Bike Suspension Design Help

[Drunkskunk]

I'm trying to design and build my own frame. Its a stretch of my abilities and tools, but sounds fun.
But I've run into a snag and would love some advice.

I need a DH worthy rear suspension pivot design that can be made of steel, and built from basic hand tools and a welder. I don't have access to any percision machining equipment. I've come up with a few designs in my head, but most are overly complicated or couldn't be built with the tools I have.

I'm probably overthinking this, but I need it to withstand some extream abuse, and still be light as possable.

Anyone built or designed anything like this? Anyone got any ideas?

The bike will be a Downhiller style full suspension, running 4" wide tires. I'm going for a minimum clearance of 110mm in the rear suspension arms. it will be running a 5404 at 100v so needs to be stable at 60mph, down a rocky mountian side without any flex or give.

Any thoughts would be appreciated.

 

[Lenk42602]

ds,

might wanna check out Farfle's builds, he fabbed up a few swing arms for a commercially available walmart fs bike.

also take a look at the build "evolution of skill" (mine)

or Hal's trellis swinga ar on his albino.

I went with mild steel - so fairly heavy, but robust.....

Len

 

[AussieJester]

I would be looking at snagging the suspension pivots off a donor a frame, you could replace the chain and seat stays with designs of your own but use the donor bikes linkages, like how Farle has done with those swing arms Lenk mentioned... Get a frame jig of some sort made up, you will struggle to get the frame straight/true without one if you haven't done alot of metal fabrication in the past...take it slow measure twice before cutting anything and you will be fine, its really not that difficult just need to get in and do it Best of luck *subscribed*

KiM

 

[MadRhino]

...
I need a DH worthy rear suspension pivot design that can be made of steel, and built from basic hand tools and a welder. I don't have access to any percision machining equipment. I've come up with a few designs in my head, but most are overly complicated or couldn't be built with the tools I have.

I'm probably overthinking this, but I need it to withstand some extream abuse, and still be light as possable...

The design of a rear suspension for DH purpose is a very complicated task, and the fabrication of this design has to be precise to make it very stiff laterally, with fluid vertical progression.

All the major players of the DH racing industry, have yet to come to a design that is solving all the problems that need to be addressed.

The main problem is that in a downhill situation, the rear is not loaded and very little weight is available to trigger the action of the suspension. It is easy to understand that, if a suspension is made to be efficient to absorb hard hits with extreme load, it would be naturally ineffective to absorb small bumps when unloaded.

You are very unlikely to build a DH worthy rear suspension, unless you copy one that has proven good and put alot of time into building it precisely. Most bicycle companies can't produce a DH worthy rear suspension, or aren't willing to invest the time and money to do it.

 

[dogman dan]

Gonna go ride the stairway at the basketball arena? I'd say Farfle is on a good track with a simple steel swingarm based on motorcycle designs, using a decent DH rear shock. I don't see anybody complaining about the similar greyborg suspension.

But the 4 bar suspension link common on mongoose blackcomb, and Genesis wallbikes should be pretty easy to copy.

 

[fractal]

Try this software, it is specifically made for designing rear suspension bikes. http://www.bikechecker.com/

 

[MadRhino]

Try this software, it is specifically made for designing rear suspension bikes. http://www.bikechecker.com/

Very interesting. For someone who knows the goals to target, this is a good tool to help.

If you copy a system, please copy one that proved a winner. Very few frames are successfull DH racers and among them, you can count on the fingers of one hand, those that could keep winning more than a single season. Rear suspension design is the main reason why some frames are winners, and others are losers in the hands of the same riders.

If you plan to ride the stairs like Dogman says, you don't need a suspension design that is DH worthy. People are happy with wallbikes suspension, or motorcycle style swingarm designs like the Greyborg, because they don't ride them DH or never tried better at any speed.

Fortunately, the suspension components manufacturers have made their part of the work already, and the high end DH shocks can compensate alot for a poor suspension linkage or frame design. This forgiveness though, will never be enough to make a bad frame, a winner.

