Power Question for BrianG, or other who may know

[TexasSP]

I am trying to figure how much horsepower/watts it will take to sustain 70 mph in an e-maxx. Anyway, it's something I got myself too deep into and need some more experienced help in figuring out. I am guessing that 2000 watts seems reasonable, but I am not sure.

Also, I am not talking acceleration to 70 mph but 70 mph sustained.

The main thing is that all this got started because I was challenged that under sustained load nimh cells out perform lipo. I don't see how as this is not my experience but anyway.

Thanks.

[aqwut]

Aerodynamics, wheels & Tires plays a big role..... but 2500 Watts should be enough with any body for an E-maxx...

[jzemaxx]

I went 74.1mph with my 3905 emaxx and a 1521 Neu. That was on 8S......so just doing quick math, that is around 2900 watts assuming I'm drawing 100 amps at least.

[TexasSP]

Thanks guys. I was trying to explain this on the Traxxas forum in my poor layman's terms but not getting it out well. A certain battery seller who frequents there and some others were trying to tell me that to go 70 mph in an e-maxx and pull 100 amps continuously was ridiculous. They seemed to think you could do it pulling very low amps with only a quick burst around 100 amps, I just didn't see how.

[jzemaxx]

They don't know squat then....

[BrianG]

It's true that the majority of the power needed will be during take-off and heavy acceleration. But, as the drag increases at the higher speeds, current will go up again. 100A is a pretty decent figure. I would think something as heavy as an emaxx geared tall enough to get to 70mph without two speeds could draw well over 200A to initially start, go down to around 50-60A during the "middle acceleration", and then go back up to around 100A towards the end where drag is higher.

Also, since you are looking for sustained speeds of 70mph, radio range quickly becomes an issue so you are going to want to get to that speed really quickly to have enough radio range to sustain it, and that kind of heavy acceleration will draw some serious power.

The current will also depend on the voltage used. It's true that higher voltage setups will pull less current than lower voltage ones, but the power levels needed to do what you want are going to take a combination of both high V and high A.

[TexasSP]

It's on the new e-revo forum, the thread title is "LiPo advantages and disadvantages". The battery dealer I am referring to is Jakey aka www.specpointbatteries.com.

I will admit that I did a poor job in trying to articulate my reasoning, but my initial point and thoughts were correct. This is why I spend my time on here reading about 99% and only posting 1%. I love this forum, I learn a lot here.

My original theory was based on Patrick with CC testing the e-maxx out at 70 mph on the Monster. I theorized 6s since that is the maximum the controller is rated for.

My original point though was a lipo producing a continuous 100 amps. I also know that my brushless setups suffered with nimh batteries but that went away when using lipo.

[jzemaxx]

Point those guys over to this board...they will learn something....hehe

[TexasSP]

I tried, then Jakey called me an rc lemming or something like that. See, he is the type when proven wrong or disagreed with to resort to name calling, I am the type to learn from it. I always point people over here to learn, some just know that their "no it all attitude" will get damaged by reality.

[tashpop]

you mention 100amps on the esc. could you list your setup. i think it would help figure some things out. also houstonian!, do you ever make it out to the drag races down hwy6 on sundays?

[TexasSP]

I have seen ya'll out there before, and snellemin invited me to go, Sundays are just tough for me somtimes. I would like to go sometime though.

The setup I was speaking of was effectively referring to Patrick with CC's MMM on an e-maxx going 70 mph. The real conversation came from the fact that a guy on the Traxxas forum insists that nimh hold better voltage under load than lipo while everything I have seen and experienced suggests the opposite. Anyway, the main deal is that he is a vendor who has a lot invested in nimh technology (although he does sell kokam's on a small scale) which I believe is the real motivation for his "theory". Also, in all his tests (which I have never seen data for) he claims to test at 20 amps constant. Well for me I think that is a joke as it does not provide a "real world" scenario with spikes, bursts, braking, etc. Anyway, it's over now, life goes on.

[zeropointbug]

100 amps in WAY to high for 70mph cruising. If my emaxx took 280 watts on medium grass to cruise at 45mph, then to go 70mph, you should need ~750 watts continuous.

[sleebus.jones]

Seeing as how he is selling his NiMH cells for $8.75/cell assembled, it's easy to see why he wants to sell NiMH. In my tests with NiMH (GP3300, IB4200) LiPo can do CONSIDERABLY better than NiMH. NiMH drop voltage like a rock under load. I have a pack (5S 8000MAh) that I've got 156A out of. Good luck getting that out of NiMH. That vendor is a fool, and now goes on the "don't buy from this idiot" list.

One final point you might want to make to this misguided fool: how many cars in the "worlds fastest R/C" contest are powered by NiMH? Hmmmmmmmmmmmmm? :)

[TexasSP]

sleebus, your post really got me rolling.

I have tried to reason with this guy and it's pointless, I just hate that some people will take his word at gold and that's the only reason I even bothered.

[TexasSP]

Originally Posted by zeropointbug

100 amps in WAY to high for 70mph cruising. If my emaxx took 280 watts on medium grass to cruise at 45mph, then to go 70mph, you should need ~750 watts continuous.

If you look at the link below, you can see though that the resistance and power required going from 45 mph to 70 mph goes up substantially. While I know this is in reference to cars it the math is applicable. Then factor in the added drag of the e-maxx body and poor aerodynamics.

http://autopedia.com/stuttgart-west/...Physics06.html

I am not saying your wrong, just saying that there is a lot to factor and I actually learned a lot from that page.