Rebel Scum II : 2500W Ultra Max Ludicrous DIY Torque Sensing Full Susp 10sp Ebike ~$2805 w/out Battery

This is officially my 16th documented ebike build on this site. Sweet sixteen turned out to exceed my every expectation for performance and usability. I decided to throw my beloved new Ultra Max 2500W peak ludicrous drive unit on a crappy old Bikes Direct Sniper full suspension fat bike that I couldn’t give a rat’s ass about. I wanted to build a trail bike that I wouldn’t care about if it fell apart and that I could ride without having to constantly lean on the throttle. For years I have dreamed of having a high power torque sensing ebike, after years of dreaming it has finally become a reality. This article documents this build and helps people understand what is involved if they want to build something similar (just please don’t use the Sniper, it is a crappy bike).

Powerful, Torque Sensing and Triple Suspension the Rebel Scum II build really has it all

Welcome to the second installment of Rebel Scum The Empire Strikes Back “The Entire Bike’s Crack”. I’ve written a separate article on modifying the Sniper frame for the Ultra Max mounting bracket and a second article on how to custom program the Ultra Max motor. I highly recommend that any potential Ultra Max buyers read both articles before purchasing any Ultra Max motor or ebike. Be aware that welding aluminum without heat treating it only gets you about 40% of the strength of the original metal. If you try to mount an Ultra Max bracket to a frame with a chainstay, you will almost certainly need to heat-treat it to keep it from self-destructing while you ride. You have been warned.

I used a cheap C691 display because that was what I had lying around. Love them orange sherbert flavored grips.

There are a handful of Ultra Max ebikes and frames on the market right now, but I’m too cheap to buy one and I already have too many ebikes and unused frames kicking around. I decided that the Sniper frame would work well due to it’s lack of chainstay/bottom bracket junctions and because I really hated the frame and didn’t feel good about selling it on ebay (didn’t want to hurt my flawless 100% feedback rating). Ironically when I finished this build and took it out for a ride my first thought was “man this backend is loose” but then after riding it for a while I quickly forgot how much I hated all the flex in the frame.

The new speed sensor can be as far as a cm away from the spoke magnet

The Ultra Max allows you to go out in the woods and ride without ever touching the throttle. I generally ride on the trails on power level 3, on higher power levels it really feels over the top. The Ultra Max ludicrous edition joins the BBSHD, Cyclone, Lightning Rods Big Block and the Tangent Ascent as being true 40mph+ ebike mid drives. Although there are plenty of Direct Drive motors in the 25 lb+ range that can handle 40mph+, the problem is that trail riding with a 25lb+ hub motor on your ebike kind of totally sucks. Of all the torque sensing drive units that are currently available on ebikes the Ultra Max ludicrous 2500W peak motor is the only one I know of that is a torque sensing drive unit that will really go over 40mph.

If you buy your Ultra Max from Luna make sure to get the 2500W Ludicrous option

I made this build with a 42T Sunrace 10sp cassette so I could crawl up super steep slopes, but in retrospect, I probably didn’t need it. If you do go with a 42T cassette make sure you get the CS-MS3 which is all-steel and doesn’t have the aluminum grannies of the CSMX3 (taco city). If you’re building a commuter then you could go with a larger chainring in the front than the 42T Race Face N/W that I used. I recommend using the Race Face rings because the stock all steel Ultra Max ring is 294g as opposed to 56g for a 42T Race Face. Since the steel ring is no extra charge with the Ultra Max you might want to test that one on your build first to see if you want to go larger or smaller. For more advice on selecting the right chainring size check out this article here. Being able to custom select the size of your front chainring is one of the things that makes high power mid drives far more desirable than hub motors.

Since you have to use a larger chainring with the Ultra Max I opted for a massive 42T steel granny

I got 99 problems and the 42T granny is one

One of the only real issues I had with this build is trying to mate a standard 10sp X7 derailleur to the massive 42T granny. The problem is that most 10sp derailleurs are designed to line up with an 11T-36T cassette max. Going from an 11T to a 42T granny means that you need to put an extra long adjusting screw on the derailleur which pulls the derailleur wheel pretty far away from the cassette to get it to clear the 42T gear. This means in the higher gears (like in the 11T) you have a lot fewer teeth engaged than you otherwise would have. This means I got obnoxious skipping under load which really took away from the enjoyment of this build. I’ll almost certainly go back to just running a 36T-11T cassette on this build because chain skipping under load is something that the dude just cannot abide.

