Kickstarter product campaigns often aren't that promising, but this one caught our eye. The Datum from Digit Bikes is the brainchild of
Tim Lane, the man behind
DirtBaggies and several successful road bike designs. It's a trail bike with a pretty interesting suspension arrangement. The bespoke rear shock/strut is housed inside the top tube and is held rigidly in place with a pair of fork bushings; it forms a structural and integral part of the suspension system. The rear triangle is attached to this with a pivot, while a short link connects the bottom of the rear triangle to a bottom bracket concentric pivot.
Digit Datum Details• 140mm Analog suspension with shock strut claimed to be lighter, stiffer and simpler
• Intended use: Trail
• Wheel size: 29" front / 27.5" rear
• Designed for 150mm-160mm fork
• 12" shock with 2:1 leverage ratio claimed to offer more consistent damping and spring rate
• Made in the USA
• Pricing TBC
•
digitbikes.com The design is called Analog because the sliding strut is an analog (equivalent) of the rocker link at the top of a conventional four-bar design. Digit aren't the first to use a sliding element in place of pivots as part of the suspension linkage:
Naild and Yeti have done that before with their R3act and Switch Infinity designs, respectively. But Digit's design is different in that the strut is both a structural element and a shock, which cuts down on the number of parts required.
Sliding suspension elements have been proven to work before.
Digit aren't the first to do this either, as
Resistance Bikes,
Maverick's Monolink and
Boulder Bicycles all used integral shock struts, but the Datum is a little different to all these designs.
When asked to categorize his design, Tim Lane says he started out referring to it as a "3-bar link". He admits that's "not quite engineering correct [but] it's a snappy name that conveys the point that there are fewer parts." He added, "I think it could be called an '
offset slider-crank mechanism with the wheel riding on the connecting rod'." Probably a good marketing decision not to call it that.
There are three pivot locations in total - two on either side of the short lower link, and one where the swingarm drives the shock/strut. That may sound more complex than an Orange-style single pivot, but even those have three pivot locations if you include the two rotating shock bushings. Conventional four-bar bikes (including Horst-link and twin link designs), as well as "faux bars" like Kona use, have four bearing pivot points plus two on either side of the shock, so six in total.
Digit claim that fewer rotating parts mean they can save almost a pound when compared to a four-bar or faux bar design, plus superior stiffness, reliability and lower environmental impact due to the lower part count.
The design also makes it possible to use a straight seat tube, which makes room for long dropper post insertion lengths and two water bottle mounts inside the frame. There's also very little seat tube offset, so the effective seat angle is almost the same no matter the saddle height.
One thing which could be seen as a downside is the need to design a proprietary shock that also functions as a structural frame member. Digit sees this as an advantage, however; because there are no conflicts for space inside the main triangle shared with rockers and water bottles, it allows them to make a bespoke super-long shock.
The shock strut is 12" long, which apparently allows for larger oil volumes for better damping consistency and larger air volumes for a more linear spring curve, meaning less beginning-stroke stiffness and more mid-travel support. "Because my shock is so long, life is easier for the oil and there's much more of it," Lane explains, "I can have a nice long air spring, and the low leverage ratio puts much less load into the frame and pivots, plus the shock is supported by fork bushings so there's no risk of sideloading the shock shaft."
The damper offers external low-speed compression and rebound adjustment, while the air spring can be fitted with volume spacers to adjust progressivity. The shock is said to be fully self-serviceable using no proprietary tools, and the first service is free to frame buyers. The average leverage ratio is 2:1, meaning the shock moves 70mm over the 140mm of travel. That's a lower leverage ratio than most bikes, which should improve reliability but could increase friction.
The aluminum frame is mullet-specific, offers 140mm of rear travel and is designed to be paired with a 150 or 160mm fork. With a 65-degree head angle and 75-degree seat angle, the geometry is modern without being boundary-pushing.
If the funding round is successful, Digit will make the frames in Southern California and hope to ship the first frames in March 2022. Pricing is still TBC.
Check out digitbikes.com for more.
Funny Beta doesn't have the first ride behind a paywall, but your reply is.
