HPI Jumpshot MT v2 into Flux (brushless) project thread

[Johnniedarko]

Hi all,

I'm fresh to RC. I'm yearning for a project, so when my buddy introduced me to RC (planes) I soon after bought a used HPI Jumpshot MT v2. Fun and well controllable, but it definitely can be a tad quicker to really rip. So I'm going the brushless route. Here is where I'll post updates, and also random information that I come across, that another beginner might find useful too. I use a lot of words and ramble on, but I'll try to remember to post pics as well.

The plan
The goal is mod this truck so that is fun to bash everywhere, from street to forest, with moderate jumps, good run-time and durability to keep playing. The project is as much fun as the driving, and I don't know where it will end.

The trivial answer is to just sell this, and buy the Flux version. But DIY is cheaper (Flux new is €350, I bought this for €130. Going brushless can be done for €70, and upgrading the diff+gear is about.. €100?? So I can save about €50. But most of all I'll learn about this thing and have a project, which is what I'm really after.

(In my introduction post I said I wanted to make huge jumps. What I meant was maybe a meter high at best. Since then I've seen on youtube what "huge jumps" mean, and I'm definitely not going to launch of a skate ramp, to see in how many pieces I can break the thing).

The plan is:

• Brushless Motor and ESC (size / power tbd)
• Upgrade gearbox from milk chocolate to at least pure chocolate
• Get a 3S battery to see if I can break the space-time fabric (I now have two HPI Fuzion 2S 5300mAh's 40C.)
• Install some LEDs for visibily
• Get nicer rims and tires. Looking at the belted Protires. (Grip is important but mostly I find the stock plastic chrome atrocious)
• Perhaps get a different body.

Has it been done before?
Yes. In 2016 user PraetoR got the MT v1 and put a Castle + Sidewinder ESC in. And promptly stripped his spur gear.
https://www.msuk-forum.co.uk/forums/topic/211549-hpi-jumpshot-mt/?do=findComment&comment=2215991

HPI Jumpshot MT v2 first impression
Positives:

• Now this truck is somewhat of a good base for more power, as proven by the brushless Fuzion (discontinued) and Flux versions.
• The frame seems sturdy and has a (imo) nice symmetrical layout.
• Seems to handle well on youtube vids compared, or at least it rolls over less on asphalt than it's Traxxas and Arrma counterparts.
• In my first run it was easy to get the hang of it, and it modulates nicely on the throttle.
• While driving head first into a wall (oops), one steering arm popped loose, but that was it.
• The frame itself looks amazing. So good infact that most people comment I should ride it like that (and I agree if not for the fact that as soon as I did that, I flipped it, and it chafed off a whole lot of the body mounting pins.
• Integy and other companies make aluminum/aluminium parts, including a metal spur, which might be needed.

Neutrals:

• The gearbox is mostly plastic, but the Fuzion and Flux models have an improved metal diff and a full gears. (The housing and spur gear are plastic). So that should be an easy swap.
• The body looks okay. Red and blue with white stars and strips is an obvious US flag, and the stars are actually little HPI logos. Looks good. But then the Square black/orange HPI logos slapped on the side don't match them at all and stand out sorely.

Negatives

• Pretty weakly powered. It'll go on asphalt, but on short grass, it's already not that fast, and overpowering the rear for a power slide is impossible.
• There's quite a bit of slack in all the wheel knuckles, giving the wheels a free toe-in-out movement of about 5 degrees. On the rear wheels it's the connection between hub assembly and the A-arm that has a lot of slack. On the fronts it's on the wheel-size steering knuckles. And also in the upper suspension struts, but that only rotates the wheel in its natural rolling direction so that doesn't impact steering (much).
• The stock 'rims' atrocious, but then I've always hated chromed plastic. It just looks like fancy disposable plates.
• The gearbox housing is the rear skid "guard". And since it sticks out like a weber-carbureted engine out of a beetle, it'll hit the ground a lot.

Pics of the release party of these rims:

Phase A: brushless motor and ESC
Motor
As any engineer knows, the first phase is always more power. Then control. And reliability follow later.
(It is actually painful for me to joke about this as I've worked as reliability engineer for years )

With help from Tudordewolf I figured out this first step and left Castle and Hobbywing and looked into Hobbyking and Surpass.

