main cause for your lack of braking power too.
If you wish to hang fire, I should be making efforts to get mine modded in the next week or two, so can let you know how it was done.
