Suspension Forks, Talk to me

[LewTwo]

The length, width and depth of my ignorance has no bounds.

The Model "T" Ford used cross chassis leaf springs for its suspension.
Some early motorcycles also used leaf (or half-leaf) springs.
Some of them used parallelogram coil spring forks.
The traditional Vespa Scooter had a single sided cantilever front fork that some sources claim originated from a tri-wheel aircraft suspension.
Then there was the original Schwinn Springer fork. A number of clones exist today (mainly for the bicycle 'chopper' market) with single, dual and/or quad spring variations.

Today most Bicycle Suspension forks look to be a set of telescoping tubes on each side of the front wheel. Sometimes they extend the full length of the head tube (triple-tree) and others terminate just short of the bottom of the head tube.

What is actually enclosed in these tubes?
I would guess the simplest have some sort of mechanical coil spring and perhaps some neoprene rubber bumpers. I believe this is what 'Chalo' calls 'pogo-sticks'.

I believe that there are also hydraulic suspension forks (some combination of coil spring(s) and hydraulic shock absorber?).

What about air-over-hydraulic?

What is actually needed for paved streets (please bare in mind the poor condition of pavements in US cities these days).
When is 40 to 50mm travel adequate.
When does one need 120mm (or more) travel ?
What is a 'down hill' suspension fork'?
What is an 'inverted' fork ?
For rear suspensions, I have seen components with external gas cylinders. Do these exist for front forks as well?
Many automobiles today use nitrogen filled gas cylinders for hoods, bonnets and trunk lids rather than springs. Anything similar used for suspension forks?
Materials: Magnesium, Aluminum, Steel vs. Carbon Fiber ,.. advantages, disadvantages?
What are the best brands and why ?

 

[dogman dan]

Hmm, prepared for a reply about 5 pages long?

One of my favorite commuter bikes was a decent, for 2005, Giant full suspension MTB. Sturdy bike, still riding it today, but as a dirt bike.

I would say don't bother with the pogo stick forks. These sell on e bay for around 50 bucks. They have a spring inside, and thats it. no dampers at all. There are other ways to get a good ride on bad streets better than a pogo stick, like longer bikes. I went from the mtb for commute, to longtails with no suspension. For street, I liked the longtail ride better, and it could carry SO MUCH more cargo, especially huge batteries.

These days, you can get a real nice mtb suspension fork that fits the old standard mtb headsets, no taper 1 1/8 " steer tubes. They are on e bay or whatever, for about 100-150 bucks. About 100 mm travel is enough, unless you are jumping the thing in dirt. You don't need a 200 mm travel DH fork for street, and in any case they sell way over 500 bucks, and are now only in tapered steer tube. I just put one of the 100 mm ones on my old giant, and its fine for the kind of dirt riding I do, or street potholes. Its a copy of a marzzochi bomber fork circa about 2008. Its gas on one side, and oil damper on the other side. Really nice to have the gas, so you can tune the fork for your weight. Add a big motor and a battery, and you should adjust both shocks to ride well, now that you bike is unbalanced in weight.

So unless you are using a front motor, you can get a good front shock to replace the broken one on any older MTB. You can upgrade a bike shaped object mtb a lot, by replacing that pogo stick with a decent shock.

We can get a lot more specific, if we talk about a specific bike. But a mid range price MTB can make a great city bike because it can handle a few potholes with ease. And a piece of crap mtb can be improved with a hundred buck fork.

Lastly, DONT put a shock fork on a frame that never was intended to have one. It will ruin the geometry of the frame, lifting the front way too much.

 

[LewTwo]

Hmm, prepared for a reply about 5 pages long?
...

Affirmative. I have tried locating information or a tutorial but thus far most of what I have found is marketing propaganda.

...
I would say don't bother with the pogo stick forks. These sell on e bay for around 50 bucks. They have a spring inside, and thats it. no dampers at all. There are other ways to get a good ride on bad streets better than a pogo stick, like longer bikes. I went from the mtb for commute, to longtails with no suspension. For street, I liked the longtail ride better, and it could carry SO MUCH more cargo, especially huge batteries.
...