Running a full suspension bike with a Thudbuster LT post means your coosh factor is over the top

Pros

  • Torque sensing is just more fun, your thumb will get less exercise
  • Bike goes over 40mph with the Ludicrous option (much slower stock)
  • Decent singletrack ebike, reminds me of the Apex, if the Apex had a crappy suspension that wagged its tail a lot (the Apex frame is a whole lot stiffer and the ride is much improved)

Cons

  • Make sure your Ultra has a steel gear in it, the nylon ones have a very high failure rate even at the stock power levels
  • The BD Sniper Full susp fatty frame is to die for (meaning if you ride it long enough it will probably kill you)
  • The build is heavy ~55 lbs, without using a backpack battery I probably probably wouldn’t want to take it on trails
  • Sucks in snow, geared too high
  • Because the motor is geared much higher there isn’t the wheelie popping “oh my god I’m out of control” feeling you get with the BBSHD ludicrous and a 30T chainring
  • The Bluto shock won’t work in extreme cold, I wouldn’t take it out if its lower than 30 degrees F
  • With a 42T granny you get skipping under load in the higher gears

I had to put a spacer under the speedo and then use 2 zipties to secure it in place, zipties hold my world together

So the question that is on everyone’s mind is do I prefer my 3000W tangent ascent build on Rebel Scum I or do I prefer the 2500W Ultra Max Ludicrous on this build? That is not an easy question to answer, the Tangent ascent was about 10 lbs lighter than this build and it felt a lot more intense to ride. I really like the torque sensing on the Ultra Max, but the extra weight is very noticeable on the trails. The Ultra Max also smashes much more on logs and log piles while the Tangent ascent was up and out-of-the-way. The flexibility of the Sniper frame was really pronounced with the Astro powered Tangent Ascent and when I hit the throttle it would wheelie and the frame would flex so much that when the front wheel came back down it was often not pointing in the direction I wanted to go. The Ultra Max doesn’t do that, it has a very smooth power delivery that makes riding a crappy trail bike much more tolerable. I would say that the best part of the Tangent Ascent is that you can install it on a normal frame whereas the Ultra Max means you have to chop it up to make it work.

Lunacycle has an exclusive dealership on a new magnesium bodied Ultra Max that is 3 lbs lighter than stock

Cost breakdown for this build is $1500 for the BD Sniper, $995 for the Ludicrous powered Ultra Max, $80 for custom welding, $45 for custom 42T Race Face Ring, $55 Sunrace 42T Cassette off ebay, Thudbuster seatpost $130 for a total of $2805. This doesn’t include a battery which will probably run you another $500-1000 bucks. I highly recommend using a 52V nominal battery with the Ultra Max drive unit. If you opt for the Ludicrous option then make sure your battery can deliver at least 50 Amps continuous, as most frame packs can’t do more than 30 Amps cont.

If I were to do this all over again I would make the bracket tilt much farther up in the front (~25 degrees) to keep the motor from smashing on logs and to give the mounting bracket more surface area to attach to the frame

The big question is should you buy a prebuilt Ultra Max bike or try to build one yourself. The Rebel Scum II build shows that you can do it yourself if you’re willing to work through some issues on your own. I’ve followed Luna through the path of building their own custom Carbon fiber ebike with the Apex and the Apollo and I can tell you they have sunk hundreds of thousands of dollars and thousands of man-hours on developing those two bikes. There is no possible way that you could ever come close to creating a build with that level of refinement. That being said if you have a chopsaw and a crappy old bike frame that you don’t care about, you can still build something that is fun to ride that won’t make you cry too much when it breaks or gets stolen.

Destroy everything you touch before you get attached. Attachment is the root of all suffering.

Ride on.

Mike Eatough takes frame modification to a whole new level when he chops up his trek to mount the BBSHD out of the way and moves the shock in the process, I’m not loaning any of my ebikes to you Mike, ever.

9 thoughts on “Rebel Scum II : 2500W Ultra Max Ludicrous DIY Torque Sensing Full Susp 10sp Ebike ~$2805 w/out Battery

  1. Karl, Any idea if the Luna version had Steel gears too? Having it lighter and metal gear is awesome but there doesnt appear to be anything on the Luna website or the forums to confirm these.

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      • It’s likely to be a 130bcd spider, Karl. Roadies use 39s on there but that would preclude a narrow wide option but I know that 38s were available about 5-10 years ago. With a little custom tweaking of the outer part of the spider arm (a bit of file/dremel work) I’d put money on the 38T being the smallest you could get on there. The issue would be….is a narrow/wide available in 38T 130 bcd?? (otherwise try an older plain ring…I have some oldish [pre 10 speed] rings that work quite well…they may even be 3/32 level of old….must measure them some day)

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  2. Hi Karl,

    FANTASTIC ARTICLE ON YOUR ULTRA-MAX MOTOR BUILD!

    I am interested in following your lead and cutting up an aluminum frame and ‘dropping in’ the cast aluminum ultra max motor mount. Did you consider ‘brazing on’ the motor mount, instead of taking it to a welder?

    Have you seen anything like the following video? https://www.youtube.com/watch?v=FDLPxqkHOyk The producer of the video calls it soldering, but it is clearly ‘brazing’. There are several of these ‘magic brazing rods’ around ‘dura fix’ is one brand, you can watch several videos if you google their name. The manufacturer claims that the resulting ‘alloy weld’ is stronger than the base metal. The process uses MAPP gas or propane torches, making it accessible to most builders.

    Can everyone shout out to Luna that they should sell a steel ultramax motor mount? Much easier to weld, super strong, simple, if you have a steel frame!

    I am very interested in hearing your thoughts (and that of your followers) about the ‘magic brazing rods’ for use on an aluminum bike frame.

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  3. Pingback: To Boldly Go … The Christini All Wheel Drive BBSHD Electric Fatbike | ElectricBike-Blog.com

  4. Would a non-bafang spider (something with a smaller offset) fit the ultra max? I think the current spider offset may be an issue for non-fat bikes.

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