Front calliper on Rear?...
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@MH-Bikes-N-Bits Yeah, I done this like 10 years ago when I was just a wee nipper. Nowadays I do like to file off markings like that. Pretty sure Aprilia don't make them and they are actually Brembos.
On the Cagiva they run the same brake calipers as the RS but with Brembo stamps instead of Aprilia/Cagiva.
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That is definitely food for thought! Though I’m having to put my upgrade ideas on hold till I can sort out my head again! Shame, the old thing was going so well, picking up smooth and quick through the band and all. So much so I proper noticed how woeful the brakes are on these bikes!
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@SpookDog I had some old front/rear pistons accessible so was able to measure them, stock DTR front is 35mm and rear is 30mm dia. so:
( ((17.5 squared) x 3.142) - ((15 squared) x 3.142) ) / ( ((15 squared) x 3.142) x 100% ) = front caliper has 36.1% more piston area than rear, meaning you need a rear master cylinder which displaces 36% more fluid than the stock 12.7mm bore item to run a front caliper on the rear and retain the rear brake action/feel/pedal travel.
So reversing the master cylinder equation gives us:
(Square root ( (((6.35 squared) x 3.142) x 1.36) / 3.142) )= New rear m/cyl piston radius
= 7.40mm meaning you’ll need a master cylinder with a bore just under 15mm.
Check by comparing with the stock setup (12.7mm dia. m/cyl and 30mm dia. caliper pistons):
STOCK
((15 squared) x 3.142) / ((6.35 squared) x 3.142) = 5.58 (m/cyl. piston moves 5.58 times the distance of caliper piston because caliper piston has 5.58 times the m/cyl. piston’s area)FRONT CALIPER AND 14.8MM BORE M/CYL
((17.5 squared) x 3.142) / ((7.4 squared) x 3.142) = 5.59 (m/cyl. piston moves 5.59 times the distance of caliper piston because caliper piston has 5.59 times the m/cyl. piston’s area).FRONT CALIPER AND 15MM BORE M/CYL
((17.5 squared) x 3.142) / ((7.5 squared) x 3.142) = 5.44 (m/cyl. piston moves 5.44 times the distance of caliper piston because caliper piston has 5.44 times the m/cyl. piston’s area; a 15mm dia. bore m/cyl will slightly reduce brake feel/pedal travel but probably not noticeable. 15mm bore is probably available somewhere).FRONT CALIPER AND 14MM BORE M/CYL
((17.5 squared) x 3.142) / ((7 squared) x 3.142) = 6.25 so if you run the 14mm bore m/cyl. the rear brake will be noticeably spongy; pedal might hit the frame/footpeg mount before locking the wheel which won’t please the MOT man!I’ve never tried anything like this and the numbers surprised me so if anyone has anything to add don’t hesitate (checked as best I can but it’s late!). However it does seem logical as anything cylindrical has a big increase in displacement for a small increase in diameter. Hence the twin .50 caliber Brownings in a B-17 tail turret could do a lot more damage than the four .303s fitted to the Lancaster, even though all four .303s had a much higher rate of fire and could pour out a lot more rounds.
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That’s some impressive math! Sorry I was so far out on the measurements
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@SpookDog Thanks bud no worries, I've been meaning to take a look at this for a while as front calipers are a lot easier to get hold of than rears. And if you can find a suitable master cylinder you'll get the added benefit of a proper dust seal which the stock rear caliper doesn't have!
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@HOTSHOT-III so I am running the stock Nissin rear master cylinder and those RS brakes stop on a dime!
Always Originate, Never Pirate!
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@HOTSHOT-III it’s a twin piston calliper I think. The ones that bolt together usually are...
The front m/cyl on the DTR125 is 14mm yes? I got one coming off of fleabay. Looks just like a standard rear but 14. Don’t know if the holes line up but will see!
I’m not sure if I can see the benefit of a bigger piston diameter on the slave, not when the pads are the same surface area on the front and rear?
I’m having trouble understanding where the beneficial ‘fulcrum’ is, if that makes any sense?! Where does it maximise PSI over a bigger surface area? That’s what brakes are about no? Plus added ‘leverage’ from a bigger diameter disc?
I’m really sorry if I’m not making sense! This is all new thinking to me
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