Long tail bikes certainly have a place but unfortunately they also have some "transit limitations". You can not take one on mass transit (i.e. bus or train). I believe the official limit is usually 6 feet end to end.

 

[2old]

I ride off road with Manitou Markhor(s) (26", 29"), which cost $225 or so and they are perfect for me (6'1', 180). They can be adjusted for 100mm or 125mm of travel which will be more than adequate for street.

 

[LewTwo]

I summarize what I have been able to learn and let folks correct my mistakes. This is a diagram of a double crown fork (aka Trippe Three) and a single crown fork (it actually started from a drawing of an old “NL Kalin” 24 inch downhill fork). I am using it to illustrate the nomenclature of various pieces parts.


Double Crown forks (aka Tripple Three) are usually only found on downhill bicycles, mini-bikes and motorcycles.

Single Crown forks are are more commonly found on bicycles as they are somewhat lighter than their double crown cousins.

Inverted Forks have the Stanchion tubes below the sliders (opposite of the diagram above). That means both the Stanchion Tubes and the Sliders have to be thicker and heavier. Thus inverted forks are rare on bicycles … and very expensive.

Brake Arches (sometimes referred to as the 'Bridge') and the Lowers are usually combined into a single piece casting on modern suspension forks. Also the Brake Arches no longer function to hold caliper based rim brakes. Most modern suspension forks are limited to using disk brakes.

Steerer Tubes may be 1 inch straight threaded, 1-1/8 inch straight, 1-1/8 tapered (1-1/2 inch) or 1-2/2 inch straight. The 1 inch straight steerer tubes are likely to be found on cheap department store bicycles. Modern Mountain bikes will usually have 1-1/8 inch straight steerer tubes and with the 1-1/8 tapered steerer tubes used on the higher end mountain bikes. The Steerer Tube Length is the distance from the crown race to the top of the steerer tube. Sometime the Steerer tubes are replaceable.

Fork Length is the distance from the crown race to the centerline of the axle measured along a line parallel to the center of the Steerer tube.

Travel is the amount of distance that the telescopic fork can collapse it length. There is a second component to this length called “Sag” which is the distance that the fork will collapse under a normal static load. The travel distance can be anywhere from 40mm on some 16-20 inch children bicycles to over 200mm on high end downhill bicycles. The travel distance will inevitably affect the bike’s geometry because the headset has to be set higher to accommodate the upward movement of the front wheel. Longer travels also increase the weight of the fork.

Offset (and the steerer tube angle) determine the amount of “trail” (distance from the vertical center line of the front axle to the parallel line to the steerer tube to the ground). A shorter trail gives quicker (some people say more agile) steering while a longer trail tends to be more stable.

Dropouts can range from those designed for 9mm QR axles to 15mm or 20mm tubular axles with clamping lugs. The standard spacing is 100mm between the inside faces of the dropouts with 110mm variation sometimes called ‘boost’. “Fat Bikes” typically have a 170mm face to face distance.

Coil Spring based forks tend to be heavier and more linear verses Air-Spring based forks because air-springs inherently have progressive spring rates. The far biggest disadvantage of coil spring based forks is one usually has very little choice for the spring rate. The manufacturer sticks in whatever spring they think is appropriate for the average customer for given bicycle. ONE SIZE DOES NOT FIT ALL!!! There is a huge difference between the spring rate needed for 100 pound rider verses a 250 pound rider. Air-springs allow the end user the vary the air pressure to compensate for various riders weights as well as the terrain. Air-Spring based forks usually have other adjustments as well such as lockouts and rebound rates.

Replacement Parts can usually be found for major brand name forks (i.e. Suntour, RockShox, Manitou). Generic or department store coil spring based forks are usually best scrapped when repair or rebuilding is needed.

