Cutting frame on Big Hit to fit battery

[B1gD]

Ok so this is my first post so hi guys
I’ve picked up a 2003 Specialized Big Hit for £150 so keep this in mind (not much to loose)
The space would be for a 72v 30ah battery maybe 20x13 for a 3 or 5000w rear hub
Has anyone tried cutting and redirecting bars or removing a bar and replacing it with a reinforced box.
As you can see in the attached images. The blue line is a welded bar shaped to take the load.
Give me your thoughts

 

[dogman dan]

You are going to have to make two batteries, that strap to the sides of the frame, like little saddlebags. It will make it pedal funny, but at 72v you wont be doing a lot of pedaling.

 

[Punx0r]

That kind of mod is fraught with structural difficulties, doubly so on an aluminium frame. The proposed box even with stiff side covers is unlikely to be as stiff as the tube you cut out. Even with an ideal battery box you're still up against the fact that welding will destroy the heat treatment of the frame in the weld area, leaving it much weaker than before. There's no real way around this: however you do it, it will never be as strong as stock. The difficult question is whether this will be strong enough for your needs.

It looks like a great frame and a good buy, but sadly Y-shaped frames are awkward for ebike conversions.

 

[goatman]

what if you welded 2 solid identical frames that sandwiched and bolted through the bike frame. it can bolt straight through the cross bar and also on each side, something like 1x2 tube. if you drill a 1/2 inch hole through the frame and use a 1/2 inch od pipe as a sleeve cut to the same width of the frame or a hair less so it wont squish. make everything a tight fit you might be ok but 200-18650 cells is 25 lbs so maybe that's a bad idea.
does your rear swing arm have 135mm spacing with a 5mm offset to the left of bike centerline? or is it right? I bought a Kona that had a specialised rim/hub and the tire wasn't centered, my local bike shop guy said specialised does that but look to see if you do and wich way its offset, it might be an issue or maybe not to get a motor/tire in the swing arm centered with the frame

 

[dogman dan]

Still a real nice frame. You did right to buy it. But don't cut it up.

Another option would be to split the battery in half, much like I suggested before, but carry half of it on a rear rack, and then the other half on a handlebar carried mount. If you put a tall set of BMX bars on the bike, it gives you a top bar that can be used to hang a battery box or bag from.

Fastest E bike I ever rode had the whole battery mounted there, like 120v. Even with that weight up front it wheelied way too much. It needed a longer rear swing arm. That is a modification you might consider, make a steel swing arm, or steel attachment to lengthen it.

 

[macribs]

The Big Hit has been the "donor" for several clean and very nice builds here on ES.

Here are two. If you search the forum you will find more. Click images to read full story.
Here is a quick search of the forum via google, you can even browse the various builds by photos, just choose the "images" after clicking the search link.

 

[macribs]

That kind of mod is fraught with structural difficulties, doubly so on an aluminium frame. The proposed box even with stiff side covers is unlikely to be as stiff as the tube you cut out. Even with an ideal battery box you're still up against the fact that welding will destroy the heat treatment of the frame in the weld area, leaving it much weaker than before. There's no real way around this: however you do it, it will never be as strong as stock. The difficult question is whether this will be strong enough for your needs.

It looks like a great frame and a good buy, but sadly Y-shaped frames are awkward for ebike conversions.

Yet a lot of clever builds have been done with this exact frame, the Big Hit was kind of a hit here on ES a few years back. And samd did a few builds that was almost stealth. Really nice builds. He used a welder well used to working with aluminium and AFAIK they didn't run into any problems. Welding slowly, short beads/short stack of coins and using a jig was the key iirc.

If you fear degradation after welding you can always take the new frame to someone to have them heat threat it again.

I say cut it, slice it, dice it any way you feel like. Either it works or it don't. The great thing with welding is that you can always take a grinder and start over again. And should you see crackling after taking a big jump you can always fix it and then get it heat treated. Sure welding alu is not like welding black steel or mild steel but it's still welding. And mistakes can be fixed.

 

[Punx0r]

I say cut it, slice it, dice it any way you feel like. Either it works or it don't. The great thing with welding is that you can always take a grinder and start over again. And should you see crackling after taking a big jump you can always fix it and then get it heat treated. Sure welding alu is not like welding black steel or mild steel but it's still welding. And mistakes can be fixed.

Those are great builds and I wasn't saying it couldn't be done. It's like you say, not like a steel frame which is very easy to hack about.

The possibility of getting frames re-heat treated is often mentioned but how often is it actually done? Careful welding is better than uncareful welding but no matter what you do there will still exist an area of weak metal right where the original frame material meets the weld (the HAZ). Depending on the original heat treatment, you're reducing the strength of the frame material at this point by around 2/3rds after welding. Ongoing crack inspection is a wise precuation but doesn't preclude unexpected failure without warning.