A good pivots and linkage system design has to be matched with an existing shock action characteristics, in order to complete and improve its functions. All the work that a shock has to do to forgive a bad suspension design, is work that is not available to the bike's handling. Then, all the work that a good suspension system is doing to complete and synchronize its action with the shock action properties, is a neat advantage to the bike's handling. This finds its best evidence in a downhill frame's cornering ability on roots and rock gardens. This aptitude to absorb a series of small bumps when the rear is not loaded, yet being capable of taking the big hits of DH racing, is the trademark of a winning design. It is easy to make it stiff to take the big hits, it is easy to make it smooth and linear to keep traction on the rough, but it is very complicated to make it good at both. The easiest is to make it bad at both, rely on the shock itself to do all the work, and hope that the rider doesn't know better.

 

[fractal]

check out this design. It is a home made dh frame. This guy's design is spot on. He decided to move up the pivot point on the rear swingarm to make it work in conjuction with the fork so the bumps feel the same on the front and on the back. He's probably not the first one to do this but its very clever.
http://www.pinkbike.com/forum/listcomments/?threadid=39840&pagenum=526

 

[MadRhino]

This high pivot design was made some 13 years ago. Balfa had some frames with this design, we were talking about it with John last week.

This was a 10 in travel that could take big drops, but never been a winner. They made it from steel here in Montreal for a few years, then went for alu without more success, before the company was sold and moved to UK. Other companies have tried this design in the following years without racing success.

 

[Drunkskunk]

Thanks for the ideas so far.
I've got Len's build tagged for reference, as I'm doing something roughly similar. Its a great build.


This bike will have a 35lbs motor and another 5 pounds of brakes, dropout, rim, tube, and tire all on the steel swingarm. with 40lbs of unsprung weight it will eat a walmart bike bushing for a snack.

I'm getting some ideas from this, and more for the swingarm. I Downloaded the free version of that bikechecker and played around with it. good stuff. Those links are giving me more ideas too.

And stairs, yeah. and more.
Here's a rough sketch of what I'm planning so far with the frame. ignore the tires and fork, I'm going for something much beefier.

 

[dogman dan]

Stairs at the arena was a joking reference to the terrain in Dallas. I'm sure though, that there are steep places and hucks to drop off of aplenty.

 

[deVries]

The bike will be a Downhiller style full suspension, running 4" wide tires. I'm going for a minimum clearance of 110mm in the rear suspension arms. it will be running a 5404 at 100v so needs to be stable at 60mph, down a rocky mountian side without any flex or give.

What 4" tires will you use -rim?

Where are you attaching the motor? Mid drive or on that swing arm?

What is the exact terrain the bike is *really* going to be used on & how often?

What is the average speed range the bike will typically be riding at & to do what purpose exactly?

How good are you at welding CrMo or Mild Steel?

 

[Drunkskunk]

Well, we have flat land, and more flat land, and some cows.. Most of my riding range locally is flat. But we do have Erwin park, and a few BMX parks near by. And I travel with Family all over the country. I plan to be riding this bike down ski slopes in the Rockies and the Cascades, and over beaches on both coasts.

Basically this bike will be a jeep. I'm taking everything I learned from my Kona, and trying to improve it.


> Devries The motor will be on the swing arm. The rims are Snowbike 65mm, Same size and build as a Surly Large marge, but without the machined sidewalls.
The normal speed will be >30mph, but I want stability up to 60, I'll hit that going down my father's road from gravity alone. Brakes will be as important, but this pivot point is, well, pivotal to my success.
I can weld mild steel. No where near professional level skills, but my welds hold. I've done everything from an engine cradle for a custom truck application to welding the ends of paperclips together. I've welded up parts of bike frames, but never a whole frame.
As for chome moly, never tried it. I've got a wire feed welder, not a tig. It will be tricky but I want a challenge.

 

[MadRhino]

From what I see of it already, you have to design your linkage to fit the longest shocks that are readily available on the DH components market. That would be 9.5 X 3.0, longer than that is possible but expansive, with very limited choice.

So, let's say that you use a 9.5 in shock with 3 in travel, considering the wheel weight that it will have to tame, you'd have to design the linkage to multiply its travel by no more that 2, that would give you 6 in travel rear suspension. It is the most that you can reasonably expect to work good using a (common, high end) DH shock. For rough DH riding at speed, one would want 8 in travel, but you'll be better with 6 in that is tuneable, than 8 in that will be demanding too much from the shock.

