New hi-speed project

[BrianG]

I'd like to get well over 100, maybe 130ish, but I can only get a calculated theoretical speed of around 140, but with wind/drivetrain drag, I'll be lucky to get 110. I probably could get a bigger pinion if I went with a single speed setup, but I want to retain the two speed for just playing around too. A single gear set up for high speed is a lot for a motor to pull considering the size/weight of the vehicle...

[What's_nitro?]

That's a 1515 right? Seems like a 1521 would fit if you're not sure it will have enough top-end speed. Also, on the brake issue- why not use both mechanical AND electrical? Sure you need a more expensive ESC since airplane controllers have crappy brakes, but you will never have problems bringing the car under control.

[BrianG]

The 1521 will fit, but I was able to get my hands on the 1515 at a very decent price so... If you have a 1521 of a similar kv (1700) you wouldn't mind trading/bartering, let me know.

The mechanical brakes lock up pretty good. The linkage I modded allows me to adjust the "progressiveness of engagement" (if that made any sense) so, it should be ok. Even so, I wouldn't want to use elec brakes on something this large/heavy running at 10s. The back-EMF voltage would be too high for the lipos IMO.

TBH, I wonder if this ESC "regenerative braking" effect is what causes lipos to heat up even on lower voltage systems. Just on the bench running on 4s, I saw some pretty substantial reverse voltage spikes and negative battery current (which means current was flowing into the battery). If braking hard enough and depending on gearing, I can imagine ample potential for this "charging" current to be well over the recommended 1C charge rate. I'm seriously considering some type of transorb/voltage clamping device to limit the amount of voltage getting to the battery on all my higher powered setups. Basically, I would choose a device with a voltage rating about 1v higher than the max battery voltage. When the voltage exceeds this rating, the device would simply shunt the excess voltage to ground. I'd rather replace a $2 part every so often than shorten the lifespan on $600 lipos!

[aqwut]

Originally Posted by BrianG

The 1521 will fit, but I was able to get my hands on the 1515 at a very decent price so... If you have a 1521 of a similar kv (1700) you wouldn't mind trading/bartering, let me know.

The mechanical brakes lock up pretty good. The linkage I modded allows me to adjust the "progressiveness of engagement" (if that made any sense) so, it should be ok. Even so, I wouldn't want to use elec brakes on something this large/heavy running at 10s. The back-EMF voltage would be too high for the lipos IMO.

TBH, I wonder if this ESC "regenerative braking" effect is what causes lipos to heat up even on lower voltage systems. Just on the bench running on 4s, I saw some pretty substantial reverse voltage spikes and negative battery current (which means current was flowing into the battery). If braking hard enough and depending on gearing, I can imagine ample potential for this "charging" current to be well over the recommended 1C charge rate. I'm seriously considering some type of transorb/voltage clamping device to limit the amount of voltage getting to the battery on all my higher powered setups. Basically, I would choose a device with a voltage rating about 1v higher than the max battery voltage. When the voltage exceeds this rating, the device would simply shunt the excess voltage to ground. I'd rather replace a $2 part every so often than shorten the lifespan on $600 lipos!

Maybe you're on to something.. ever since I did Nitro ---> brushless converisons with Mechanical braking... I have never had any heat issues at all....

[Mod Man]

Good idea on the back EMF issue.

I have never had a problem in real life, though. The lipos can take sort spikes of high back current, but not long duration.

Matt

[What's_nitro?]

Instead of a transorb, why not just use a zener diode with appropriate voltage/current ratings to protect the batteries? Seems simpler to me anyways and the diode will last forever so long as it can handle the current produced.

[lincpimp]

I guess my anchor idea is stupid.

I have noticed that mech brakes do seem to lower the esc temps quite a bit. I have a quark 80 airmaster on 5s in my tekno revo. It runs a aveox motor very well. I have a feeling that the motor is loaded quite a bit,maybe even slighly undersized, and it will get the esc warm, but it never gets much over 130. The quark air esc is nice as it does not require a pixstix, and stars just like the car model. No proportional brakes, but it has all of the other features.

[BrianG]

Originally Posted by What's_nitro?

Instead of a transorb, why not just use a zener diode with appropriate voltage/current ratings to protect the batteries? Seems simpler to me anyways and the diode will last forever so long as it can handle the current produced.

Yeah, whatever will work I guess. Even a transistor or FET could be used...

[What's_nitro?]

Retro-rockets?

[MetalMan]

Wait a second, you were in SoCal and didn't notify us?!!?

One thing with mechanical brakes at those speeds is heat. Don't forget KE = 1/2mv^2, so stopping at 100mph creates 2.5+ times the amount of heat as the intended ~60mph stock top speed does. On my ~13-14lb Savage with dual front disc brakes and no rear brake the fiber pads were melting from high speed-only running.

[BrianG]

Yeah, but the few days before I went, I was busy and forgot to tell everyone. Sorry. Regardless, it was a really packed week since I left Monday morning and came back Sunday night. There was only a period of about 4 hours on one day where I could've done anything, but I didn't have access to a PC (it was torture!!). Oh well, maybe next time. It was a really nice week weather-wise. 70-75 during the day and about 60ish at night. A far cry from the 30's in good 'ol Des Moines!

Yeah, I know heat will be an issue, but I'd still rather replace pads than expensive lipos. Besides, running the two speed prevents me from running elec brakes, so the point is moot really...

[MetalMan]

Originally Posted by BrianG

Yeah, but the few days before I went, I was busy and forgot to tell everyone. Sorry. Regardless, it was a really packed week since I left Monday morning and came back Sunday night. There was only a period of about 4 hours on one day where I could've done anything, but I didn't have access to a PC (it was torture!!). Oh well, maybe next time. It was a really nice week weather-wise. 70-75 during the day and about 60ish at night. A far cry from the 30's in good 'ol Des Moines!

Yeah, I know heat will be an issue, but I'd still rather replace pads than expensive lipos. Besides, running the two speed prevents me from running elec brakes, so the point is moot really...

Hopefully you will get to make it out here again. Maybe RCX? I bet you loved the weather .

Good point about the 2spd vs motor brakes. I just wanted to point out that you might want to increase the number of discs to increase the surface area of heat dissipation.

[What's_nitro?]

Would mounting a fan over the brake pads do any good? They may still heat up but would probably be cooled off by the end of the next pass...

[lutach]

BrianG,

I going to let one of my ideas go here. Just give me some time to make something that will make some sense and I'll post pictures. I've been trying to do this for the last 6yrs and never got to it. I was going to patent the idea, but I think it wouldn't be right for other companies to not try it. One company so far has done it, but not in a 4WD 1/8 scale. How good are you in fabricating things?

[BrianG]

Depends on how complicated/intricate it is, and what it needs to be made of. Due to the lack of "proper" equipment, I'm pretty much limited to sheet metal bending/cutting, and some fiberglass work.