Behind the Numbers: GT Force Suspension Analysis
If you'd like to know more about the Behind the Numbers series, aren't familiar with the terms being used or want to know why we're doing it then check out our Introduction article for all the information.
Up next in the series we take a look behind the numbers of the GT Force.
It was exciting times when GT released the Force and Sensor. Their previous bikes were always a bit wacky, and while some of the theories they used were interesting, they were probably best left as theories. Which is why when they quietly dropped all their prior quirks and adopted a proven layout lots of us sat up and looked a bit harder at their bikes.
Force Analysis Details
Travel Rear: 150mm to 152mm
Travel Front: 160mm
Wheel Size: 27.5
Frame Size: L
COM Height: 1150mm
Chainring Size: 32T
Cassette Cog Sizes: 50T, 24T and 10T
Travel Rear: 150mm to 152mm
Travel Front: 160mm
Wheel Size: 27.5
Frame Size: L
COM Height: 1150mm
Chainring Size: 32T
Cassette Cog Sizes: 50T, 24T and 10T
Many bikes of today are converging on a similar Horst link rocker layout, and for good reason. It’s naturally a stiff and light layout, it can generate good suspension curves, and amongst the details there’s room for a water bottle. But the Force could be seen as a good example that simply just changing to this layout is not job done. No matter what layout you go for, you have to work hard to position all the pivots well and get good curves, surrounded by good geometry and packaged in a form that is durable, reliable and up to the task of mountain biking.
Leverage Ratio
The Force has a leverage ratio progression of 12.9% with an average ratio of 2.75 in the low chip, and there's 13.4% progression with an average ratio of 2.77 in the high chip. The bike has a similar shaped curve to the Specialized Stumpjumper EVO, with a progressive to linear curve in low chip and a linear/progressive/linear in high chip.
There’s more progression than the Specialized, which is good, but overall the ratios are a bit on the high side, down to their choice of a 55mm stroke shock. For example, a 60mm stroke shock would have given an average ratio of 2.5, and a 65mm stoke would have further lowered it to 2.3.
Lowering the leverage ratio would bring more balance to the spring/damping equation - right now it might be a touch too much in favor of the spring. And longer stroke shocks are easily available. Perhaps design got priority and they didn’t want to push the rocker link farther up the seat tube, but it looks like there was room to do it and line up the seat stays really nicely with the top tube.
To be a fly on the wall in the development meetings would provide the reasons as to why they chose this.
A touch of over-springing might be needed on this bike to provide some support. Again, it’s dependent on the aggression level of the rider and their desired output from the bike, but it’s not as much of a predicament as the Specialized. If the rider wants to use most of their travel most of the time, and never really hucks to flat, then it should all be okay. But the pilot style riders might be looking to add some psi and a volume spacer or two to help hold the bike up.
More progressively damped shocks are going to help out on these lower progression bikes. There would still need to be a bit of over springing, but the amount of Newtons of damping support that can be added with a progressively damped shock will help support the bike and resist its want to over use travel.
Anti-Squat
The GT is constantly below 100% anti-squat in all settings and gears and even in the least regressive setting, 32/50, it still quite low at sag and only getting worse the more into travel you’re going.
Riding along a smooth climb, with smooth pedaling and this might be okay. But, factor in impacts while climbing and non-smooth pedaling technique on a technical climb, for example, and more of the rider’s energy input into the pedals is going to be wasted rather than being used to push the bike forward.
Again, we have another bike in the enduro category that dips below 0% anti-squat. The industry really isn’t doing much to help Mike Levy’s hate of lockout levers.
Anti-Rise
The anti-rise curve is pretty nice, rising as it goes through the travel. If it was a bit higher up in percentages then this would further reduce any of the vague feeling at the rear wheel that comes from really low anti-rise figures, as well as give a touch more support to the rider.
However, it’s a smooth curve with reasonable numbers and doesn’t drop off the farther into travel you go. That should provide some nice mechanical support when the going gets tough. And given the combo of leverage ratio and anti-squat, you might be spending more of your time at the second half of travel rather than being nicely supported and held up higher in the travel.
Axle Path
The axle path is predominantly forwards in its trajectory. But really only a true high pivot bike is going to show a vast difference here, and then it’s going to have some compromises in other aspects of the suspension.
But, the Force hasn't got the big forwards travel of the Stumpjumper. Among all the bikes analysed so far, the axle path could be the least perceivable characteristic. Someone would have a hard time singling out the axle path differences between the GT, Specialized and Marin on the trail. The combination of leverage ratio, anti-squat anti-rise and geometry are going to have more of a say in the characteristics of this bike, and it’s only when you go to an extreme that the strength of the axle path characteristic flavor is going to be more noticeable
There’s a more pronounced difference between the high and low chip settings, but this also is apparent in the geometry change between the two settings. Due to the long link lengths and the chip being in line with the shock it doesn’t translate into vast differences in suspension characteristics.
Final Thoughts
 | GT took a positive step towards creating a really good bike with their shift in layout on the Force, and with some tweaking of the details they could create something that performs with the best of them. Criticisms of this bike might be easily pushed aside given its current winning form, but it does have a few less-than-favorable characteristics. Hopefully GT stick with this layout and refine it into something to help a certain Belgian go even faster, if that is even possible. |
Previous Behind the Numbers Articles:
Marin Mount Vision Suspension Analysis
Stumpjumper EVO Suspension Analysis
Introducing Behind the Numbers - A New Suspension Analysis Series
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I love these articles and want to see this analysis done on as many platforms as possible
Or getting better, when considering pedal kickback on deep impacts. @dan-roberts have you ridden flat pedals recently?