With so many motor types, I had to think of some constraints and settled on:

• No higher battery cell count than 3S, because those are easy to get.
• No larger motor than 3650 for now, 900W seems plenty and it'll keep it lightweight.
• Crawling or excellent low speed operation isn't important to me. I play Snowrunner for that

Given that. I think their 3650 3900KV is optimal. It's missing from that Surpass data chart but they do sell it.

Why 3900kV?
If I go lower, it'll just make less power. Let's say 3650 / 2300KV, which is also rated at 900W, but only with 22V (22V * 41A = 902W). But as I don't have a 22V battery, so with 3S it will only 'make' 12.6V * 41A = 516W, quite a loss, that you can not get back with gearing.
If I go higher, I lose the ability for 3S. In itself not so bad, but in order to make 900W it's coming from the amps. In general, higher current means higher losses, so that's also less ideal.

ESC
Then for the ESC, there's a 80A that they combo it with. But I wanted one step up to 100A, because then I can fit almost all Surpass motors between 3650 and 3670 (1400W), although that last one will require 4S LiPo's. So that's just a bit of futureproofing. There's no Surpass 100A (goes from 80A to a too large 120A), but there's 100A Rocket that fits As an added bonus, the on/off switch is on a cable, so I can mount it somewhere that doesn't require me to get the body off.

I'm waiting for the parts to arrive.... so let's move on to

Phase B: metal gears, solid.
As mentioned, the gears need to be updated, which I will do with the HPI parts for the Flux. I also need to change the diff because HPI changed it from 4 straight roller gears to 4 45-degree angled gears, and the old diff won't fit the new gearbox. For anyone who wants to do this same thing, this is the list of parts that you need to get:

Spoiler: HPI parts list to upgrade Jumpshot MT v2 to Jumpshot Flux

• 160045 - GEAR BOX SET V2101298 - DIFF GEAR SET
• 101301 - DIFF SHAFT SET
• 160038 - DIFF CASE 5x30x16mm
• 160040 - HD DIFF GEAR 60T
• 160041 - TOP SHAFT GEAR SET V2 (22T)
• 160042 - HD IDLER GEAR V2 (27T)
• 160046 - WASHER 12x15x0.3mm (10pcs)
• 160065 Spacer 5x8x6.5mm
• 101221 Diff case gasket (3pcs)
• Z263 Pin 2x8mm (12pcs)
• Z663 Lock Nut M3 (6pcs) [1 needed]
• 6921 Pinion Gear 21 tooth
• A020 O-Ring P-5 (4pcs) [2 needed]
• Z181 1000CPS diff oil
• Z478 TP. Flat head Screw M2.6x12mm (12pcs) [2 needed]
• 160037 Diff Shaft 5x22.7mm (2pcs)
• 160064 Lock washer 3x6mm (10 pcs) [2 needed]
• Others:
• Grease (I got Traxxas TRSX-1647)
• Z159 Threadlock

In the Netherlands, this was about €115 total, shopped at 3 different RC's shops (Toemen, TopRC and Wetronic). Quite a number of these parts are only carried by one of these three, even though they all claim to "have all the parts". And some listed this as being the last parts. So definitely check beforehand if you can actually get all the parts.
I haven't located 160037 Diff Shaft 5x22.7mm (2pcs) yet, but I'm going to ask a shop here to import it, and otherwise I see I can get it from the UK.

I'm also now waiting on these parts to arrive....

They should be here faster than the Motor+ESC from China so I'm going to start on the diff+gearbox first.

As some extra's I ordered some cheapo 2.8" tires with road/street thread, just for the looks, and some Leds. But led placement will have to wait until I figure out what to do with the body. I quite like the idea of running the bare frame, with a rear wing to give it a buggy look. The rear wing should then also act as a sort of roll bar. And some top cover protection for the electronics would be wise. The frame absolutely lends itself for interesting single-seater designs, so I wonder why HPI didn't do anything with it

This is all for now. I kept it short. Updates hopefully soon. Cheers!

Edit: Measurements
Adding measurements here for future reference.