In most modern telescoping forks the Springs are located in the Left Stanchion Tube and the Dampening components (if any) will be located in the Right Stanchion Tube (as viewed from the seat of the bicycle).

Most modern suspension forks do not provide mounting bosses for such luxury items such as lights, baskets, painers or fenders.

 

[LewTwo]

We can get a lot more specific, if we talk about a specific bike.

Most bicycles today are designed for persons that are 5 feet 4 inches to 5 feet 6 inches tall. They 'accommodate' taller riders with extension seatposts that move the CG further back and higher off the ground. This may be acceptable for an athlete crouched into the most aerodynamic position but it is a damned poor concept for a comfortable riding position on a city commuter. I decided some time ago that if I ever built another e-bike it would have a custom frame designed for a full grown adult (6 feet tall).

As I also decided that I wanted the overall length to be no more that 72 inches, I decided to use 24 Wheel assemblies. This is to give a me a slightly longer wheelbase and to trim a bit of weight (maybe).

Parameters:
Paved streets, no trails unless I ride down the rare footpath next to a drainage canal.
Rider Weight: 130-150 pounds
Rims: Sun "King Pinb" 507 x 34.5mm
Tires: Schwalbe Big Apple 24x2.00
Rear Hub: 3 or 5 speed IGH
Target Mid Drive: Bafang BBS02
Target Speed: 25 MPH
Battery: Headway LiFEPO4, 12 Ahr, 48 volts nominal

Thus I have been collecting pieces parts toward that end for some time. Among those bits is a six pound "NL Kalin Downhill Boxer Fork" (pogo stick) .... brand new and never mounted. I since come to the conclusion that beast is likely only suitable for someone building 20 to 24 inch mini-bike.

I am now revisiting the suspension fork and considering the a SR Suntour XCR Air 24 Suspension Fork.

 

[dogman dan]

Sounds like you know more than I do now.

You can find triple tree, cheapo pogo forks. They just aren't typical.

Not all forks can get parts to repair them. Sure, for awhile, but they change shit yearly for a good reason, to make you buy new. I swear, the only reason they phased out straight taper steer tube was to make you buy new bikes in the high end.

But its a lot like motorcycle stuff in a way. I have a 250 with a cheap, relatively short stroke front fork. It works fine for what I am able to ride. I simply don't need the longer travel and cost of a fork that will win you a motocross race. I just putt around on trails and jeep roads.

So think a lot about what your needs really are. Do you really need a fork that costs a thou? Likely not for street. But you would enjoy a fork that does not bottom out when you sit on the bike stationary. A worn out pogo stick can do that.

Re the cargo bikes. One reason I started building my own longtails, was to make them a bit shorter, and able to fit the local bus bike racks. Or my own bike rack on the car. That fit my needs. My needs are not yours. But there are alternatives to riding a 4,000 buck MTB on the street, just because of potholes. You can of course, just stand up for the big ones. But a moderately decent front shock can help you not go over the bars, or break front wheels.

 

[dogman dan]

I was typing while you posted that last one.

63 mm travel is not enough IMO, for the big potholes. It will help with rough pavement. Bear in mind, sit on the bike, and you just lost half your travel, at least on a short fork.

The fork you have though, if its got real long travel, will have to be put on a special frame that is made for travel that long. The 63 mm travel fork though, could be put on any bike pretty much. Once on the bike, only 30-40 mm taller.

The cheap, china made copy brand forks tend to be about 100 mm travel, which is just about right for a bike primarily for street, or easier dirt.

Those bikes made for short people, many call them bike shaped objects, and they definitely are made for people who are not old enough to get a car. But real bikes these days really cost. Hunting down a used real bike in a larger frame size on the cheap is part of the fun for some. Its also part of why I started building long bikes, that fit me.

There are bikes out there though, that have geometry for a large person, but are built to have a very long seat post. Its the relationship of the angle of the post tube, the steer tube and its height, etc, that make it look like a kids bike, when its not. You won't see these in wal mart.

 

[MadRhino]

Most ebikes don’t need a good (or any) suspension on the street.

