Critique my design

Hey everyone, I stumbled upon this website while searching for a use for my rarely used cannondale gemini 900. I used to go downhilling often but I just haven' had the time recently. after looking at some of the awesome projects that have been completed on this website i was inspired to see what I could come up with in photoshop. I wasn't interested in building something that can be pedalled, rather something similar to a motard or trials bike.

I took a picture of the same bike and made the following changes:
-replaced 26inch wheels with 20inch bmx wheels
-lowered the front suspension to ~4 inches (from 7) -- the blue behind the front wheel represents where the wheel will be at full fork compression
-removed drivetrain, replaced with large rear sprocket
-small 4.25 (inch (dia) motor behind bottom bracket. -- note: I realize that as the rear triangle+ wheel moves through the travel the tension on the chain will change, therefore a chain tensioner would be needed, or replace it all together with another chain from the motor to the rear arm rotation point then to the rear sprocket.
-rear travel is now ~5 inches, down from 8
- yellow circles represent batteries, as much as possible towards the bottom of the bike to keep it manuverable
-torquise box is the controller
-off yellow circle represents the footpegs.

any helpful suggestions are appreciated, especially towards component selection. I would like this to have a range of 10km, and a top speed of 60km/h (not sustained, but please let me know if this is unreasonable), although low end torque is more important to me.

Eamonn said:

note: I realize that as the rear triangle+ wheel moves through the travel the tension on the chain will change, therefore a chain tensioner would be needed, or replace it all together with another chain from the motor to the rear arm rotation point then to the rear sprocket.

Click to expand...

On the first plan, the chain tension will also hinder the motion of the swingarm...

If you run via a jackshaft on the swingarm pivot AND both sprockets on the chain between it and the wheel are the same size, the effect on the suspension will be minimised.

The alternative is to mount the motor on the swingarm, somehow.

I thought about mounting it on the swingarm, but I would like to minimize unsprung weight. I think a chain tensioner would work so long as the chain is long enough to accomodate the suspension at its peak travel.

ultimately the best way would be to mount another chainring at the pivot point.

Eamonn said:

I thought about mounting it on the swingarm, but I would like to minimize unsprung weight. I think a chain tensioner would work so long as the chain is long enough to accomodate the suspension at its peak travel.

ultimately the best way would be to mount another chainring at the pivot point.

Click to expand...

The problem is that the tension of the chain stops the effective working of the suspension...

If you go via a second sprocket on the pivot point, it will solve the chain extension problem but you will still have interference with the suspension if the front and rear sprockets are different in size.

The closer the motor is mounted to the pivot point, the less the significance it has as unsprung weight so, I think this would be the best compromise.. How much does your motor weigh?

What does the motor weigh! Many of us used geared Bafang which weigh in at about 7 lbs. Hardly noticable as unsprung weight. Mounting on the swingarm looks fairly straightforward.
otherDoc

You could use an intermediary sprocket on the bottom bracket.

If you want to go ahead with your first plan, I would move the motor up a bit....

Right on, Miles! As usual!
otherDoc

I suppose it would be easier to mount the motor on the underside of the swingarm, avoiding chain issues all together. I havent decided on a motor yet, but I thought it would be heavier than 7lbs... that's why I wanted to mount it on the frame.

If the motor weighed 7 pounds it would be about equivalent to having a 3 pound hub motor in the rear wheel.

Wow that frame is a spitting image of the old Marin Alpine Trail and East Peak frames! The with your current motor mounting location the motor's power will interact with the suspension. It'll cause 'squat' or 'anti-squat' depending on the exact location. If a line is drawn from the pivot point of the rear suspension arm to the rear axle in the fully extended and compressed positions, keeping the motor midway between those lines is a good starting point.

Regarding power/speed. Since it looks like the pedals will be gone, how about adding an old 3-speed geared hub. A few people on the endless-sphere use them and so far the little hubs have held up to electric power just fine.

Also, why bother reducing the suspension travel? I've ridden trials motorcycles and 6-8 inches of "pogo-stick" suspension is loads of fun.

Marty

Hi,

Miles and OtherDoc:
Would there be any problems mounting a motor on the swing arm and running one chain from the crank to the motor shaft and one chain from the motor shaft to the rear sprockets (mid motor) on a system with no derailleurs (geared rear hub and one sprocket at the crank)?

Emonn:
If you want light weight and good power and are willing to jump through some gear reduction hoops you might want to consider an RC Motor. Here are some related links:

http://endless-sphere.com/forums/viewtopic.php?f=3&t=5156
http://endless-sphere.com/forums/viewtopic.php?f=6&t=5168
http://www.wisil.recumbents.com/wisil/shumaker/default.htm
http://endless-sphere.com/forums/viewtopic.php?f=6&t=3904#p57685
http://www.endless-sphere.com/forums/viewtopic.php?f=1&t=6163

Thanks!

Mitch

MitchJi said:

Hi,
Miles and OtherDoc:
Would there be any problems mounting a motor on the swing arm and running one chain from the crank to the motor shaft and one chain from the motor shaft to the rear sprockets (mid motor) on a system with no derailleurs (geared rear hub and one sprocket at the crank)?

Click to expand...

Hi Mitch,

Yes, it would be difficult to avoid interference with the suspension....

Miles said:

MitchJi said:

Hi,
Miles and OtherDoc:
Would there be any problems mounting a motor on the swing arm and running one chain from the crank to the motor shaft and one chain from the motor shaft to the rear sprockets (mid motor) on a system with no derailleurs (geared rear hub and one sprocket at the crank)?

Click to expand...

Hi Mitch,

Yes, it would be difficult to avoid interference with the suspension....

Click to expand...

Hi Miles,

Thanks!

Would you be kind enough to explain the problem more fully? How difficult would it be?

Thanks!

Mitch

MitchJi said:

Would you be kind enough to explain the problem more fully? How difficult would it be?

Click to expand...

Do you have a particular bike in mind?

Miles said:

MitchJi said:

Would you be kind enough to explain the problem more fully? How difficult would it be?

Click to expand...

Do you have a particular bike in mind?

Click to expand...

Have a look at this: http://www-personal.umich.edu/~jtalliso/files/bikereport.doc

Hi Miles,

Thanks! I'll check it out. If I have any remaining questions I'll let you know .

I was given a Fisher Joshua (Stock Photo):


I need to decide wether to use it or sell it and get a bike that will work better for what I want to do. I might decide to do that anyway to get a bike with more space in front of the rear tire. So I need to know if this will be a problem with the Joshua and I might need to understand the issue for selecting a replacement.

Also I'm planning on mounting a rear fender that is strong enough to mount the motor. This will give me a wide range of mounting positions (I think - pending review of the bikereport.doc).

Thanks Again!

Mitch

The Fisher Joshua seems to be a URT design - in that case, there shouldn't be a problem with what you're proposing.

Miles said:

Have a look at this: http://www-personal.umich.edu/~jtalliso/files/bikereport.doc....

The Fisher Joshua seems to be a URT design - in that case, there shouldn't be a problem with what you're proposing.

Click to expand...

Hi Miles,

I think I understand.

If the suspension causes distance between the sprockets to change the chain will inhibit suspension travel.

Thanks!

Mitch

I just read your orginal post.
Undersized rims, been there, dun that, wont do agian.
Dont remove drive train USE it. Slap a Cyclone on that pup. http://www.cyclone-tw.com/order.htm 1500W for round 1600 Bucks includin batteries & shipping. $1 per Watt! Cant be beat!