Full suspension frame mod - ideas

[veloman]

I picked up this $50 Specialized Stumpjumper at the bike swap meet today. Tons of triangle space for batteries, and what I think is an easily modifiable rear suspension.

My goal: 5-7" of cushy soft rear travel. I think it's about 2.5" travel stock as it is now. But with a larger shock and some easy machining of aluminum to replace those two small arms, I think this is possible. Pushing the ride height higher would be okay, as with more travel you need to start higher so you don't bottom out hitting the pedals.

Any ideas? I'll post a rough sketch of what I am thinking by tomorrow.

 

[dumbass]

WOW, now that's the frame I'm looking for. I found one I liked but but it wasn't near as good as yours. Would love to find one like that.

Bob

 

[DAND214]

Looks like you got a sweet deal. $50, around here that thing at a swap would be 2 to 3 times what you got it for.

We have a swap coming up the 21st, hope I can find somrthing decent for that money. Most of the good stuff is $300+
Dan

 

[veloman]

This thing is beat, needs a complete overhaul and new front shocks. But still a deal.

 

[motomech]

I don't think the chainstay dimensions would allow that kind of travel without hitting the frame.

 

[Drunkskunk]

Nice find!.

Angles are a bit off for a long travel, so it may be hard to get it to do more than 5" reliably, But I bet you can get that if you cut some new side plates. longer between the chainstays and the frame, and deeper so a longer shock can be fitted.


That bike begs for a Girvin Fork. I'd fit it with an air shock, but for cool factor, this would rock. 1" steer tube.
http://www.ebay.com/itm/90s-Girvin-...Cycling_Parts_Accessories&hash=item19cf58267d

 

[dogman dan]

1" headset? That will limit things quite a bit. What's the point of 5" rear when the best shock you can find for 1" headset might be 80mm.

I'd have bought it too, to make a street bike. For that the stock shock should be enough. Very nice triangle space for a big range battery.

 

[veloman]

1" headset? That will limit things quite a bit. What's the point of 5" rear when the best shock you can find for 1" headset might be 80mm.

I'd have bought it too, to make a street bike. For that the stock shock should be enough. Very nice triangle space for a big range battery.

I don't need much front travel, just rear. The weight of a rear hub bashing back up at me is still an issue at times. I thought with 5" travel, I could adjust to be really soft. Horrible for pedaling, but who cares.

I don't understand how I"m the only one who has issues with how much the rear hub beats you up, even with suspension. I'm running 25psi now on the big tire to help.

 

[veloman]

I didn't notice the 1" steerer tube.... that will limit my options a lot, damn.

 

[John in CR]

as-is should be a big difference compared to a hardtail hammering that hubbie up your spine. I don't find big travel much benefit on the street...gotta dodge those big holes suspension or not..

 

[dogman dan]

Well, I have to admit I do notice it a lot myself. Bad back. And now that you mention it, my Giant that I did most of my commuting on does have about 5 inches of rear travel. It was a huge improvement over the cheap mongoose FS bikes I had been riding.

It's not that I ever use the 5" on street though. I do set up the suspension pretty stiff with preload, so I don't have excessive pedal bob. But the occasional big bump likely does use 3" of my travel. Mostly it's just that the bike has a tunable suspension.

I find myself wondering if you still have some learning to do to get suspension tuned. Too stiff or too loose on one wheel will make a bike kick. I do it pretty cude myself, but it helps me. First load the bike with everything, the motor, the battery, and any cargo you may nomaly have. Get on the bike and while rolling slowly, bunny hop it. Both the front and rear suspension should respond the same. Don't have the front or the rear dipping more than the other.

Depending on the bike, you may be limited to partialy balancing it. This was the case with my earliest cheaper FS bikes that had non adjustable front forks.

The other thing bothering you could be that some of your bikes never really fit your body in other ways, I have that problem too. I'm long legged for my height, so I end up with handlebars uncomfortably low on most bikes. For me the fix was forgetting the tuck, and putting riser bars on the bikes. You may have something completly different that bothers you, but the problem may still be one that some kind of bike fitting adjustment can help.

 

[MadRhino]

I have 4 inch suspension travel front and rear on my road racer. With the bad streets here and all the potholes, it is still enough travel not to bottom it. I use almost all the rear travel, but only 2.5" in the front.

Your old stumpjumper has poor suspension compared to the recent ones, but it was calculated to work with the design and won't ride better if you mod it. Your best bet is to buy a better shock, as fat a tire as the frame will accept, and ride it as is.

 

[veloman]

as-is should be a big difference compared to a hardtail hammering that hubbie up your spine. I don't find big travel much benefit on the street...gotta dodge those big holes suspension or not..

But if you set 5" of travel to max out for something like jumping off a curb - imagine how cushy that would be over normal bumps. It's not the potholes that bother me, I can avoid them. It's the many small bumps that my suspension nearly doesn't move at all on.


Oh, this steerer tube is 1 1/8, not 1". I just need to get a threadless headset and stem.

 

[dogman dan]

Well that helps. I tend to agree, replace the shock if it's possible and put a nicer fork on it.

Sounds like eventually, you'll need a better bike. Didn't you have a specialised FSR? I can't picture one of those not plush enough. Mine rides so creamy I nearly need depends underwear. Mine is about 03, FSR XC. I think it's only about 3 inches of travel, but the rear suspension is very nice. Some kind of marzocchi air shock up front.

This is my fsr bike, similar to the one Oatnet put a motor on.

 

[veloman]

My current ebike is a KHS (see signature) same design as the FSR you have. I put a lighter spring on it too. It is pretty soft, but I still feel the bumps due to the hub weight. The worst is when I'm coming back from the grocery store with a heavy backpack and on some road roads. Even standing seems rough on it. Maybe the pivots just have too much stiction.