Then, you can make your linkage and pivots design to act as a virtual cam, to make it progressive. It could start multiplying 1.5 times the shock travel, up to 2.5 times at the peak (that would be 3/4 of total travel), and back to 1.5 at the end of the course. This would let the shock have a lot of force at the begining and end of course, thus extending the effective rebound tuning range and bottom out resistance. This is a way to make it good for 8 in travel, using a commonly available DH shock with the weight that you have to deal with. This would also give you a true progressive suspension feel that you've never experienced with your Stinky. :wink:

The other (simpler design) solution to have 8 in travel with that weight, is to spend 1K for an 11.5 X 3.5 DH shock. It would be easier, and eventually cheaper to use a motocross shock, but those are not very good for DH purpose.

 

[deVries]

Basically this bike will be a jeep. I'm taking everything I learned from my Kona, and trying to improve it.

The motor will be on the swing arm.

On the side or centered in front of the tire?

I'd suggest practicing welding some CrMo from "trashed" bikes. Maybe get some frame junkers to practice on. Then just use a single width wall tubing not double butted, which will be more forgiving to weld. Consider brazing with lugs. Otherwise...

If weight isn't such a big consideration & you're willing to build a few frames & you want to save money initially till you know what you're doing & you have the time long-term, then go for mild steel.

I would check with Len to see how he handled the jigging & what tolerances he got doing it, and find out if he would suggest some better method next time.

Len's bike could be called a jeep... super heavy rear suspension, overbuilt mild steel, so I would key off his design. He is very happy with it's performance. You might wait and see what Phasor(?) will come-up with their CrMo design too, since it will be based on a DH FS concept too.

You are certainly one of the long-term experienced eBike dudes here on ES, so it's going to be fun to see what you come-up with. That CrMo frame Etard found recently... see his post about it... the one with the carbon fiber swing-arm is a beauty... & that swing-arm was supposed to be CrMo originally.

Personally, IF I could idealize your dream DH jeep bike situation, then I would learn building on cheap steel doing a learner frame to test concepts & obtain skills jigging & alignment. Then do a second bike using CrMo IF I had proven I knew what the hell I was/am doing. :lol:

Hope you take this challenge w/build thread, as this is the ultimate "total build". 8)

 

[Hangdog98]

Great to see you developing this off-road E-Bike. I have developed rear suspension on DH bikes for some unusual clients with some success. The issues for you, as I see them is the differences in suspension forces in leg powered designs Vs motor powered design. Unlike the competitive DH designer, you have the benefit of silky smooth torque from a motor instead of the jerky motion of a leg pumping athlete.

One of the main challenges facing MTB suspension design is the compromise between supple travel and the isolation of pedalling forces which act on the swingarm. Those giant pistons attached to your hips really pump out some torque in big lumps and the pivot position of the swingarm relative to the chain-pull can turn the whole affair into an inefficient bucking pig, yet while coasting the same swingarm allows the bike to bomb downhill beautifully. That is the compromise. All those fancy linkages on DH-MTB's are designed to isolate pedalling forces from bump forces.

I would suggest that, assuming you will be fitting a powerful hub motor, that you won't be pedalling too much. That being said, you should be focussed on optimizing for motor power, not leg power. This means dismissing any notions of four bar linkages, Horst links (FSR link) or floating, virtual multi anything. A simple single pivot is what you want. You may want to make life harder by creating a link for the shock absorber but in all honesty, a progressively wound spring will get it done on a really good shock. I would put the swingarm pivot on the frame close to, or just above the top dead center of the chainring (I assume you're only using one). I say this for when you are pedalling so your leg forces aren't pulling the swingarm up or down too much.

Your biggest challenges will be taming the unsprung weight of the rear wheel/hub motor and stopping the powerful rear disc brake from causing the wheel to hop all over the place under brakes. I had a similar (but easier) challenge to dial the suspension in on a Downhill bike equipped with a Rohloff 14 speed hub for the National DH Championship round at Mt Buller back in the day. The extra weight of the Rohloff was really noticeable over the bumps and required a change to a much harder spring from 450lbs to a progressively wound Eibach spring topping out at 600lb with modified internals and higher viscosity oil. A big rear DD hub will require even more. For this I would suggest a motocross shock absorber from an 125cc size bike. The springs will probably be cheaper and easier to obtain than MTB-DH springs as well as you may need to do some experimenting with spring rates.