Weights and Sizes

	weight in grams (ounces)	size 1 in mm (inch)	size 2 in mm (inch)
HPI Jumpshot MT v2 total (no battery)	1906 (67.23)
HPI Fuzion 2C 5300mAh 40c-80c (my battery)	277 (9.77)
HPI Jumpshot MT v2 total (with battery)	2183 (77.00)
parts
Stock wheel (115324 6-shot MT wheel with 115315 Jump T2.8 tire )	154 (5.43)	diameter: 117 (4.61)	width: 66 (2.6)
Front shock (115288)	15 (0.53)	length eye-to-eye: 102 (4.016)	travel: 30,0 (1.181)
Rear shock (115289)	17 (0.60)	length eye-to-eye: 119 (4.685)	travel: 30,0 (1.181)

 

[biggman100]

Phase B: metal gears, solid.
As mentioned, the gears need to be updated, which I will do with the HPI parts for the Flux. I also need to change the diff because HPI changed it from 4 straight roller gears to 4 45-degree angled gears, and the old diff won't fit the new gearbox.

Are you sure yours is a V2? I have a V2, and, the only thing i changed in the diff is the ring gear, from the plastic to the metal one, and, running that truck with a 4300KV motor and HobbyWing 10BL120 ESC, on 2S, i havent stripped a gear yet. Its so fast, it barely stays on its wheels anymore, so, i am thinking of converting mine to the Jumpshot SC chassis. This is literally the only part i replaced in the diff on mine: https://www.ebay.com/itm/115596451402 and, yes, you can replace the idler and top shaft with metal, but, unless you are going for crazy 4S power, it really isnt necessary. Also, both the V2 and the Flux only have a 3 gear transmission, so, if yours has 4 gears, it definitely isnt a newer V2 or Flux. Now, if you do swap everything to the Flux all metal transmission, then yes, you will need the housing, as well as the gears, diff, and idlers, but, its not really needed.

This is the Flux transmission:

And, this is the V2 transmission:

 

[Johnniedarko]

Oh yeah, about the 4 gears, I meant that the design of the gears in the differentials are different:
v2 left, flux right

I see I made wrong assumptions. Since the metal gears were introduced with the Fuzion, I just assumed plastic gears wouldn't hold up, let alone with the more powerful flux. Same with the different differential design in the flux. Great to know that they aren't really necessary, the parts will be spares then...

Are you planning on using the SC frame but keep the larger MT wheels if possible, or go full SC?

 

[biggman100]

Oh yeah, about the 4 gears, I meant that the design of the gears in the differentials are different:
v2 left, flux right

View attachment 167214View attachment 167215

I see I made wrong assumptions. Since the metal gears were introduced with the Fuzion, I just assumed plastic gears wouldn't hold up, let alone with the more powerful flux. Same with the different differential design in the flux. Great to know that they aren't really necessary, the parts will be spares then...

Are you planning on using the SC frame but keep the larger MT wheels if possible, or go full SC?

I rarely keep my SCT's stock. The only ones i have that have SCT tires and wheels are my Arrma Fury and Associated ProSC10. The rest of mine, 2WD and 4WD Slash, Losi TEN-SCTE, have MT tires and wheels. In the past, every SCT i owned had MT tires and wheels (DHK Hunter, Slash 2WD and 4WD, ECX Torment, and a bunch i can't even remember at the moment).

As for the HPI diff, you actually need to take the transmission apart to see which diff you actually have, since the V2 early models used a different diff than the newer ones. The newer ones use the same style diff as the Flux, except, instead of the 160040 metal diff gear, the V2 uses the 160039 plastic gear.

This diff, shown below, is the one used in the original V1 Jumpshot, which is now discontinued, and some of the early V2 trucks, but, they no longer use that diff in any of the trucks:

If your truck has this diff, then, to upgrade to the Flux diff, you will need the entire Flux transmission housing, diff, idlers, shaft, slipper, and spur.

 

[Johnniedarko]

Nice list Are there any tires you recommend for allround grip (asfalt and grass mostly) and longevity?

Interesting stuff about the diff. I tried opening it up today to see, but a worn allen head stopped me halfway. It's one that's in the caster block, I tried epoxy welding it but it didn't stick. Need to find a moment to drill it out. But I think you can also tell the diff without opening it up, because the diff-out-shafts are different:

LEFT #115290 MT V2 (the old type diff shaft), RIGHT #160037 (MT V2 new type and flux)

The shape is difficult to judge without opening it up, but the material of mine are definitely the type on the right. So prematurely, yaay.