To the OP, just be careful. Aluminium is a difficult material to work with anyway. Reworking heat-treated material, even more so. If you do this over-build the interface where new material meets old. I would maximise the size of the welded joints to spread the load. I.e. design the interface so you have 3-4x the material in play and you'll hopefully offset the weakened material.

 

[macribs]

You are right. Bad welds can be a point of failure even suddenly during riding. So "stack them coins"slowly, aka precise welding with good control of temperature and speed. And at least in the start of riding, do close up inspection of the seams. Even later after hard riding. Safety is important. But so is the joy of a completed DIY project.

Yeah I just get really excited when I see people wanna do a build rather then ordering online. As great as the sur ron bike is for its price I feel it has also been maybe the go to solution for a lot of people the last two years. So much so that many people don't venture into the DIY part of e-bike anymore. And that is fair. The sur ron is great. And it is hard to build something better for less money.

Guess I am just a sucker for reusing rather then online shopping.

As far as more e-bike then e-motorcycle with pedals I think some of the most stealth full suspension builds on this forum is actually done reusing the Big Hit frame. And some of them got all the torque and power needed even for wheelies and hooligan riding. And that is amazing. Price wise, depending on batteries, controller etc. If people are truly honest when they do their final inputs in the spreadsheet, those numbers add up way to quickly so if you workshop contains nothing e-bike related or previous builds you can strip for parts, you can easily end up with a build costing close to or even more then a sur ron. But the flipside is that if you are a little lucky picking up a used Big Hit those can often be had for a very low price. And often previous owners have done upgrades too, like better brakes or even suspension upgrades. And careful shopping for the electric drive train, and choose the correct motor/controller combo can really save a lot of money.

When completed you got you own build. Designed and made by you. And you have reclaimed and reused rather then gone shopping online. I think it is great, and it is sustainable. So much so that I tend to focus more on what can be done then on all possible problems.

Btw for your viewing pleasure here is one of samd's builds, was not the one I had in mind but I couldn't find back the the white Big Hit he did. But this is pretty stealth too.

 

[B1gD]

Thanks guys, I appreciate the feedback & the knowledge/experience.
This bike is for road use so I won’t be doing any ‘big jumps’ on it.
I don’t weld, but I have a friend who does ....it seems if that is the way this is to go then slow and steady with the correct heat seems to be the universal advice. Plus keeping a close eye on the welds after to see if there are any cracks.
I don’t know what stress factors that frame can handle but I’m sure it’s many times what it faces in reality. I can’t remember seeing a frame fail catastrophically on one of these bikes. I would go as far to say the frame would survive on a flat road for a long time if that bar was removed(don’t worry it’s out of the question). I saw a guy on YouTube the other day cutting out half the structure of a bike then taking it off road, hard, the bike survived and then some. I will try to find the link.

macribs give me your thoughts on this ‘easier’ alloy weld please...*note he cannot break the frame weld without massive force https://youtu.be/k21JDVdGalY
This guy welded an ebike frame for £30 ($50) https://youtu.be/PWiPdSqZfhw
Tig weld aluminium bike frame repair https://youtu.be/axrLCn4QSIM

 

[Tolkano]

Why can't you just put a box under the downtube? You could probably do around 120-130 cells that way without it being too unwieldy

It will look low profile and still look like a regular bike, hardly any welded frames look good except for that guy fpvlee who did the mondraker summum and he's already inserted that frame into the garbage bin after it cracked from welding

If you want to build a 20s13p battery then it isn't even the right frame for that, you should look into an eeb frame or something

 

[macribs]

I started to watch the delboy video and then I realized this is something like modern day voodoo and I couldn't say anything worth saying about it as I am not a welder by trade, I go to work at an office each day and I am no engineer, heck I hardly know any advanced math so I likely couldn't check if claims are true if someone gave me the formula. When it comes to welding I am self thought, or I took a class back in the days in high school and the rest I learned bu trail and error. But I also got various grinders so it always work itself out. If I miss the first time, I bring out the grinder and go again. This "alu rod soldering" might work for you, but why take the chance if you can get help from your friend?

As for breaking strength for the metal, I would be very careful not to reduce the original strength, like removing the top bar of the frame or something similar. As long as you replace what you cut out I am sure you will be just fine. I base that on the result others had with just the same Big Hit frame.

But remember when electrifying a bike said bike lends itself for some radical riding. And you might find yourself ending up rather quickly in sticky situations just because it is so fun to ride WOT. And given the quick acceleration and high top speed possible that can lead into some gnarly situations you didn't even think about when cracking the throttle wide open. So building for longevity I think is a smart move.

When it comes to proper welding and cutting the frame to allow for including a battery box I wouldn't worry too much. Make a jiig and go slow. Alu melts easily....so control and nice and steady wins the race.

 

[B1gD]

Talkno,
Thanks for your input....I like to be different m8What can I say ....
I need space for 20x10 30q 18650 prob 2 batteries deep

 

[B1gD]

Thnx for you thoughts goatman, macribs, & others very much appreciated. I will keep you informed of decisions, progress and final build.