You need a good suspension if you ride hard and fast, carry a lot of cargo, or the streets in your city are looking like off road. Good suspension components can be very simple or very complex, depending of the usage requirements.

They have one main reason to exist: keeping the wheel in contact with the ground. The harder you make it for your wheel to stay on one ground, the more complex a suspension you need. Suspension is implicitly dynamic, thus it is the result of a combination of factors and parts. A shock absorber is not a suspension, only the finger tip of one that starts with frame design and geometry, cockpit tuning, weight distribution, etc... If you put a shock under a wagon, you still have a wagon. :wink:

 

[LewTwo]

... or the streets in your city are looking like off road ....

Well it is Houston, Texas so that amounts to asphalt 'washboards', unfilled cracks, and/or uneven concrete slabs along with any number of half-ass asphalt repairs. The 'bike lanes' here are the open rain gutters on the right hand side of the roadway (and they tend to be in worse condition than the street pavements) ... I kid you not. I can generally avoid large pot-holes but some of the other stuff, not so much.

 

[LewTwo]

63 mm travel is not enough IMO, for the big potholes. It will help with rough pavement. Bear in mind, sit on the bike, and you just lost half your travel, at least on a short fork.

Seems that there is a newer model "XCR AIR LO 24" that is:
.. 63mm travel
.. Internally adjustable to 80mm travel
.. 415mm axle to crown measurement set at 63mm travel
.. 54mm maximum tire width
.. 1950g
https://www.srsuntour.us/products/xcr-air-lo-25

A bit heavier and a lot smaller maximum tire width.

 

[ben2401]

You can run a smaller wheel in a fork designed for larger sizes, as in, a 24” wheel in a 26” fork or 26”/27.5” in a 29er fork. As others have said, a single crown fork with 100-120mm travel will work nicely. Better to have something decent quality than heavy junk. That said, if you scour your local used markets (fb bike groups in your area are a good place to start, they’ve pretty much replaced CL here), you could get a very nice DH (dual crown) fork for not much money if you’re lucky. I ride a 2004 Rocky Mountain Switch DH bike with a 1500W hub motor around the city (with city ebike tires). The travel is comfy and lets you drop off curbs and ride down stairs with ease. I got the complete bike for $350 CAD used. Came with a Marzocchi Super T Pro with 170mm (7”) travel and a Fox Vanilla coil out back (6” with the geometry). My first conversion was my 26” dirt jumper hardtail with a 110mm travel Marzocchi fork. It really depends on what you want, even no suspension is absolutely fine if you know where you want to ride and choose your tires wisely. The used market will have mostly 26”, 27.5”, and 29” forks. All can work as long as the geometry (head tube angle) isn’t too far off. If you plan to use disc brakes, make sure it has disc mounts. If you buy used, are you prepared to service the fork? Fork oil/ATF is cheap but the job is messy. Try to make sure it doesn’t leak so you don’t have to replace the seals right away.

 

[MadRhino]

... or the streets in your city are looking like off road ....

Well it is Houston, Texas so that amounts to asphalt 'washboards', unfilled cracks, and/or uneven concrete slabs along with any number of half-ass asphalt repairs. The 'bike lanes' here are the open rain gutters on the right hand side of the roadway (and they tend to be in worse condition than the street pavements) ... I kid you not. I can generally avoid large pot-holes but some of the other stuff, not so much.

Then you need enough power and speed to ride the street. Doing off road on the shoulder is not a safe option. A good suspension frame is a good investment to build a fast ebike. Many DH frames are stiff enough to build a fast and powerful bike, plus most of them are built with good brakes and suspension components already. You are looking at 10-15 years old DH racing or Freeride top of the line bikes from one of the high end brands: Santacruz, Specialized, Trek, etc... those that have MTB racing teams. Find one that is the next size after yours, because they are made compact. You will need some extra length for comfort, space and stability.

 

[LewTwo]

Then you need enough power and speed to ride the street. Doing off road on the shoulder is not a safe option.

