Strong off-road frame for mid-drive. Need your advices

[220volf]

Hi! My name is Alexander. I'm nurting idea of building custom frame for some months.

Spoiler: Now some background.

I riding bicycles for about 10 years. Two years ago I bought first electric kit about 350W front drive for my hardtail Merida. It was good but I had cheap components especially fork and off-road roding was unpleasant. And I desired more speed, more power... so I decide to buy full suspension bike with large battery and powerfull motor. First impressions was positive, but when I tried to go off-road It was unbearable vibration. When I ran over gravel, roots, sticks or small stairs I got kick in rear wheel. When speed was great this kick became to wheel damage.

So I analyzed situation and decided that hub motors is for sleek roads only and ful suspension is not cure-all. Hub motors is very easy way to convert existing bike. But due big unsprung mass massive hub motors give very strong shock transfer to vehicle and rider. And if I need power and speed when going off-road, I need durability. I can't hang all these kilograms on standart frame - it will crack. So If i need frame for my bike it will be:
- custom
- strong
- full suspension
- for mid drive
- without pedal transmission (like scooter, like motard yeah) and with bike seat
- with low center of mass
- with place for big battery
- maybe motorcycle fork, brakes and wheels. (strong, heavy and cheap)

So, I want build my first frame. I have no expirience with welding and metalworking, but I have internet, I have youtube with lots of educational video. I have arms and brain (a little And it my project will be successful and frame will pass tests I will make some for sale. This is my plan

Now I have concept and need your advices and criticism. I drew inspiration from some good projects. Here goes links:

https://endless-sphere.com/forums/viewtopic.php?f=12&t=49359

https://endless-sphere.com/forums/viewtopic.php?f=12&t=55592

I like idea to use engine axis as axis for rear suspension. I think this is good because this is only way to use single chain.

My first step is to define basic geometry - angles, wheel size, wheel base an so on. Here my first draft.

Need your opinion for these moments:
- Did you interested this project?
- What about tube angle?
- I want to use 20'' bike wheels not 26.
- What do you think about wheel base? I have ~1040mm in my old bicycle, about 1200 in my full-suspension e-bicycle and 1400mm in my gasoline bike. What is the best length?

 

[ferret]

Hi Alexander.

-Interested enough to read your post and reply :wink:
-Do you mean head tube angle? Big travel bikes these days run 63-65 degrees.
-For series off roading I would go for 27.5" wheels, certainly no smaller than 26". My own new build (from bare frame up) is 27.5".
-Wheelbase depends on your height and the agility/stability mix you are looking for, but around 1200 sounds close, especially if you are happy with the size of your current full-suspension e-bicycle.

Regarding head angles and wheel base, they are always a matter of compromise and personal preference, there is no magic number that will work great for all conditions and all riders.

A lot of R&D goes into the design of frame geometry and suspension kinematics. That is one of the reasons that I decided to base my builds on tried and tested off the shelf conventional mountain bike frames instead of designing or buying dedicated e-bike frame, even if integrating the electric drive components would have been easier.

Of course I can understand the appeal of designing, building and riding your own frame (regardless if it is electric or human powered), just keep in mind it may not perform as well as one deigned by experienced pros.

Avner.

 

[220volf]

ferret, thanks for your reply!

Hi Alexander.
-For series off roading I would go for 27.5" wheels, certainly no smaller than 26". My own new build (from bare frame up) is 27.5".
Avner.

Hm... what is the reason that all enduro motor bikes have weels like 20'' not 26'' ? I think there is complex reasons:
- weight. Less unspured weight - less vibration and more smoothness
- strengt. Smaller spokes - more strength
- and (but i can mistake) if I have big wheels high frame, i cannot reach ground by my legs. I have same problem now - the bike is too high with 200mm fork.

A lot of R&D goes into the design of frame geometry and suspension kinematics. That is one of the reasons that I decided to base my builds on tried and tested off the shelf conventional mountain bike frames instead of designing or buying dedicated e-bike frame, even if integrating the electric drive components would have been easier.

When we talk about lighweight bike, this is true. But when we add huge components (10-20 kg battery or even more) and 8 kg engine, when we increase speed twice frame became to work in not suitable conditions. All tested kinematics fails because of changed mass distribution. And you can without ceremony crack some bike part (dropout for example). And usually bike frame does not have enough space for battery. So I think bicycle frame not suitable for me.

Could you show your project?

 

[ferret]

Hm... what is the reason that all enduro motor bikes have weels like 20'' not 26'' ? I think there is complex reasons:
- weight. Less unspured weight - less vibration and more smoothness
- strengt. Smaller spokes - more strength
- and (but i can mistake) if I have big wheels high frame, i cannot reach ground by my legs. I have same problem now - the bike is too high with 200mm fork.

The terminology is different for motor bikes wheels and bicycle wheels. For a motorcycle wheel, 20" means that the RIM diameter is 20". For a bicycle wheel, 20" means that the TIRE diameter is 20". Do a search and you will find that an off road motorcycle TIRE diameter is pretty close to 26". A bicycle 20" tire is very small (kid bikes/BMXs/folding bikes). 26" was up to a few years ago the standard for mountain bikes and now 27.5" is becoming the new one.
Most frames are designed for specific wheel size, fork length and rider hight. If the frame is correctly sized for your height and built with the wheels and fork it was designed for, your legs will reach the ground.

When we talk about lighweight bike, this is true. But when we add huge components (10-20 kg battery or even more) and 8 kg engine, when we increase speed twice frame became to work in not suitable conditions. All tested kinematics fails because of changed mass distribution. And you can without ceremony crack some bike part (dropout for example). And usually bike frame does not have enough space for battery. So I think bicycle frame not suitable for me.

It's true that adding 30 kg will certainly effect the behavior of any bike (although the kinematics will stay the same). I don't know how much you weight. Lets assume you weight 70kg and the electrics another 30. There are many riders weighting more than 100kg and their bikes can handle the weight.

Take a look at some of the videos below and you will see just how much speed and abuse a modern downhill bike is capable of withstanding.

https://www.youtube.com/results?search_query=downhill+wc
https://www.youtube.com/results?search_query=red+bull+rampage

I'm sure a stock frame is not the best option for every build, but modern mountain bikes are extreme high performance machines that took decades to develop to their current state. I think only a genius could match their performance on his first try.

My signature contain a link for my first build's thread (http://endless-sphere.com/forums/viewtopic.php?f=28&t=56178) which is build around a 2003 frame with 26" wheels.
I posted a teaser to my current build (http://endless-sphere.com/forums/viewtopic.php?f=28&t=74157), 2015 frame with 27.5 wheels.

Avner.

 

[Sean9002]

is it going offroad?
bigger front wheel required if so. A 24 inch front matches well to a 16inch moto rear (same size wheel as a 20inch bike, but way more tire)