Will an overly worn rotor cause brakes to heat more and not work correctly?
Kart is a cadet size with 65cc shifter, rear brake only. At a track that he was doing a 60s in a mini, he was able to pull of a 57.5s with the braking issue in the shifter. This was after adding the OTK brake duct, which doesn’t fit well and he still complained of the issue. Earlier in the day I rebuilt the master cylinder because the pad was not returning off the rotor - that seemed to fix the pad problem.
Any other ideas to explore before simply saying the motor is too big for the brakes?
Since you don’t describe what the brake issue actually is, the answer to the title question is “yes”. A thinner rotor will build heat faster since it has lower thermal mass.
The issue from my standpoint is the heat. The issue for the driver is extended braking distance in 2 corners. He stated that he needed to brake MUCH earlier than usual to make those corners (6 and 11).
No change in color. But I want to throw something else in for reference. Same track, but different kid had a rotax mini as well as a ka junior on a cadet chassis with no braking issues. So I think I can rule out the idea of too much motor, too little rotor
I did measure the overall rotor thickness. A new rotor is close to 10.2mm, the one on our kart is 9.5mm, and a really worn one I saved was 9.3mm. So without a doubt, it is time to replace the rotor.
One final point - I think OTK changed both the pad and the rotor with the introduction of the evm/evs chassis. The older rotor has more drilled holes in it than the newer one (which is now floating). And I was also told the pads had changed. Which I sort of believe as they don’t show as many copper flecks when deglazing them on the asphalt. I have the older rotor with newer pads on the kart.
I will try to get a look at data in the next few days. I do not have any pedal sensors, and I am not used to looking at shifter data, so it may take some thinking.
What is the condition of the other parts of the braking system? Is there any spongy feeling in the brake pedal, has the brake fluid been replaced with the correct type, how worn are the pads, is the rotor cleaned before each session, and has different positions of the brake rod been tried ?
This is a comparison of the shifter in red to the mini in black. Unfortunately I don’t have a good lap in the shifter because he was complaining of this instantly. On the brakes almost 100ft earlier, which close to twice the normal distance. Is this brake fade? Is this something new to shifter drivers face with braking and down shifting?
I will double check everything - but I usually leave the brake rod on the top hole of the pedal. This is how he was always driven a kart and he can lock the brakes no problem. During the morning of the session I rebuilt the master cylinder with all new seals and bled with fresh fluid based on his complaints. There was no sponginess, and when he applied the brake, the pedal was stiff. There are no leaks at the cylinder or caliper. I switched to a second set of pads, which were a little less used, same problem.
The only thing I did not switch was the rotor and I can tell it is worn. So no matter what, I will be doing this for the next time out. Just curious if there are any other ideas.
Perhaps? In the swift he can lock the brakes which effectively stops the drum from rotating as the clutch is slipping. The kart slows and RPM plummets sharply until he releases but still holds pressure and the RPM climbs back up as the clutch and drum are now friendly again. I would guess with the kx65 that this is not possible. Maybe the friction of the kx65 clutch is so high that it is not possible to slip it by way of braking? The engine is overpowering the brake is what it seems like. So he is on the brakes longer hence the heat build up.
It isn’t something silly like the pedal stop backing out and limiting brake travel? Pretty common little annoyance when you build up a kart.
100 ft. difference in brake point seems very extreme, so I would say something is up mechanically. I don’t think you can put that down to weight or a different braking style for shifter.
I haven’t scaled it, but I think slightly. It doesn’t feel much different from his mini when lifting to the kart stand other than the right side bias. When he picked it up in the front he didn’t think it was any heavier. My guess is 5 to 10lb more at most?
Do you mean that little guy? I will look again, but when I was bleeding them, it felt normal, like there was no mechanical getting in the way. I can always get them pretty stiff and then get like one tiny little micro crick out of them if I press harder.
Mechanically, I keep thinking it is the rotor since it has worn. And the cylinder has been rebuilt. I don’t see a need to rebuild caliper seals and all 4 pistons are popping out. Only thing left is rotor or the fact that they could be overpowered. Or the rod location.
Anyone ever use carbon pads? Not legal for most national series, but that doesn’t matter with what we are doing.
Some of this doesn’t add up. The engine should be providing braking assistance when you lift. With the clutch still locked, the engine braking is going to aid the brakes, not fight them. That is unless there’s a bind in the cable and the engine is actually trying to still accelerate the kart and fight the brakes. Which sounds very plausible.
I will take a look at the throttle cable tonight, but there did not appear to be any binding. It revs up/down crisply on the stand. Maybe I will pull the filter and look at the slide to make sure nothing is hanging up there.