The next challenge is the rear brake. Many motorcrossers use a floating rear caliper to arrest rear brake chatter. This is where the movement of the swingarm, usually as it compresses, draws an arc that shortens the wheelbase a bit. The net effect is that the rear wheel must speed up a little over the front of a bump and slow down on the other side. The floating caliper with its linkage allows the caliper to move independently and smoothes out the braking quite nicely. You'll have to make this set-up or modify one from a small motorcrosser. I'm not sure but I think some Honda brake calipers are narrow enough on the inside to fit between a Clyte hub and rotor. You have the luxury of making the over locknut dimensions as wide as you need so this won't be a huge challenge.

After reading all this you may have come to the same conclusion that acquiring a small motorcrosser swingarm complete with brakes, shock and all linkages would be the easiest way to set this up. It would probably be no heavier than anything you might make from steel too. The downside is that the brake/sprockets are on the opposite side but that's half the fun 'eh.

 

[MadRhino]

Interesting input Hangdog98

Some misconception though: Suspension linkage systems on DH frames are not meant to make them pedalable, for we don't pedal DH bikes much. DH bikes are made to be climbed at the top on the chairlift, all of them DH racing suspensions are bobbing alot and terribly inefficient for pedaling. The suspension design on DH bikes, is made to make them working good in downhill racing situation, where the load on the rear whell varies from very light to very heavy. No motorcycle style, single pivot suspension does come close for this specific purpose.

I believe you are confusing with XC bikes. Those are made to pedal, and their suspension design has no progressive linkage. Most are monopivot, and their pivot is set as close as possible to the BB. They have stiff suspension, not applicable to hard and fast DH riding.

Then, by your reflexion about "giant pistons" leg power, I guess that you have no cue about the torque that is applied to the swingarm by an overpowered hubmotor. That is enough to spin through the dropouts of any DH bike, and when they are reinforced with thick steel torque plates, can flip the bike and rider upside down. Pedal bobbing is the least of our concern, for one can't apply enough leg power to make it bob, as long as the motor supplies torque.

Then, the spring weight has to be selected for the weight of the rider and bike, not the unsuspended weight of the hub. It is the rebound control of the shock that need to be adapted for the weight of the hub, thicker oil may help but this is not a practical, conveniently adjustable solution. The shock need an external rebound control adjuster that has a wider/higher range of setting, and the suspension linkage can be designed to help in that matter.

Using a motorcycle swingarm assembly would add alot of weight. The benefit of a motorcycle floating rear brake is minimal, for the rear brake is mostly useless when riding DH, and with a DD hub motor that can give stong regen braking. There are floating brakes for DH bikes, but we use them in the front mostly.

No doubt that your suspension design suggestion is good for dirt riding, likely to be very good at junmping too, but not for DH riding IMO.

 

[Hangdog98]

That was a disappointing and condescending read Mr MadRhino. My experience in developing DH bikes for MTB World Cup competition and having had my designs adopted by industry giants such as KHS and boutique firms such as Lahar, as well as having competed at the elite level suggests to me that I do indeed 'have a clue'. I also have acute knowledge of the dynamics of suspension on mountainbikes as well as Superbikes and Supercross bikes by working for many years in racing teams at the highest level. I also have a full understanding of the torque forces acting upon various parts of the DH bike suspension and the differences between DH and XC having managed teams in both disciplines at UCI world cup level.

Sorry that I wasted your time with my clueless input MadRhino. If DrunkSkunk would like some advice from a professional he can always PM me.

 

[MadRhino]

That was a disappointing and condescending read Mr MadRhino.

Sorry that you feel that way, maybe translation made me use inadequate expressions.
I hope it's not for the only reason that I disagree with you.

My experience in developing DH bikes for MTB World Cup competition

Surprizing that you say that suspension designs that made multi DH world cup winning frames, are not suitable for an Ebike that is meant to ride DH, then suggesting to build with a much heavier motorcycle swingarm.

Sorry that I wasted your time with my clueless input ...

You didn't. Any input is appreciated and open to discussion, when the poster is willing to discuss and give arguments to explain his solution. Experience is not a valid argument. I would have liked better to know why adding the weight of a motorcycle suspension can help a DH ride, because everything I built and ride tells me that the next one has to be even lighter.

 

[Drunkskunk]

All good points, even when they don't all agree. After reading ya'lls responses, I've thrown out every preconceived idea and plan for this swingarm and pivot point and restarted, twice now. I think I'm back to the drawing board yet again.
I thought about a motorcycle swingarm, but it won't fit my rim and tire size.