"Pinkbike appears to be broken.
1) The comments are positive.
2) It doesn't #lookslikeasession
3) And nobody complained about Outside Magazine paywalls, even when there's a Beta MTB article cited.
We're living in a bizarro opposite world, and I like it."
...and then you posted about the paywall.
(oops, I replied to myself above, not you guys). I'd recommend making appeals on todays podcast page.
First I have to get past the funding stage, then build the first production, and then I can think about longer and shorter travel models. If I don't get fully funded however, I won't have to think about any of this (the campaign was more than 50% funded @30 hours, so quite optimistic).
I've tested a strut with a big rubber coil-shock bottom-out cone installed, that could be installed as a tuning option, but I don't plan to include it for production as the progression resists bottoming very smoothly already.
mombatbicycles.com/MOMBAT/BikeHistoryPages/Boulder.html
ep1.pinkbike.org/p4pb18931156/p4pb18931156.jpg
I owned a flaming hot pink one (with matching hot pink Rockshox RS1 fork). It rode okay. Ridiculously heavy frames though (like 8 pounds for a medium).
www.balfa.wooyek.pl/balfa-belair.html
Also don't forget that the rear wheel has leverage over the shock in the range to 2-3x, so there could be at least twice the friction at the shock before it's as noticeable at the wheels.
"This" only has to do part of aligning the rear suspension. The lower links aren't like ball joints or something, they're 1 degree of freedom, so they will also guide the movement.
Yes forks take other loads, momentarily, and when they do their performance drops proportionately. But the lions share of the weight of the rider and normal head on bumps, is supported by the spring and damper.
I'm not saying that the bushings cant handle this well, they are down hill from the shock so can probably be really well lubricated compared to a fork, but it definitely isn't a given that they will cope well. Would need to do the calculations.
Accomodating these considerations is what engineering is all about, and why struts use real bushings (shock manufacturers often call the glide-rings "bushings", but that's quite a stretch IMO, they're only really there to prevent the seals from burping out of place if the piston moves eccentrically in it's bore, or from heving metal-on-metal if the eccentric movement is too big. There are glide-rings in the Integer strut for these purposes too, but their supported by long, large, hard built-for-purpose bushings).
I've not done their math, but I imagine the side loading on a Maverick Monolink was much higher than in the Analog system. Those struts have typically outlasted the chassis parts, and are also servicable in the home shop. The side loads must be horrific in a Chapman or MacPherson on a car.
Shock in the top tube
Dropper post & cable in the seat tube
Mulitool in the head tube
Tubeless kit in the handlebars
Doughnuts in the downtube Swat box,
A motor in the bottom bracket and battery in the downtube
All that's left are the seat stays & chain stays and even they can have internal cable routing in there., but I'm sure we could fit something else in-
lift a little flap in the seat stay and have a trash port so you leave the trails pristine?
Hermetically sealed chamber for smuggling drugs, cheese or dinosaur embryos across the border?
We live in a brave new world of bikes with thinks stuffed inside the tubes
The 'loading' of the shock that some are putting forward? It's absolutely Sweet FA compared to a fork.
By comparison to 'normal' shocks, it has quite a large oil capacity. Many here would be shocked (bad pun) at how little oil is in the vast majority of shock absorbers on bicycles.
It appears to have / easily have Big, Serious Bearings at all pivot points. Those and the triangulated swingarm, and a big , boofy 'Dog Bone' make for , potentially, a very stiff structure, which combined with those Big, presumably High quality bearings (and bushings in the shock) the shock shaft and body will have little adverse loading
And, it has a Lot of potential with the 'dog bones' pivot placements and length. A Lot.
It's quite similar to some drawings I did a long, long time ago.
Oh , and for those eyeing up the length of the shock, in comparison to the gap between the seat post and top tube, look to the plate / port on the headtube. A mate of mine was trying to work out the various ways to break down the shock for rearward removal, until I suggested he enlarge the head tube area on his screen. But, he was looking at it all on a phone screen, so.............
The Kickstarter price for a frame is $2750, which is competitive in today's market.
