Extended swing arms / longer wheel base

[zombiess]

I have a full suspension bike and the wheel base is pretty short. With my new higher power settings it really like to pick the front end up so I was thinking about having a friend extend the rear swing arm for me. I was thinking 6" longer would make a huge difference, but I'm looking for input form those who have done this.

I'd have to measure my exact wheel base to see what it is right now to give you guy a better idea, but with the controller limited to 45A battery current and me leaning all the way over the handlebars it can still wheelie me off from a low speed at 110% throttle setting.

This is a 2806 motor in a 20" wheel.

 

[nebriancent]

myself i might error on the side of caution as by extending your wheel base by chopping your swing arm you will have more mechanical stress at the point it meets the rest of the frame and you may also at that point have to change out your rear strut

besides to me having it go into a wheelie at wot doesn't sound that bad .. at least you can show off to the baby boomers that a ev can have a sh%tload of torque

 

[John in CR]

I've extended the swingarms on 3 bikes now. One just 3" or so with the saddle moved as far back as saddle linkage allows. I hate that bike. The other 2 worked great with longer extension and moving the saddle rearward by at least half of the swingarm extension. Lowering the saddle is important to my ride taste too. The only real issue I've had is addressing the increased leverage on the rear shock.

On this cheapie bike extended by 8", we just left the crap shock on there screwed down as stiff as possible. It was fine for street riding, but very bumpy roads left the 25lb rear wheel airborne too much. LFP and I had lots of street fun on this bike at 120A/300A. The extra force broke the frameside attachment of the rear shock, so I hacked up the bike for parts, a mistake I still regret.
View attachment 1

For my SuperV I built a completely new swingarm about 9" longer than the original. I increased the height at the front of the swingarm in the same proportion as the increase in length, so the mechanical leverage on the shock would be the same, and made several connection points on both the frame and swingarm for fine tuning later. The swingarm itself is a bit heavy made out of angle iron for easy welding for me, and the entire assembly with wheel motor and battery weighs over 60lbs. To me it rides like a dream after fabbing the seat extension to move the CG lower and rearward, and the Fox Float air shock seems to be holding up fine. The only thing I'm not happy with yet is the handgrip position...stem forward was way too far forward, and stem rearward which seems to drive everyone else nuts puts my hands a little too far rearward. I need to modify the stem, which will be easy, since it's essentially a big solid block of aluminium. Something not visible is that the tremendous forces of the shock on the main tube were beginning to deform it despite a repair by the previous owner. My solution was I solid plug of epoxy and fiberglass inside the main tube centered at the shock attachment point, so I could jump the bike out of a plane now without hurting that part of the frame.

 

[zombiess]

My rear shock shouldn't be an issue. It has an 850lb spring and I have it set as light as I can get it without it going slack just to get about 20% sag. The longer swing arm should fix that and let me get more sag + more travel. I've got a few ideas, but I'm thinking I'll go with 6" over and see how that does. Power wheelies are ok as long as they are controllable, but this comes up too fast for at 25 mph if I'm not leaned over the bars.

I'm going to lower the seat 6 inches for racing so that should help too. I'll give that a shot tonight.

Over all, I'm just really surprised by the power this 9c 2806 in a 20" wheel makes with a 45A limit. Haven't even been able to get it very hot yet, windings look new, needs more abuse.

My friend who will be doing the mods are familiar with the stresses involved as well as proper geometry. They like to build some crazy stuff (turbo liter bikes, turbo LS1 sand rails, roll cages, etc) and fab really nice safe stuff.

 

[nebriancent]

that is some nice professional looking work

i guess you have taken all the mechanical aspects into consideration and the results are impressive

being a millwright (mechanic metal fabricator plumber welder electrician carpenter etc.) myself i would like to think i have a bit of a eye for it

 

[spinningmagnets]

I put the rear swingarm from a cheap full-sus bike onto the rear drop-outs of a hardtail with front suspension. It was pretty easy, just find two bolts with the same threads as the pedal retainers, add a couple washers, and it bolts right in. There are several ways to secure the front of the shock to the rear of the hardtail, but it would be specific to the particular parts that you choose.

Dogman made a longtail full-suspension, he kept his seat fairly normal, but mine sits lower and farther back. He has stated that it was very easy for him to modify two frames to fit together, and I can verify that mine was very easy. I specifically used only a drill and hacksaw to make the custom braketry in the shape that I chose. dogmans method was clearly easier, and I would use his method if I did it again. Its really nice having my feet flat on the ground when I come to a stop.