Anyway I go at this, I'm still stuck with one fact. I have an 80lbs bike with 40lbs of unsprung weight on the rear end. Its never going to be a great performer. I just want it to be better than a simple bike conversion.

 

[RWP]

I've got to say that for someone, like myself, who has no real experience designing bike frames let alone full suspension, the idea of a simple swing arm with no links and a quality spring/shock combination sounds just the ticket for a first project.

 

[panurge]

Hi Guys

Unfortunately, My bad English don't allow me to explain extensively what I could and would, from my experience in building DH frames, swingarms and even shocks.

Luckily I have the really pro HangDog98 post to XTRAQUOTE.....

What I can say easily and clearly is that a "Bike" that has 18Kg of weight suspended on the swingarm, plus the swingarm itself (so like a complete ready to race DH bike) simply cannot get a true DH track......intact you can make it by whatever CroMo-Ti-Kriptonite material you want but unlikely it reach the right strength/rigidity to do well the job......(with a typical bike design-thickness-weight).....
The suggestion to use a motorbike swingarm design (or parts) and single pivots, sounds to me the only reasonable advice (other than to forget the idea of a 30+ lbs motor laced in the rear wheel of a DH bike)....there are also some inverted drive-brakes motocross/enduro models (like Husky or KTM) to use to avoid the problem of the chain side....:wink:....also I subscribe that an Alu swingarm from a small cross-enduro motorbike could be not so far in weight from a CroMoly one even more if made without top skills and resources.....

Personally I love single pivots and this is what we make for our bikes, mostly, and that's not for easiness but for preferences, performances and simplicity (hope the difference between easy and simple is the same as in Italian or German)....

Some considerations:

- What happen with an high position (true or virtual) pivot during hard braking when the wheel is lock?
- What to a 20 Kg bike's swingarm when an extreme side load is applied during a fast DH corner, even if made by whatever CroMo-Ti-Kriptonite you want?
- 10+years projecting and realizing racing FS bikes and shocks (well, not to win a world cup but yes to race in some UCI championship) lead us to the idea that for solving the High-or-Slow response of a rear suspension and other typical issues, the best thing is to work mostly with the shock itself, and not (or not only) with exotic pivot geometries.
- Yes, lightness is what we are looking for, everyday, but this is mostly for racing purpose, where a gain or a loss of even 1 second in a track could mean win (or loose) the race...... (so a 500grams could mean a lot in the same rider's hands, don't forget that is the Rider who win, not the bike)

The Linkage software is a good instrument to understand FS behavior, however, for single pivots, a basic 2D CAD or even analog draw skills + knowledge + ride experience, could be enough.... :wink:

Just my few cents to the topic...

Jules

 

[Drunkskunk]

I think I have my head around a pivot now. But placement is still eluding me.

If this was going to be a peddel bike, then putting the pivot up higher made sense, but since the peddels aren't going to be used often, I'm wondering if there is any bennifit to that. as panurge points out, a higher pivot point can cause chatter when hard braking. The other issue is the leingth. the back wheel will be 6-8 inches further back than a normal bike to clear the peddles, meaning a longer swingarm. Longer swingarms make for smoother rides, but I have no idea what the down side will be.

So the question is, High or low, Long or short?

I have a few weeks still to learn all I can. I'm still working on building the tools to build the tools I'll need. Right now I'm finishing a welding table needed to make the frame jig.

 

[panurge]

Hi Drunkskunk

If a bike style (DH) single pivot swingarm, then low, short an possibly with some sort of triangulation....

When you say DH I mean DH, so trust me, during fast DH corners, a single pivot long swingarm with such a weight laced, will flex at least with unpredictable movements.....
This is my point of view.

Jules

 

[MadRhino]

...during fast DH corners, a single pivot long swingarm with such a weight laced, will flex at least with unpredictable movements.....

+1
Thus, the interest of a 2nd pivot with a link. The link design must be calculated to take best advantage of the characteristics of the shock that you chose, for the extra weight of the hub slows down the damping.

An extended swingarm will improve stability at high speed and hard acceleration, but will handicap handling precision and cornering ability. I don't recommend a wheelbase longer than 47 in for speeding in the mountain trails, for it would slow down the reaction time to unexpected obstacles. 46 in is considered stable, for smooth riders who like to float over obstacles, while an aggressive rider will feel much better in control with the responsiveness of a 42 in wheelbase.