Good luck to Tim. Hopefully it'll get funded. If I were in the market for a new bike I'd consider it.
ESTA and rear center numbers are the same across the size range. Less cool.
Made in the USA means there's a possiblity for a custom program. I hope they consider this as there are very few FS custom shops.
My immediate thought on seeing this was 'Boulder Starship' - a lot has changed since then. This looks light and clean, and if it works, you're onto a bit of a winner - best of luck!
Looks like a very interesting bike in general. I feel like it is a good example of Colin Chapman's mantra of, "Simplify, then add lightness." I hope to see you get to market soon.
Listen, I'm stoked that people wanna create these cool ideas. I would love to see these small companies succeed though. Find a way to use current off the shelf suspension, and you would have my vote. As of now, that's an investment that I personally don't feel I would get any return on.
If my funds weren't so tight right now and I had a use for a 140mm bike I'd give it a shot.
Tim do you think this could be adapted to a 160 or even 170 design ?
I took part in the Tantrum kickstarted and while my frame was about 8 months behind schedule everything worked out well. Thie bike was interesting and in many ways did what it said it would. I ended up moving on due to weight and some other drawbacks but overall it was a positive experience, although the company did fade away and not make anymore bikes, so there is of course something to the properitary problems argument.
The important thing about knocking another product off the throne is to excel in ONE important metric. This one isn't that cheap, isn't that light, isn't that stiff, and the rear end isn't going to be as good as current offerings.
I can see a good use for the design in cross country or marathon. A 1500g carbon frame including shock with remote lockout would be a killer. Two water bottles, superior weight, who cares about stiffness.
Making a well rounded package was better for what I wanted from a bike, my riding style and terrain, and apparently for all the test editors who've ridden the bike so far. All design is a bowl of soup, too salty for some isn't salty enough others - this is a terrible analogy, I don't like soup.
Just a thought :
Have you thought of Cannondale's Lefty fork technology when building a shock into the frame like this? It may result in a stiffer build, more compact internals and runs on needle bearings so its butter smooth.
Again mate, great job! All the best!
It’s a very stiff feeling frame as is.
Whatever happened with that story about the university that made filler rod that allowed the welding of 7075 alloys?
Kinda looks like it was welded up after the strut was installed...
I like the idea of integration seatpost and shock into the frame as such;
It needs gearbox integration
mombatbicycles.com/MOMBAT/BikeHistoryPages/Boulder.html
www.youtube.com/watch?v=d76PFjH1SJA
Fixed it for ya. Lots of neat stuff comes through Kickstarter, but the bike stuff on there is usually hot garbage.
cyclingtips.com/2019/06/what-happened-to-speedx
This outfit actually produced something (which was hot garbage)... more than other KS initiatives did.
www.pinkbike.com/news/anamoly-launch-crowdfunding-for-their-new-flat-pedals-shoes-with-bold-claims.html
Bringing leverage ratio into the discussion was regarding “stiction” only ——- as in:
When your fork compresses 4”, you’re overcoming seal drag/ stiction on 2 seals traveling 4” each.
When this bike’s rear suspension compresses 4”, you’re overcoming the drag of 1 seal traveling 2”
Sure, forces could be higher than a fork. Or maybe less—- Leverage ratio does not come into this question at all.
As the lower link is pointing more or less toward wheel contact point (and obviously the IC), it doesn't hold much in term of vertical forces, so the vertical forces at rear wheels are more or less directly transmitted as radial loads on the shock.
The horizontal fore/aft forces will be more or less transmitted in line with the shock, with or without leverage from the lower link. And the horizontal lateral forces will be held by both lower link and shock, depending on their relative stiffness.
A fork's leg mainly sees fore/aft forces, but with leverage, but they are two, so it's more or less the same order of magnitude. And well, a factor 2 would still be the same order of magnitude.
Also, if your Fox/RS has short-lived internals, you can curse them out and rely on them still being there to buy overpriced parts from. If you bought a no-name novel fork that got started on Kickstarter 3 days ago, and it turns out to have the lifetime of a junebug... that's all you.
Which situation between the two does this seem like to you?