Teslanv's Giant DH Comp Ultimate Off-Road Ebike

[teslanv]

I picked up this frame from my local Craigslist yesterday.

I was pleasantly surprised at how beefy this frame is, considering it's weight.

As many of you know, the Giant DH frames are one of the best frames to build a high-performance ebike with, due to the superior rear suspension design and large triangle space for a battery.

Now I'm trying to decide what kind of system to install on it.

Thus begins my quest to build the ultimate off-road Ebike.

My requirements are:

Power of about ~3000W-5000W Mid Drive if at all possible. (Will be looking at the LR Options, probably, or maybe one of the New Cyclones.) The Frame & Rear Suspension system take up a bit more space around the Bottom Bracket (100mm wide) so I am not sure what systems will fit without some modifications...
Light-Weight and agile for superior off-road/trail riding. - (I would like to keep the weight in the center, with a Mid-Drive.)
Probably a LiPo Battery. Range is not as critical as is current discharge capability. Probably Nano-tech in the 18S Voltage range.
IGH. I am hoping the Shimano Nexus 3-speed IGH I have kicking around will be able to handle the Power of the motor kit I choose. Your experiences with a Nexus 3-Speed IGH is greatly appreciated, or other lightweight alternatives???

The Nexus IGH has three ratios, .733, 1, 1.364 which multiply the cog count of the single speed freewheel that mounts to the hub. If I Use an 20T Freewheel, I can get an effective tooth count of 14T, 20T and 27T, which should be enough range to do what I need on the trails, exspecially considering the power available.

DH Fork, Double Crown, like a Marzocchi Bomber or similar.

Probably 26" Bicycle (Downhill) Rims & Maxxis Holy Roller Tires, to keep the weight to a minimum.

Let the research begin. 8)

 

[teslanv]

Confirmed. Michael Backus' Big Block will fit.

Oh, Baby.

https://endless-sphere.com/forums/viewtopic.php?f=28&t=57720&start=925#p916574

 

[WoodlandHills]

With a comparatively lower powered BBSHD and a Nexus 3 I find that I need bigger cogs than the Shimanos, Sturmey-Archer cogs are the same and they go up to 26t. I am currently running a homemade 32t cog with a 42t chainwheel for off-road only, but the 26t is a good all-arounder for mixed street and dirt. I've not had any longevity issues, but I am running only 1500w and am a relatively gentle rider, I suspect.

Essentially these are ultra low powered ultralight motorcycles and gearing is critical. I can't wait until we can buy a mid-drive with a 6-speed internal gearbox like a real motorcycle and just change sprockets once to dial in for your area and riding style.

 

[teslanv]

Batteries:

A quick search on HobbyKing found these 8000mAh 25C/50C LiPo packs:

http://www.hobbyking.com/hobbyking/store/__21439__Turnigy_nano_tech_8000mAh_5S_25_50C_Lipo_Pack_US_Warehouse_.html

Rated for 25C Continuous, 50C Burst. Using Icecube57's derating factor of 1/4 of rated continuous, that would net me 25C X 8 Ah X 0.25 derating = 50A Output continuous, and nearly double for short periods. Building these 3 in series for 18S would net a max voltage of 75.6V.

50A Discharge would put me right in the 3500W Range for power.

Weight of three bricks is 2.772 Kg or 6.1 lbs. Total of 532 Watt-Hours. Depending on how hard I ride and terrain, range would be anywhere from a 5 miles to maybe up to 20 miles.

 

[upek]

Are you planning to look into suspension bearing? I would be very happy if you can measure for me suspension link marked on photo. I don't have orginal one so I can't copy existing one.

For my frame I will be using BHT Big block motor connected directly to the hub (no freewheel) with 0.5in pitch chain.

 

[teslanv]

Are you planning to look into suspension bearing? I would be very happy if you can measure for me suspension link marked on photo. I don't have orginal one so I can't copy existing one.

For my frame I will be using BHT Big block motor connected directly to the hub (no freewheel) with 0.5in pitch chain.

Not sure what you are after, upek. Do you need a CAD of the pivot arm shap, or just the center to center distance?

 

[teslanv]

Gearing Notes:

Per Michael Backus, the Big Block motor has a Kv of 62.5 RPM per Volt at the motor's shaft. At 75V, that would be 4687 RPM, unloaded.

The Primary reduction is 5:1 - Gets us down to 937 RPM

The Secondary Reduction stage has the following possible combinations:

Small Driver Sprocket: 12T, 15T or 18T
Large Driven Sprocket: 64T, 75T or up to 93T

I'm thinking of going with the 12T Driver and 75T Driven sprockets on the secondary for an additional 6.25:1 reduction for a total of 31.25:1 reduction, and a crank RPM of 150.

Then with a 42T Chainring and an 18T Rear Sprocket on my IGH, I'll have another 2.33 reduction for the rear wheel Sprocket rpm (unloaded) of 64.2.

Assuming a 27" O.D. Tire, at middle gearing on the IGH (1:1 Ratio), that's an unloaded top speed of 28.1 MPH.

Low Gearing would be a top speed of 21.6 MPH with Massive Torque, and Highest Possible speed would be 38.2 MPH on the tall gear of the IGH.

That sounds about perfect for this bike.

 

[teslanv]

Looks like I'm going to need some of Kiwi's torque arm/adapters if I want to use my IGH.

https://endless-sphere.com/forums/viewtopic.php?t=33767

 

[macribs]

Seems that frame is just screaming for a really light weight and powerful mid drive. Have you considered the Tangent drive?

 

[Leebolectric]

^ I would second that...but ya know I'm biased :lol:

 

[upek]

Not sure what you are after, upek. Do you need a CAD of the pivot arm shap, or just the center to center distance?

CAD will be great but hand drawing with dimensions is just enough.