How to improve brakes? (Warning: rambling)
Posted: Fri Apr 16, 2021 2:26 pm
Disclaimer: Having raced cars and ridden a lot of mountain biking, along w/ being an engineer, I am a brake snob. They should have little travel to the engagement point then be very firm to be force modulated. Squishy pedals and levers suck IMO.
Ok, so on to my main question: I have some play bikes to teach wife/friends. XR100 w/ drums on both ends. Those that suck as bad as those old 70's era bikes w/ drums you'd borrow from a college friend and hoon around on (They honestly suck.) The TTR125LE is another one: Disc front and drum rear. The disc works 'ok' but the rear drum still sucks. Definitely not Brembo Ducati/KTM level but they are cheap toys so what can I honestly expect right? Enter the TTR125L (w/ 150cc BBK) project I bought this winter. Exact same year and bike as my LE except no e-start. It has AMAZING brakes front AND rear. As good feel as the Brembo KTMs. WTF?
I'll discuss the front disc first:
LE: works well but once engagement starts the lever can be pulled back almost to the bar for good stopping force. Its just squishy.
L: about the same amount of initial movement but goes super firm once pads engage. Pulling really hard will not let the lever go to the bar.
So I stare at these for a while and think - bleed the LE front brakes again, must have an air bubble. No improvement. So I stare some more - the L might have an aftermarket hose on it and that could be the difference. Braided cables make a big difference in feel in a car. That must be it! $50 and a Russel braided brake line later I was disappointed to find it didn't make much difference. I know pad material plays a big role in how much force you need to apply to get good stopping power but they shouldn't affect how firm the lever is with the bike stopped. What gives (literally)? Does the LE master cyl need a rebuild? Do I need to take the whole system off but intact, hang it vertically and bleed again? (is there an air bubble in there somewhere I can't get out?) FWIW, I've reverse bled it using syringes and that typically works great getting bubbles out. How does one make a play bike brake feel so awesome? I will slowly replace every part one by one until I can make the LE feel as good as the L if thats what it takes (or finally give up and swap the brakes between the bikes )
Now the rear drum:
I've never ridden drum brakes that works very well. There is a reason bikes uses discs today. But I'm sure they can be tuned for racing and such and work much better than your never-adjusted-or-serviced mini bike drum. So what do you do to them? I'm too young to know these speed secrets. I put new aftermarket front shoes in the XR100 and its 'better' but not great. There are no adjustments in there. I've seen inside the TTR LE drum and no adjustments in there either. So you need to space out the end contact points so the shoes fit closer to the drums? Do you machine the shoes to better match the drum radius? What are the old-school racing drum secrets? I haven't taken the TTR L rear apart yet to see if there is 'special stuff' in there.
And while I'm discussing brake problems: The kid's SurRon rear brakes have a ton of travel until the pads contact the rotor. I know disc brakes have square seals that are used to retract the pads slightly. His seem to retract a LOT. Seems stiction may be causing the pistons to be stuck to the (cheap) seals? I'm wondering if I can lift the caliper up up off the rotor, and squeeze the lever a LITTLE and try to move the pads to a closer static location. He has small hands so has to run the levers fairly close to the bars. Coupling close levers w/ a lot of free play puts the levers into the grips by the time good braking kicks in. Not very easy to use or confidence inspiring.
Ok, so on to my main question: I have some play bikes to teach wife/friends. XR100 w/ drums on both ends. Those that suck as bad as those old 70's era bikes w/ drums you'd borrow from a college friend and hoon around on (They honestly suck.) The TTR125LE is another one: Disc front and drum rear. The disc works 'ok' but the rear drum still sucks. Definitely not Brembo Ducati/KTM level but they are cheap toys so what can I honestly expect right? Enter the TTR125L (w/ 150cc BBK) project I bought this winter. Exact same year and bike as my LE except no e-start. It has AMAZING brakes front AND rear. As good feel as the Brembo KTMs. WTF?
I'll discuss the front disc first:
LE: works well but once engagement starts the lever can be pulled back almost to the bar for good stopping force. Its just squishy.
L: about the same amount of initial movement but goes super firm once pads engage. Pulling really hard will not let the lever go to the bar.
So I stare at these for a while and think - bleed the LE front brakes again, must have an air bubble. No improvement. So I stare some more - the L might have an aftermarket hose on it and that could be the difference. Braided cables make a big difference in feel in a car. That must be it! $50 and a Russel braided brake line later I was disappointed to find it didn't make much difference. I know pad material plays a big role in how much force you need to apply to get good stopping power but they shouldn't affect how firm the lever is with the bike stopped. What gives (literally)? Does the LE master cyl need a rebuild? Do I need to take the whole system off but intact, hang it vertically and bleed again? (is there an air bubble in there somewhere I can't get out?) FWIW, I've reverse bled it using syringes and that typically works great getting bubbles out. How does one make a play bike brake feel so awesome? I will slowly replace every part one by one until I can make the LE feel as good as the L if thats what it takes (or finally give up and swap the brakes between the bikes )
Now the rear drum:
I've never ridden drum brakes that works very well. There is a reason bikes uses discs today. But I'm sure they can be tuned for racing and such and work much better than your never-adjusted-or-serviced mini bike drum. So what do you do to them? I'm too young to know these speed secrets. I put new aftermarket front shoes in the XR100 and its 'better' but not great. There are no adjustments in there. I've seen inside the TTR LE drum and no adjustments in there either. So you need to space out the end contact points so the shoes fit closer to the drums? Do you machine the shoes to better match the drum radius? What are the old-school racing drum secrets? I haven't taken the TTR L rear apart yet to see if there is 'special stuff' in there.
And while I'm discussing brake problems: The kid's SurRon rear brakes have a ton of travel until the pads contact the rotor. I know disc brakes have square seals that are used to retract the pads slightly. His seem to retract a LOT. Seems stiction may be causing the pistons to be stuck to the (cheap) seals? I'm wondering if I can lift the caliper up up off the rotor, and squeeze the lever a LITTLE and try to move the pads to a closer static location. He has small hands so has to run the levers fairly close to the bars. Coupling close levers w/ a lot of free play puts the levers into the grips by the time good braking kicks in. Not very easy to use or confidence inspiring.