Also not an option as there are no shoulders (or hips).

If I had to resort to something that is fast enough to keep up heavy traffic then I would simplify the problem and get a GY6 Scooter.

I just want to smooth out the ride on rough pavements a bit so that I my feet do not get knocked off resting on the pedals.

 

[LewTwo]

You can run a smaller wheel in a fork designed for larger sizes, as in, a 24” wheel in a 26” fork or 26”/27.5” in a 29er fork.

Have considered the 26" fork option as they are fairly common with 100mm to 120mm of travel. It is a viable option as additional height of the mismatched wheel size is only 1 inch. Then ... I do not ride down stairs or need to jump curbs.

If you plan to use disc brakes, make sure it has disc mounts.

I do not believe one can purchase a air-spring fork built in the last ten years or so that does NOT have disk brake mounts. Have to go back to around the turn of the century to find rim brake mounts.

I ride a 2004 Rocky Mountain Switch DH bike with a 1500W hub motor around the city (with city ebike tires).

I use the e-bike mainly to go to the grocery store and/or Post Office. Never can figure out where people manage carry a bit of cargo on those things and I have never needed more 750 watts even headed into the wind. FYI: I usually drag a Burley Travoy Trailer with the hitch on the rear rack.

 

[ben2401]

I ride a 2004 Rocky Mountain Switch DH bike with a 1500W hub motor around the city (with city ebike tires).

I use the e-bike mainly to go to the grocery store and/or Post Office. Never can figure out where people manage carry a bit of cargo on those things and I have never needed more 750 watts even headed into the wind. FYI: I usually drag a Burley Travoy Trailer with the hitch on the rear rack.
[/quote]

Finding space for the battery is a pain in the ass let alone any room for cargo. I’m working on finding the best way to mount my trailer hitch but for now it’s just backpacks. Not many places if any that are well-protected to mount it on the frame. I have a single 13s4p on the top tube (was terrible strapped on the front of the handlebar, it ruins bike handling). I have a 765mm wide 5” riser bar made of steel and meant for a motorcycle on it. More cockpit space and the display is protected in the middle. Long seatpost. Schwalbe Super Moto-X 2.4” wide balloon-style ebike city tires. These actually have the same low rolling resistance as much narrower tires but at a much lower pressure. For me, electrification is a game-changer. I would choose a totally different bike to ride around the city if I only had pedal power. The DH bikes are way less effort to get moving around with low rolling resistance tires and a motor. The bike is a joy to pedal with a six-speed Shimano freewheel shifted by a clutched derailleur and old-school friction thumb shifter. I set the controller at 50% current limit (~1000W) because it goes more than fast enough and helps to get the most range. The safety aspect at higher speeds is nice. Good forks and suspension is buttery smooth when set properly. I never enjoy hitting potholes on my hardtail dirt jumper with a 12lb hub motor on the rear wheel on a super-stiff aluminum frame. It’s definitely a different ride on a hardtail where you have to get light over bumps, but that can be part of the fun/it’s how a classic “normal” bike feels. Hardtails are nicest to pedal, especially on smooth pavement. To me all bikes have a place and purpose — but I do understand that not everyone wants a DH city commuter.

 

[dogman dan]

More talk about the things that made me start welding my own longtails. Including trailer hitch mounts capable of towing 400 pound loads. This one had a shock fork too, because I built it using a frame that was designed for one. It didn't need it, but without one it would have needed a special extra long fork to ride properly.

With the wheel flipped backwards, it would fit on a bus bike rack. Not as long as a yuba mundo, and the small rear wheel gave me a lower gear, and made it fit the rack as well.

The best street ride over heat cracks the size of 2x4s laying in the road is to get your ass off the rear wheel.

 

[giantfan]

One of my favorite commuter bikes was a decent, for 2005, Giant full suspension MTB. Sturdy bike, still riding it today, but as a dirt bike.

