OFNA Ultra GTP / MBX conversion

[lincpimp]

So I had a MBX buggy sitting around, and decided to convert it to a GTP on road car. It was not much good as a buggy, so I figured that it would do better as a road car.

Purchased a new front bumper and body mounts for the gtp, fit great, as the mbx and gtp share the same chassis, etc. I made an L shaped mount for the KB45 10l out of some 6061 al angle. Bolted it to the rear center diff bulkhead, and left enough of the L portion to screw it to some existing chassis holes. I also reused the upper rear brace I made for the buggy, and mounted the ccbec and the mm to it. Center diff was replaced with a solid center drive with the 51t plastic ofna spur gear (all stock gtp dirt oval parts).

Battery tray is designed to hold a pair of 2s2p emoil (drill lipo) cells. I have my trakpower lipos in there in the pics, artfully taped to fit the tray as it is too long for them. The emoli cells did not seem to have the punch and top speed of the trakpower lipos, despite being 6000mah. They must have a lower c rating, probably more like 15c.

Pinion is a 20t, got it from sdp-si. It is mod1 and has a 6mm hole, so I did not have to mod the KB motor shaft. Nicely made pinion, not hardened though, so it is only good for plastic spurs.

Only compaints so far is that the shocks have too much travel. A true gtp has shorter shocks, and I have a spare set of revo shocks, so I may have to make them work. The extra travel causes the body to catch the ground under hard braking and cornering. I was going to limit the travel of the stock shocks, but I figured that the revo shocks would be about the right length. Plus the stock shocks are crappy.

If someone else with a GTP (Briang?) could measure the length of the shocks at full extension it would be a great help.

Now for the pics!

[Arct1k]

Looking good - Are you able to lay the shocks flatter and stiffer to get less vertical travel....

LMK if you see any reasonable GTP bodies best i've seen is 48 for a porche or 45 for a viper - Could you also measure the hub to hub distances (W/L) I'd like to see how far off the mugen is... thanks

[lutach]

How are you liking your solid center shaft? I love mine and how it made my truck drive better.

[lincpimp]

Originally Posted by Arct1k

Looking good - Are you able to lay the shocks flatter and stiffer to get less vertical travel....

Not sure if that will help, the shocks need to have about 1/2 the travel they do now.

LMK if you see any reasonable GTP bodies best i've seen is 48 for a porche or 45 for a viper - Could you also measure the hub to hub distances (W/L) I'd like to see how far off the mugen is... thanks

I found my body on ebay for 15 bucks shipped. Not sure how many are floating around though. Have you looked on tower hobbies yet?

Originally Posted by lutach

How are you liking your solid center shaft? I love mine and how it made my truck drive better.

I like the solid center in this car, and oanything on road, but prefer a diff with thick fluid in my 1/8 buggies, and the revo. I find that it helps the steering feel and overall turning ability. I like to drift and goof off with my on road cars.

[Arct1k]

Looked on ebay nothing doing atm - tower is 50+

[Arct1k]

I'm 14.5" length between hub centres and just under 11.5" width to edge of hub hex.

How does that compare...

Thanks

[lincpimp]

Just gotta find a tape measure at work tomorrow and see what it specs out at.

Check these specs: http://www3.towerhobbies.com/cgi-bin...?&I=LXTCJ1&P=7

[Peter Zicha]

Really nice and clean build , that centre shaft what a wonderfull idea . How does it pull with that KB . internaly they are the same size as the 22xx series lehners just not as efficient

[lincpimp]

I am very happy with the kb so far. I have put about 2.5 packs thru it, and everything seems to be staying cool. It has quite a bit of power, and cand draw the batteries pretty hard at full throttle. I have the punch control set at 20%, and may move it back up to 50% for less wheel spin out of the hole!

The solid center gives it a different feel to a buggy, but I am used to it from driving shaft drive 10th scale cars. Still prefer the diff in a buggy, but it is cool drifing this big car around the parking lot!

The mm seems to be ok with this setup. I had a novak cap on the mm, but it fell off. I could not get solder to stick to the joints on the board, so I will attach it to the input leads near the board. The esc ran noticably cooler with the extra cap. I may also put a fan on it, as I have a few 12v units hanging around.

On a side note, what would it take to have the fan temperature controlled? Have some sort of sensor on the heatsink, and a relay or solid state elec. to switch it on and off when needed? Maybe BrianG could whip me up something, given his electrical genius background?!

[BrianG]

"electrical genuis background"??? lol, yeah right! Just a hobbyist...

Anyway, to make one out of discrete components would be fairly easy:

Maybe a Schmitt trigger (op-amp or comparator based), with a thermistor v-divider on one side and a fixed 5v regulator as a reference on the other. Drive a small NPN transistor configured as a switch to turn on the fan.

Or, see if you can find an all-in-one thermal switch IC. Most of the ones I know of are thermally controlled fan speed controllers, not on/off controllers.

[lincpimp]

Hmm, the thermally controlled fan speed thingy sounds interesting.

If it would be "fairly easy" to build, do you want to make me one? Funding will not be a problem.

How big do you think it would be? Size of a bec, or larger, like a house brick?

[Arct1k]

Here you go : http://search.digikey.com/scripts/Dk...eywords=switch

[BrianG]

If I built it, it would probably be fairly large, like the KoolFlight BEC - maybe a tad smaller. Much easier to use one of the devices Arct1k linked to (except weed out normally-closed models): http://search.digikey.com/scripts/Dk...ormally%20open

A fan speed controller is unnecessary IMO. It's not like you are trying to maintain a specific set temperature, but rather just keep it cool. And TBH, just an inline manual SPST switch to the fan would be adequate and simpler.

[Matthew_Armeni]

Originally Posted by lincpimp

I had a novak cap on the mm, but it fell off. I could not get solder to stick to the joints on the board, so I will attach it to the input leads near the board. The esc ran noticably cooler with the extra cap.

Are you using flux when you solder? I realize that some solders are rosin core and don't necasarily need more flux but I always use it and it makes the solder flow right away and makes a very strong bond.

[Arct1k]

Interesting experiment but why not let the fan run - Can't use many amps...