Yesterday I went to a skate park and had a blast. At some point the tips of the teeth of the spur gear got bent though. I have some replacements coming in, and I'll set it up a bit tigher and see if it stays intact.

 

[Johnniedarko]

Hey everyone, it's been a while! Time for an update.

Brushless install

I have replaced the brushed 550 motor with a Surpass 3650 3900KV and a Rocket RC V2 ESC 100A.

The 3650 3900 KV motor is the highest KV that will run on 3S batteries but, for now I run this on 2S.

The install was painless even for a noob like me, and I like the long cable on the power button. It's a very controllable ride, and on the stock tires, it's nice to drift around.

Durability
Durability so far has been excellent.
The only thing breaking was one of the front spring/dampers after two violent top-speed run-ins straight into a high curb. (It was not my best day). I truly expected much more damage. On hard crashes the steering rods pop loose before something breaks.

One time the servo saver did it's job. It was a bit finnicky to get it back to normal but no permanent damage.

With regards to the discussion about internals above, my HPI Jumpshot v2 has the v1 and early v2 gears and differential, and not one from the

newer v2 and Flux. I've opened the diff up and found it was lubed sparsely rather than

oiled almost full. But it looked fine, and it works so I keep it like this.

Mods / parts
Here's a list of goodies I got.

• Pinions: 21T and 25T. The stock 17T gives a speed of 40 mph / 60 kph and has enough power to move the bigger BKT-look tires.
• Street tires (9imod on Aliexpress). See the picture on the right. Pretty much the only good looking racing tires for 12mm hex on aliexpress. A bit easy to flip the car on asphalt though as the tires are (too) soft and squish in cornering.
• BKT monster truck tires, see the video. (RC Upgrades Parts Store on Aliexpress). Require gluing but they are very large, look great and are surprisingly good at bashing.
• LED lights
• Aluminum spring-dampers (New Enron on Aliexpress). Look nice and they work. Filled with 75wt oil, which seems good for general purpose soft damping.
• Metal spur (Integy)
• Metal wheel hubs and caster block. (Integy) Not installed, used as a backup.

Metal Spur vs. Plastic spur
I use a metal spur gear. The main reason to use the metal gear is that the axle of the Surpass motor is slightly shorter than the original one, meaning that the pinion gear is not properly aligned with the spur. The metal spur is slightly wider than the plastic one and this nicely corrects this issue.

I know there is a lot of debate on metal vs plastic spur. A common theory is that it is safer to have a plastic spur as the first point of failure. I'd like to add to the discussion that this metal spur is at least 5x as heavy as the plastic one (my scale isn't accurate enough). It is also holed in the center so all that extra weight is on the outside, in the teeth. Considering the moment of inertia of discs, I'd estimate the moment of inertia of the metal gear is about 10 times higher. The metal spur gear thus also works as a heavy flywheel and therefore might prevent jolts of power downstream, instead letting the motor "take the hit" (and the ESC in the form of the current draw). Something to consider.

To do
My main thing is bashing and capturing some nice shots, and for that it is already done. But I also like to see how top speed runs go, but the body shell is like a frikkin parachute. It will just lift the car and wheely at speed. The first step is to cut slits in the body shells "rear window" and truck bed, to leave the air out. If that doesn't work well enough I'll whip up a wing like shape out of cardboard or something:

Some random cs

 

[Johnniedarko]

Bodywork
I had the issue that the bodyshell caught a lot of wind at speed, and it would lift the front wheels off the ground. To remedy this I cut the top and sides of the "rear window" and folded it forward for more airflow through the shell. I expected nothing but it's quite a difference. The shell used float more if you'd drop it out of your hand, now it just falls faster. The speed 'wheelie' is gone, and the truck is pretty stable at speed. The cheapest and best mod so far.

Pinion gear
Edit: Upped the pinion from 17t to 25t. Drives very nicely, pretty perfect. But the temps even in light conditions after a couple of minutes are:
ESC: 77C / 170F degrees, motor: 30C/70F. So I'm going down to a 21 pinion. If temps aren't acceptable I'm going to swap the metal spur back to plastic and see what that does.