Is my buggy slow

HI
I have a converted hyper8 with a medusa 36/70/2000 , MMM, 5s 20c 5000mah zippy. Gear ratio 18/46. I measured the top speed with a garmin etrex 48mph with onroad losi tires on asphalt. I want 60mph top speed. Is there anything wrong with my electronics and what can I do to reach 60mph top?

[Lauri]

Two things can limit your top speed with that combination:
- too short runway ie time to reach the top speed
- too low / slow gearing

If you want to drive the car for longer time and not only go for top speed then you have to compromise. Usually the limiting factor is that too fast gearing will overheat the motor. That's why there's the calculated 40mph speed rule of thumb. You will use gearing that will get up max 40-45mph but overall this is the maximum you will get out of the combination over longer time.

If you "overgear" your car to do speedruns for maximum top speed, you'd still have to take care not to overheat anything!

Basically the faster the gearing = the more power you get = the higher top speed .... until you overheat the engine, burn the ESC, melt the plugs off the wires or kill the battery ;)

[BrianG]

That setup calculates to around 55mph, depending on actual tire size. You are getting into the zone where air drag is playing a larger role. You're gonna need to gear higher to compensate for wind drag and a decent area to get up to speed gradually. Gearing high and WOT starts is a recipe for disaster. Make sure your tires are balanced and there is no excessive drivetrain drag (too tight mesh, worn bearings).

[ReV:-O]

Just out of curiosity, why is gearing high and WOT starts a recipe for disaster?

[BrianG]

High gearing means the motor is loaded much more (think of it as being closer to a stalled condition). That means it pulls more current on startup. Couple that with the inherently high bursts on WOT starts can add up to HUGE amounts of currents.

[ReV:-O]

but surely if you have a system that can handle it like the MMM combo, and GOOD 6s lipo to match, this shouldn't be a problem?
Going straight to WOT would get it up to speed quicker

[What's_nitro?]

Originally Posted by ReV:-O

but surely if you have a system that can handle it like the MMM combo, and GOOD 6s lipo to match, this shouldn't be a problem?
Going straight to WOT would get it up to speed quicker

The MMM does not like 250A+ spikes of current. Trust me... That's what you get when you:

1) Run 6s 5000mAH 25C Lipos
2) A Neu 1515/1Y
3) Gear a GTP for 140MpH
4) Try to dragrace with it.......

Ask me how I know.

[ReV:-O]

well if ur running it in a buggy, with the MMM motor being designed for monster trucks, you should be able to gear it higher for being in a buggy. and i dont mean gear for 140mph, but maybe gear for 70-80mph. I'm sure the MMM could handle it, if not it should just thermal

[BrianG]

Thermals generally happen when heat is built up over time. If the current bursts are too high, it can draw enough to instantly turn FETs into fuses; the ESC just doesn't have time to catch that quick of a rise in temps.

Yeah, a buggy on 6s geared to 70-80 may be ok, but I know you are going to be pushing the ESC hard. Not to mention the brake spikes at that speed/voltage!

[lincpimp]

Going up to 10s or more would be the best bet if you want to continually run high speeds. That will drop the amp draw down quite a bit, and the system will be much more efficient (you must also use a lower kv motor).


Another factor is that you really need to be spinning these motors faster. A 2000kv motor on 5s lipo is spinning about 37k, Going up to 6s would be the best bet and then you can gear it down a bit. Not sure what rpm max medusa is rated for, I know the 15 series neu motors can do 60k. If i planned to do a larger speed car, i would spin the motor close to the 60k and gear accordingly.

[ReV:-O]

whats the difference between having i faster spinning motor and having to gear down, than having a slower but torquier motor and gearing up?

[BrianG]

Mostly startup. Since these systems are sensorless, the ESC needs to get a good rotor positioning signal from a dead stop. When geared high, the motor isn't turning as fast and so the signal is not as good until it starts to spool up. This could lead to cogging, and therefore very high currents which could kill an ESC. When geared lower, each rpm doesn't move the vehicle as much so it is easier on the ESC.

[lincpimp]

Load is load. If you want to move a 8lb buggy to 80mph you have a set load. For that application I would like a motor that has plenty of torque, but one that can handle something like 60k rpms. A 1515 motor would be good for torque needs for that speed, and spin the motor about 50k and gear appropriately.

I think that would be a better option than a larger motor spinning slower. The lower gearing with the faster spinning motor will help sartup as brian mentioned, and it will also have a better "feel" when driving. Motor spool up will be faster as well. Not sure if acceleration will be better, as that depends on traction.

So a 1700kv motor on 8s with 1515 dimensions would fit the bill. You could try 2200 kv motor on 6s but you will need good batts and some airflow over the motor/esc.

[ReV:-O]

Originally Posted by BrianG

Thermals generally happen when heat is built up over time. If the current bursts are too high, it can draw enough to instantly turn FETs into fuses; the ESC just doesn't have time to catch that quick of a rise in temps.

Yeah, a buggy on 6s geared to 70-80 may be ok, but I know you are going to be pushing the ESC hard. Not to mention the brake spikes at that speed/voltage!

How comes that guy (Over_revO) got his e-revo which is a pretty darn heavy monster truck, to 91mph using the MMM on 6s?

I thought cogging was only really a problem for rock crawlers, where they spend most of their time at low rpm. For speed runs, the amount of time spent at low rpm is minimal so would cogging really have an effect?

Also, what are brake spikes?
cheers

[SnoopMaxx]

I punched the no. in to Brian's calculator

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 18
Total Voltage: 18.5
Motor KV: 2000
Tire Diameter (inches): 4.5
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor Coil Resistance: 0.005
Spur/Pinion Ratio: 2.56 : 1
Total Ratio: 8.45299 : 1
Tire Circumference (inches): 14.14 inches (359.08 mm)
Rollout: 1.67:1
Total Motor Speed: 37000 RPM
Vehicle Speed: 58.6 mph (94.13 km/h)
Effective KV Value: 2000
KT constant: 0.68 oz-in/A
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...
Final Power: Amperage not specified...

If you use a 22T you should reach the 60mph you want