Strongest seat post rack

I built a "sprung" rear rack that holds all the elecronics and batteries behind the seat.
The rack holds two bags- total of 4 12Ah SLA's, two inverters (15V for lights, 5V for cooling
fan and aux power, contactor, fuses, circuit breaker, controller and room
for bike tools. The load is about 50lb, and broke two other racks.

The design is similar to spinning magnet's diagram, except I used a coil- over spring from mtn bike fork as shock absorbers. The rack has a couple of aluminum angle pieces attached to the bike at the rack front (this is a recumbent. I drilled thru the frame and fit brass bushings which fit around a 1/4 in bolt.

The "struts" are 3/8 in threaded rod which telescopes into aluminum tubing (got at L Ace Hardware). A washer is fixed to the top of the strut, there is a slot cut in each upper half
to allow the threaded rod to move up and down without coming apart.

The springs are a single replacement spring from a MTV fork (10$ at bike shop). I cut it in half with a Dremel tool to give two springs of the correct length and force absorption.
The lower end of the strut has a 3/8" nylon locknut and washer on each side to adjust preload.
The bottom of the strut threads into a connecting nut which has been driiled to accept a small
bolt that connects to the dropout (I had to fabricate a piece of aluminum stock to attach to the
dropout since I use the threaded hole in the dropout to hold rear fenders on.

This rack works great- I have about 250mi on it. Not only does it show no sign of bending, but the ride
is much better. I have been used to punishing rides even tho the bike has a rudimentary rear swingarm
suspension. We have lots of potholes and rough roads. I have ordered a lithium barrery and
BMS, and could not imagine subjecting the expensive purchase to this kind of physical shock. The ride is so
much smoother that I feel safe going faster on most routes.

I think this simple shock- absorbing rack would work fir any bike that has
to carry a load on the back. I have been very pleased, since the ride
is so smooth, it's like getting a new (better) bike.

I will try to post pics.

Here is a pic with the batts off. There is a receiver tray on top that clamps onto the battery bags so they can be quickly removed.IMG_0147.JPG

Oh crap it didn't work I am such noob...
I am trying this with a cell phone, will try later when at home.

IMG_0147.JPG

Sacman said:

Very clever and interesting that you used a gooseneck (that's normally attached to the steerer tube of front forks) but instead have it attached to the seat tube. The gooseneck's clamp mounts are definitely strong and fasten securely to the seat tube. But how does the rest of the rear rack attach to the gooseneck? It's difficult to see from your 1st 3 pictures because they are zoomed in to close.

Click to expand...

just a simple solid aluminum or steel tube with two bolts to hold it together.

karma said:

Sacman said:

Very clever and interesting that you used a gooseneck (that's normally attached to the steerer tube of front forks) but instead have it attached to the seat tube. The gooseneck's clamp mounts are definitely strong and fasten securely to the seat tube. But how does the rest of the rear rack attach to the gooseneck? It's difficult to see from your 1st 3 pictures because they are zoomed in to close.

Click to expand...

just a simple solid aluminum or steel tube with two bolts to hold it together.

Click to expand...

Ahhh... I see it now. Thanks!
BTW your ebike looks like one heavy beast! :wink:
That battery pack on the toptube doesn't get in your way when you pedal?

Doctorbass said:

Thanks guys.. every of your suggestions are nice! :wink:
Spinningmagnet, I love your desing... if only they could exist for sale ready to use! :| the mecanical desing facilitate movement and distribute force in teh right direction!
Doc

Click to expand...

Doc it already exist and is for sale. Try these 2 racks. I've used them on suspension bike and they work great

http://www.niagaracycle.com/product_info.php?products_id=13695
http://www.niagaracycle.com/product_info.php?products_id=1547

Back to the dogman solution. Yeah, the post got bent after about 3500 miles of riding with about 20 pounds on just the stock post rack. It was a good seatpost, and took a long time to bend, but some of the bumps on my commute route get both wheels off the ground. They put a lot of whoop de doos in the bike trail to let water cross when it rains. At 25 mph, you pound em hard. I've taken to pounding broomsticks up the seatposts of my other bikes since then to use a seatpost rack.

The Delta Mega Rack Post Porter Seat Post Rack has a 40lbs max load version (non quick release) works best on my full suspension ebike. The mass is now a sprung weight so my bike handles better. My previous two Axiom Odyssee Rear Suspension racks added unsprung weight and bent at the same spot under the modest 27lb weight of my 48V 20 amp hr battery pack. I only ride on city roads so the previous axiom racks should have held up better.