 

[John in CR]

that is some nice professional looking work

i guess you have taken all the mechanical aspects into consideration and the results are impressive

being a millwright (mechanic metal fabricator plumber welder electrician carpenter etc.) myself i would like to think i have a bit of a eye for it

Thanks nebriancent,
I think I got the geometry right, at least it feels good at all speeds, and the suspension compresses freely with no binding, so pivots are aligned well. Cannondale offset the swingarm pivot to one side to make room for the chainrings, so that was a real bear to nail down, however, the low res pic hides my noobish metalwork that took a lot of angle grinder time to smooth out. It may have a couple of pounds of excess steel, but at least I know it strong. I even have 20mm wide pieces of leaf spring steel welded horizontally in the dropouts to form a wide contact surface that clamps to the motor axle so I'm confident that axle will never spin in the dropouts.

It was a great learning experience, and now I'm ready for a full scratch build with a much lighter end result using a similar design with plenty of battery capacity built into a slightly larger somewhat rectangular main and seat tubes. I definitely won't go with the offset rear pivot, and having the main batteries in the frame will enable a high power mid-drive and get that 30lb anchor out of the rear wheel and spring some of the motor weight instead of 100% unsprung for moto-like off road capability without all that weight.

 

[amberwolf]

My CrazyBike2 is rather an extended wheelbase, but no swingarm. The new one will be, but no hard data yet as it's not finished enough to ride.

I found this one:
http://www.forum.freakbikenation.com/phpBB3/viewtopic.php?f=17&t=3814&st=0&sk=t&sd=a
but hasn't got ride data posted yet.

(which I am probably going to copy most of just to see what it is like, as I already have almost everythign needed to build it, except a working welder).

 

[nebriancent]

i got one welder at home and 4 at work.... yay welding
im going to have to play some w/ the mig at work... see how it likes aluminum (broke off a v-brake mount in the rear)
see if i cant make a spiffy aluminum battery box for the doc makita pack im building

 

[nebriancent]

only thing you need to work on on your bikes now is a crash resistant battery box for your packs (duck tape thing robs from the overall beauty of the other fab work)

 

[dogman dan]

That is a beautiful bouncer to copy. But the one thing I don't like about it is the cheezy cheap front section fromt the next bike. I'd prefer a frame with more strength from side to side. I have found tail wag to be the biggest problem when loaded up with cargo. I was pleasantly suprised how lateraly strong mine turned out, having started with a traditional frame front section, and having left it uncut.

Of course, it may be stronger than it looks. It's steel, and all the cheap bikes that wag real bad I've ridden were aluminum.

A more current pic of mine.

 

[nebriancent]

how do you find you kick stands..
i had mine fold up on me a few times..
dumped my bike about 10 times b4 i dumped it w/ me on it
so i guess that makes 11?
i ended up making a A arm kick stand for my bike so i didn't have to worry about it anymore
and it makes it easier to get at my wheels as one is always off the ground

 

[John in CR]

I just use motorcycle kick stands. More than strong enough, and nice strong spring, so no rattling noises on even the bumpiest roads.

 

[dogman dan]

So far, I find the double stand to work the best for me. In that pic above, you can see both needed to be lengthened. On other bikes, I've used the stock stand behind the bb, then added one of the wallbike stands that bolt on to the rear wheel axle. There are similar ones that bolt to the chainstay also.

Others swear by the Y type stands, but I haven't found one yet on a 10 buck flea market frame. I really do a lot of my parts shopping by buying half a bike at the flea.

 

[Pure]

My only gripe about most extended bikes are the lack of upgraded suspension. I mean, if you are going to go through all the trouble of fabing up a new swing arm, you might as well upgrade your suspension wile at it. As John found, when you mess with the geometry you put added stress on the stock shock. Then add to that the added weight of the swing arm extension plus the batteries plus our motors, and you really need to look at some sort of smallish motocross type of shock.Something like this with a little lighter spring in it would be perfect. http://www.ebay.com/itm/11-280MM-REAR-SHOCK-REBOUND-FASTACE-58AR-PIT-DIRT-BIKE-/360404510406?hash=item53e9c866c6&item=360404510406&pt=Motorcycles_Parts_Accessories&vxp=mtr#ht_10562wt_1397

Stock bicycle shocks are just not meant to handle the load we put on them with our bikes. I know my 90 pound bike plus my 250 pound ass has my stock shock at it's max dampening ability and I have it set to it's stiffest level of adjustment.