I may just leave this frame stock. I played around with it and some other shocks today, it's going to take a lot of work / new parts to get it to work out. I decided it is not going to be an ebike. It's significantly harsher than my other FS bikes. Could be a good pedal bike for the trails once I tune it up.

Still searching for the perfect, affordable frame...

 

[Russell]

I don't need much front travel, just rear. The weight of a rear hub bashing back up at me is still an issue at times. I thought with 5" travel, I could adjust to be really soft. Horrible for pedaling, but who cares.

I don't understand how I"m the only one who has issues with how much the rear hub beats you up, even with suspension. I'm running 25psi now on the big tire to help.

No you're not the only one experiencing discomfort caused by by bad roads. What I really hate is when the back end drops suddenly due to sharp pavement breaks. I've tried steel frame, bigger tires, lowered pressure and front suspension and it helps but still I have this one stretch of road that annoys me with the back end going bang, bang, bang over the broken pavement. :x My present ebike (w/front motor) is the best yet over this road but my next build will have a rear motor so I'm going to use (a cheap) full suspension bike...combined with big tires and low pressure. :wink:

-R

 

[MadRhino]

It is very complicated to tune a bicycle suspension with a heavy hub motor. Most shocks are not up to the task in the first place, and mods to the linkage require a lot of try and error, even for someone experimented.

 

[dogman dan]

"With a heavy backpack" Bing! Well there's yer problem. It's not a bike issue, it's the pack pounding you to death.

They have these great things nowdays, they are called panniers.

 

[John in CR]

You guys are just spoiled. I'm still riding my hardtail everyday, with a heavy backpack. Just like a car on a gravel road, the faster you go the smoother the ride, just gotta avoid holes or bumps big enough to buck you off. Another benefit of going fast is that I'm not on the bike long enough for the backpack to become a bother. Then there's the safety factor, but once you start matching their speed you won't be able to comfortably go back to trusting every passing car not to be texting, spilling their double mocha latte, or trying to comfort a screaming kid, etc at just the wrong time.

John

 

[MadRhino]

You guys are just spoiled. I'm still riding my hardtail everyday...

Well, I have to live with the consequences of 45 years training horses. My back deserves some suspension, and i hate carrying a backpack for the same obvious reason.

 

[veloman]

A heavy backpack just feels like it's compressing me onto the bike.I can't be as nimble and light on the pedals/saddle.

John - do you have mirrors? I love my left side mirror. We have a lot of bike lanes here and to not draw attention I use them at a reasonable speed - mid 20mph range usually. Cars pass at 5-15mph and I can easily watch them in my mirror. Without the mirror - forget it - you really are trusting drivers way too much. Of course, speed is always good. The problem is that I don't think I'd get away with going 35-45mph all the time on a bike. Cops are pretty hard on cyclists in my city as it is. Though a fast ebike is a different story. They usually ticket riders for going through stops/lights, not riding too fast.

 

[amberwolf]

Teh problems mentioned above are pretty much why I am working on the replacement bike for CrazyBike2, simply because useful suspension of a heavy wheel is difficult (so far seems impossible to me with what I have avaialble), and putting the motor in the frame driving the wheel via chain (or belt or whatever) is about the only way that works well to let the suspension do it's job properly.


Moving CB2's heavy hub out of the front suspension fork to the unsuspended rear let me test whether it makes a difference to the suspension operation, and it absolutely makes a huge difference on both big and small stuff, even with a crappy 50mm travel spring/elastomer Suntour M2000 fork.

Unfortunately it makes the already obscenely heavy rear end of the bike even worse on bumps and holes, but overall the bike still feels way better than it did. Adding suspension to this frame is possible but kinda pointless, as the frame itself twists too much for reliable chain drive--as tested with my original powerchair motor drives, either the motor chain or the pedal chain ends up jumping off pretty frequently, and often ends up destroying drivetrain parts.

That flexibility does also give me some suspension in the frame itself, though, which you don't really get in a regular bike design.


If there's any way for you to do it, I'd recommend sticking the motor in the frame and driving the wheel with a chain or belt, as it will allow the suspension to react as designed to bumps small and large. If the suspension is made to do it, it'll be able to--with the hub in there it can't really move fast enough to deal with things properly (inertia and all that).

Doesn't have to be anything other than a straight 1:1 left-side drive, doesn't need to go thru the derailer or bike gears....just taking your hub and moving it to the frame will probably fix the suspension problems, if the suspension was capable of doing what you want in the first place.

That you can test by loading up the frame itself with the same weight as the batteries, motor, you, cargo, etc., and riding pedal-only with it, using regualr bike wheels rather than the hubmotor wheel. If it now responds like you want, then it'll work once really setup that way.

 

[veloman]

AF, you have the right idea. The key is putting the weight on 'your' side of the suspension. Think luxury cars, they aren't 4-5000lbs for no reason. It improves ride quality to have that mass on YOUR side. Simple physics. I notice if I put weight on a rear rack it helps a little. But yeah, getting the motor out of the rear wheel would help a lot. I am on the lookout for a good mid-drive motor, I'd hate to take apart my brand new wheel that was just built. There would be a 2-3% loss in efficiency due to the drivetrain, but ride quality would definitely be better if it's a FS bike.

In the mean time, I am running low psi for most riding where range isn't critical. Like 20-30psi, just enough to not get pinch flats and not be sloppy.

 

[Drunkskunk]

Looking at pics of your current bike, you may see some significant improvement swapping for an air shock.