Hi Doc;

Came across this review while looking for one on the Axiom I suggested earlier.

http://www.mtbr.com/cat/accessories/bike-rack/delta/post-porter-rear-rack/PRD_359129_98crx.aspx

Greg

some of the 20" folders have full suspension racks that are fixed to different points on the swing arm. thats way more secure than using the seat post on a suspension bike. i haven't seen it done on a 26" suspension bike but i can't see why not.

monster said:

some of the 20" folders have full suspension racks that are fixed to different points on the swing arm. thats way more secure than using the seat post on a suspension bike. i haven't seen it done on a 26" suspension bike but i can't see why not.

Click to expand...

ok, so i really think now that the best way to have a secure rack that have NO chance to break is to get a rack that fix direct to the upper swing arm instead of seatpost.

The bike i have, dont really have enough space to place some brace or tube that could be fixed to the frame to renforce the seatpost tube cause the pivot mecanical part that goes to the shock, obstruct the possible angled tube passage..

I know that if using a rack that is independent to the suspension and that is direct on the swing arm will get harder vibration to the rack and then to the battery but if wraping foam or packing bubble insulation around the battery that will certainly solve the problem.

having more time, that would be nice to try a seatpost rack that pivot with the swing arm like previously suggested.

Doc

Doc, on the drawing I posted, there's a lot going on. For that style of design, half the rack weight is on the seat post, and half is on the suspension arm. However, because the swinging post attaches to the back of the rack, and then angles forward, rear-suspension movement will cause the posts to swing in an arc.

If you attach everything to only the suspension arm, the battery rack will get sudden jolts as the whole rack moves up and down as you hit a bump. Any weight attached to the rear suspension will affect the way the bike feels. with ALL the weight on the rear suspension, the handling will be worse.

The optimum angle for the swinging posts is for you to fully load the bike by adding the batteries and also you sitting on it. From that frame position, the rear suspension will cycle a little up and down as you ride, but at rest, that angle is its most common position. So the angled support "swinging" posts should be connected to the rear-most rack point at the top, and angle down directly towards the rear suspension pivot.

Where the angled swinging posts are mounted to the rear suspension arm...if you move that point more forward, then hitting a bump will actually pull the rack DOWN! if you move that attachment point to the rear, a bump will lift the back half of the rack UP. The farther away you move the attachment point from the optimum position, the more rack movement you will get.

The idea of using a triangle to support the rear rack load (as spinning magnets, dogman and others proposed) is central to get a strong design. The triangle replaces the bending load on a single seatpost rack with pure pull and push loads on the triangle arms. Not only can tubing of a given size take more pull and push, but the more open angle the triangle has the lower is the load/force going to be.

Now the next problem is how to attach it to the bike. Mass market racks tend to use clamps, since they have to fit any bike, and there are no consistent brace ons on the seat tube or post.

A good solution would be to find or modify a rack to work with existing attachment points, for instance the rear suspension. Many bikes have a suspension linkage similar to the Specialized FSR

http://www.mombat.org/1994_Stumpjumper_FSR.htm
Here one could attach a triangle between the upper and lower suspension brace ons. On the upper it is as simple as using a longer bolt. On the bottom a challenge is that the bolt rotates. (The bushing is in the braceon). Not sure how to solve that easily.

These early FSR's seem like good candidates for a full suspension eBike conversion. They have good quality components, but being 10-15 years old they are relatively inexpensive now. In particular they have a decent size front triangle (where I like to keep some of my batteries for weight balance), and in general a more traditional frame design. I was thinking of buying a used one locally, so would be interested in your experience or comments.

On Doc's bike I would ghetto-up some angle-steel, to canti from the top tube rearward, then add as much lower buttress as possible. I might get a longer seatpost and brace to that.

Something like what Tyler drew is what I'd like to see built right into the frame, with gussets if needed to strenthen the area. In other words, the rack is integral to the frame. Monster cromoly dropouts front and back too of course, as long as I'm dreaming.

But back to more practial solutions, mabye a way to install a brace running up to the seat instead of down to the frame would work good enough. It would have to take up some space at the front of the rack, pushing the battery box back, unless it just attached to the top of the battery box itself.

Some kind of triangle is the answer for sure. Even very short braces would help a lot, just to relieve the stress on that weld.

Also, I think at least one person has posted a pic showing two racks, stacked one on top of the other, doubling the strength.

Spinningmagnet, I have the same rack installed on my bike. It feels solid.

ngocthach1130, where did you get those fenders? Thanks for the report on the that rack. If I got one of those and part of it failed, I would beef up that part stronger than the original because I like that design (rather than get a different one).

My plan for the next build is a rear hub on a full-suspension, and I want side panniers to hide the hub. Tyler Durden, I also like the tube-rail that you posted. Even though the triangulation is shallow, I have confidence the rack in your drawing would be more than strong enough.