 

[Gordo]

Then add to that the added weight of the swing arm.....

How does the weight of the swingarm get added to the problem? It is UNSPRUNG weight sitting on the axle. Not effecting the shock/spring as far as I can see?

 

[Pure]

True but the total mass of the swing arm plus the (in my case) 30 pound hub motor, would probably prove to be too much abuse on smaller shocks designed for much much less weight on a standard bicycle.

 

[nebriancent]

View attachment 2
fabbed my kickstand out of a lil square tubing, 2 lengths of 1" pipe and some misc hardware inc a spring i had from a old kickstand
View attachment 1


ps excuse the mess in my shop i haven't had much time to clean it up w/ my new battery build ect

 

[Gordo]

True but the total mass of the swing arm plus the (in my case) 30 pound hub motor, would probably prove to be too much abuse on smaller shocks designed for much much less weight on a standard bicycle.

The swing arm and the hub-motor do not push up on the shock. The weight of the bike MINUS the UNSPRUNG weight pushes down on the shock. That is why it is call "unsprung." The 30 pound hub-motor has no more effect on the shock/spring than the weight of the pavement under it. Think of adding 100 pounds to the rear wheel? Does the seat go down? ONLY the SPRUNG weight is held up by the SPRING.

 

[Gordo]

The only real issue I've had is addressing the increased leverage on the rear shock. On this cheapie bike extended by 8", we just left the crap shock on there screwed down as stiff as possible.
View attachment 1

Hey John,
From looking at the new seat position it appears you have moved your 200 pound arse back @ 10"? This has almost doubled the lever arm on that poor little spring.
Would that not mean you need to increase the strength of the spring by an equal amount? I have a vision of the seat snapping off, landing on the LIPO which bursts into flames. :lol: :lol:

 

[nebriancent]

True but the total mass of the swing arm plus the (in my case) 30 pound hub motor, would probably prove to be too much abuse on smaller shocks designed for much much less weight on a standard bicycle.

The swing arm and the hub-motor do not push up on the shock. The weight of the bike MINUS the UNSPRUNG weight pushes down on the shock. That is why it is call "unsprung." The 30 pound hub-motor has no more effect on the shock/spring than the weight of the pavement under it. Think of adding 100 pounds to the rear wheel? Does the seat go down? ONLY the SPRUNG weight is held up by the SPRING.

^^^^--- agreed

 

[Gordo]

It worries me a little when one who does not understand unsprung weight begins calculating the effect of the mass of the unsprung weight, on a suspension system.

When I see the "F" means I am F**KED as far as any further understanding of suspension behaviour goes! :lol: :lol:

 

[Pure]

So when we hit a bump the rear wheel in no way pushes up?

 

[nebriancent]

nope its the bike that pushes down ..
if for sake of argument you were riding upside down then yes i could see the hub motors weight affecting the spring but.. we don't commonly ride on our heads

one example could be

take a spring and put a 20 lb weight on it and watch how far it will compress

now take that same spring and weight but first put the spring on top of another 20 lb weight and go to setp one

do you think that this will change how far the spring compresses?
if so we are all f%#@ed and must take into consideration the weight that the planes has on said spring and go for something a hell of a lot bigger

**EDIT**
i meant in no way to belittle or offend anyone by this post
it is sometimes said by those who know me well that i can be a little blunt and in your face
thats not how this is intended

 

[John in CR]

Yes, moving the saddle rearward increases the leverage on the spring. No, the red bike's saddle extension isn't going to fail, because after bending progressively stronger reinforced tube I ended up making it solid with a small triangle at the bend.

Also, the rider's weight is supported by the spring. That weight is "sprung". The weight of the swingarm and wheel is "unsprung weight" with different amounts of leverage which is at a maximum at the wheel end. That weight absolutely does have a significant effect on handling, not on smooth roads, but over every bump and through every hole. The lighter that entire assembly is the more travel is possible in the suspension while still keeping the wheel in contact with the ground the maximum amount of time. Having a lot of unsprung weight will put the wheel in air more.