Hi dogman, I have a 2005 abouts Giant NRS with a stuffed front fork. It's an Extra Large frame too
I looked around ebay but can't seem to nail one with 50mm brake caliper centers, which is what I measure them as. All seem to be 75mm. A lot of the listings don't even have this spec in them? Any ideas.

 

[ben2401]

One of my favorite commuter bikes was a decent, for 2005, Giant full suspension MTB. Sturdy bike, still riding it today, but as a dirt bike.

Hi dogman, I have a 2005 abouts Giant NRS with a stuffed front fork. It's an Extra Large frame too
I looked around ebay but can't seem to nail one with 50mm brake caliper centers, which is what I measure them as. All seem to be 75mm. A lot of the listings don't even have this spec in them? Any ideas.

Those are called IS mounts. The newer style is called post mount. IS-post mount adapters are very common so you can use newer calipers with an older fork, but IS calipers were rare as most were post mount and they used IS-post adapters to space out the caliper for the rotor size. Also, post mount is a better way to mount calipers because you have a few mm to move them parallel to the rotor axis. IS-IS adapters were rare and out of production now (for a larger rotor with both IS fork and caliper). Many older forks came with IS mounts and that will let you use those brakes. If you are willing to change the front brake you can use a newer fork.

 

[dogman dan]

Yeah, get the new fork you like, then hunt down the brake and rotor you need for them.

 

[LewTwo]

Those bikes made for short people, many call them bike shaped objects, and they definitely are made for people who are not old enough to get a car.

Back in what some people call the "Golden Years" for bicycles the frame sizes were based on the the cockpit size ... more specifically the center to center distance of the horizontal line between the top of the head tube and the seat tube. These days bicycles frame sizes are specified by silly and totally meaningless seat tube length.

I happen to have an old 1984 Miyata 610 frame hanging (literally) about. I went through the exercise of trying to figure out which frame size it was. Unfortunately I could not locate a full range specification for the 610 model. Thus I used a 1000 model to estimate the sizes for the 610. Both frame sets are "Grand Touring Road Bike" series models and limited to narrow tire widths


Interesting enough the two dimensions not given anywhere on any of the old Miyata geometry charts are the seat tube length and fork length. I still keep this chart around just as a reference for real frame sizes.

 

[giantfan]

If you are willing to change the front brake you can use a newer fork.

Thanks for the info. I'll consider it, but I really want to keep the bike as original as i can, the brake system is unique, a real classic design that runs Dot-4 fluid. The bike is only used on the hardtop and at the moment the stuffed fork performs fine over the bumps I encounter. I suppose it would be impossible to rebuild with new seals? I'd have to have the legs re-chromed probably.

 

[ben2401]

If you are willing to change the front brake you can use a newer fork.

Thanks for the info. I'll consider it, but I really want to keep the bike as original as i can, the brake system is unique, a real classic design that runs Dot-4 fluid. The bike is only used on the hardtop and at the moment the stuffed fork performs fine over the bumps I encounter. I suppose it would be impossible to rebuild with new seals? I'd have to have the legs re-chromed probably.

The fittings look very similar to the older Hope stuff. If you like the brakes it shouldn’t be too hard to find a nice used fork with IS mounts like any older Marzocchi. They are dependable, easily rebuilt and plush. Should cost less than $100 for a good one, almost free if you’re lucky. Probably a lot cheaper than getting the stanchions rechromed. Probably needs new elastomers and a full disassembly and cleaning at the very least. If that Suntour fork is elastomer (like a rubber pogo stick) then any oil-damped/coil sprung or air fork will be a huge upgrade. If you can fix the old Suntour yourself for free, no harm in doing so. If it were me and I wanted to keep the brakes I would look for a good used fork that fits them.

 

[giantfan]

I'll start looking :thumb:

 

[DogDipstick]

I have rebuilt sooooo many forks.

A good fork has cartridges, coils, and oil. Rebound valves stack, compression valve stack. Shims valve stacks that are replaceable and configurable. Preload. Adjustable spring rates. Some come with a single weight in them, some have two.