Thanks for these exemples and experiences guys.. that help alot me to choose the rght rack.

But I need to better explain something here: This is not for my own mongoose ebike!... This is for the 7 ebike i need to assemble for my familly!!! I have 7 to assemble and adjust!

so i need the right rack.

The Bike I choosed for that is 7 mtb from costco: the model is Infinity Impact at 399$ CAD. designed iat Vancouver

I can't find a pics but i'll have the right costco SKU tomorrow.

Here is one that is VERY similat about the rear suspension desing:

Doc

spinningmagnets said:

Doc, on the drawing I posted, there's a lot going on. For that style of design, half the rack weight is on the seat post, and half is on the suspension arm. However, because the swinging post attaches to the back of the rack, and then angles forward, rear-suspension movement will cause the posts to swing in an arc.

If you attach everything to only the suspension arm, the battery rack will get sudden jolts as the whole rack moves up and down as you hit a bump. Any weight attached to the rear suspension will affect the way the bike feels. with ALL the weight on the rear suspension, the handling will be worse.

The optimum angle for the swinging posts is for you to fully load the bike by adding the batteries and also you sitting on it. From that frame position, the rear suspension will cycle a little up and down as you ride, but at rest, that angle is its most common position. So the angled support "swinging" posts should be connected to the rear-most rack point at the top, and angle down directly towards the rear suspension pivot.

Where the angled swinging posts are mounted to the rear suspension arm...if you move that point more forward, then hitting a bump will actually pull the rack DOWN! if you move that attachment point to the rear, a bump will lift the back half of the rack UP. The farther away you move the attachment point from the optimum position, the more rack movement you will get.

Click to expand...

Where this rack is availlable? they seems VERY interesting!

If your bike have brake boss for v brake that you're not using since you run disc brake, you can attach the bottom support of the rack to the brake boss. Just need to find the appropriate nuts and bolt to fit those holes.

So far, with a cantilevered battery on a rack, I have broken:

- A seatpost

- A rack...twice

- A frame (after I reinforced the other parts to the point where the frame was the weakest link).



The battery really needs to be in the frame triangle...like my Kona


And the handling sucks arse anyway with the battery on the rear rack.


Just don't do it.

I have to admit, I'm suprised I haven't broken the frame yet myself, with all that weight on the seatpost to the top tube weld. I don't ride slow commuting and sometimes even get airborne on the larger bumps for water drainage on the bikepath.

Seatposts are easily stiffened with a broomstick pounded into the tube.

I would think that a rack attachce to the swing arm would have to affect handling, just like too big a battery on the back does. With 15 pounds of lifeop4 on my rack it handles fine. When I add 16 pounds more nicads for range and maybe 5 more in tools and water, it handles pretty crappy and I won't go very fast like that. I do ride carefully when I have 35-40 pounds back there. Doc knows to put the battery in the triangle if you can, but for storebought batteries, this seldom works, and many mtb's have real small triangles. Two 24v batteries to put one in the frame reallly increases cost and complexity.

This shows a different frame, however the idea mentioned by tyler,dogman is to weld the rack so it becomes part of the main frame. In motorcycles, I see racks(permanent or not) are always mounted in the frame.

claimed maximum load: 40kg, not exceeding the total of 160kg(rider plus baggage front & rear)

Quoting the manufacturer:
3. Why has the panamericana a suspended rack?
Suspended baggage rack is one of the most significant aspects of our suspended travel bike. The
more weight is suspended, the better the riding quality and the function of the suspension. It can be
adapted to different load states and the suspension has always an optimum response.
4. Why has the panamericana an integrated rack?
The rack of the panamericana was integrated as it could be adapted optimally to the frame. This
special construction of the rack also serves to support the suspension and is therefore a fully
integrated part of the frame. The construction secures maximum stiffness in the rear triangle and
the rack.
5. How many fitting positions for panniers are there?
There is one fitting position by the height. The rack is provided with an inclination forwards to let
enough space for travelers with big feet. The rear tube of the rack has moved further to the back
than normal to reach a better support of the pannier.
http://www.tout-terrain.de/2/products/touring-bikes/panamericana/panamericana.html

Here's another one on a tandem bike, notice the rack is not attached to the seat tube/swingarm.

That's a nice tandem bike. I've alway wanted to have a tandem bike but never looked into it because they're all hardtail and build to look like road bike. But this is nice, with suspension and all. One thing that bothers me is in country like china and south east asia you have bikes that have racks which another person could sit on and yet here we can't build a rack to hold battery which weigh at a fraction of another person.