After riding high vs low anti-squat bikes, I'm a fan of low.
But it is also about what and where you ride. My terrain is typically fire-roads up (climb switch) and/or a bunch of steep technical rocky climbs where traction and an active suspension is necessary. Then it's pretty much all steep rocky, droppy gnar on the way down, the perfect terrain for low anti-squat.
Yes, ridden flat pedals for the vast majority of my cycling life. I prefer clips at the moment, but again, pedal kick back was never a factor in that choice.
Having ridden a whole bunch of bikes with anti-squat figures ranging from below zero to almost 200%, I can say that the low anti squat bikes were far more energy sapping and inefficient for riding around and had added harshness from needing to lock out the shock to gain some pedalling performance.
And after spending over a year riding a 160mm 29er with around 130% anti-squat at sag, it's clear to me that the on paper benefit of low pedal kickback is far outweighed by having a good chunk of anti-squat combined with a decent leverage ratio and a comfortable and efficient seated position. That combination pedals very well, has no need to be reaching for a lock out switch on or off road and gives no perceivable disadvantage to the riding experience by having x amount more pedal kick back.
Perhaps worse wasn't the best word to use for stong opinioned people. It sparks a more fuelled debate. But, if you've found a preference in bikes with low anti squat then you're one step ahead of many of the public in having a tool to find the best bike for you.
The comparisons aren't made to anything hypothetical, but from experince in engineering and riding bikes, linking these graphs to what works or falls short in the real world. And I would stand in that 80% crowd, but know that the suspension characteristics that help me have a better riding experience also benefit the professionals.
Some of the criticisms of this bike are also true of the Specialized, which shows that it's perhaps a use of words rather than the topic underneath that spark debate.
The low speed compression adjustments on the DPx2 more than make up for any pedal bob. The thing climbs quite well, and has lots of traction for technical climbs.
"Worse" is relative but in terms of JUST anti squat (technically pedal kickback is more than just anti squat IIRC) it probably is. In an ideal world we have 100% antisquat everywhere and no pedal kickback but that's a fairy tale.
The high antisquat in the platter, lower in in harder gears makes sense for enduro, but less so for XC type riding. I ride a lot of techy climbs and am pretty fit, rarely end up in the platter and the anti squat is annoying over tech features have had to adapt riding style around my current bike.
Although in my one experience doing an extended demo of a VPP bike (Bronson), I thought the anti-squat was too great - not just pedal feedback, but outright *kickback*, causing me to lose multiple pedal strokes on a technical rocky climb that none of my three DW bikes have ever had an issue with.
Many of us have ridden bikes with high, medium and low anti-squat and prefer low. Your "preference" to less pedal kick, and less active suspension designs is just that, a preference that not everyone shares. It's why when I demo'd bikes and from my previous bike experience, I settled on the new Knolly Fugitive LT... but again, it is also terrain and riding style dependent.
First, I think these articles are fantastic!! Love them!... But, I think they could be even better if you approached them from a less biased angle where your preferences take precedence. Adjust them to a more journalistic approach with zero bias and just report on the design specifics, what type of bike each design gets you, comparisons and differences to other bikes (which you do already, just remove the why you think one is better than another) and perhaps what type of terrain or style of rider each design suits best.
Of course, some will still just be wacky and you should point that out and let us know... but again inform us on what each bike suspension design will excel at and what it will do poorly and let us make the decision on which suits our own preferences best.
I think we start to encroach on the deeper topic of what makes a good bike, and the stagering multitude of factors that come into it, let alone how they all interact.
There's certainly no pushing of an agenda here. All the speculations and comparisons are coming from experience. Be that in riding, development or even in testing out theories of, for example, anti-squat.
While I attempt to be as impartial as humanly possible, removing the rest of the information and just leaving a commentary of graphs wouldn't help. There's definite links between certain undesirable characteristics and numbers, which is what I've tried to show in these analyses. If a little more of a dissapointed tone came accross in this article, then I apologise, but it was stated and explained as to why it came about.
However, I think this would only hold when there is an actual chain force, hence when you're pedaling...
On the downhills you're generally coasting, which means you're not applying a chain force, and therefore you're not affecting the suspension. In those cases, actually a clutch rear-mech is probably a bigger hindrance to the rear-suspension's movement due to the chain-growth resulting in having to overcome te clutch force. (I remember an article on pinkbike showing the Bulls DH team bikes, with Wyn Masters at the time, running a non-clutch rear-mech for smoother suspension action)
Sometimes on a hard impact, you can hear you freewheel engage due to the sudden chain-growth, and then you might feel some pedal kickback as for a brief moment your rear-wheel is pulling at the cranks. Otherwise, I think anti-squat is only a thing when you're actually pedaling, so not during most downhill riding. (But indeed it does matter for technical climbs)
Right?
To get pedal kickback during coasting, the chain-growth due to your suspension compressing must be 'faster' than the rotational speed of the wheel (or actually, rotational speed of the cog your chain is on). That basically only happens on sudden hard impacts, and you will hear it as a 'Klunk!' as suddenly your chain tensions and the freewheel engages.
If you want pedal kickback, it's best to lock your rear wheel. In that case chaingrowth due to compression is always faster than the rotational speed of the cog, because the cog is standing still. A good reason not to skid all the time?
That would help give people a bit of a look behind the curtain, as to what is driving the more abstract charts that we are seeing. Also, a discussion of why IC is not the same as CC would be cool.
If you run a lower engagement hub, and keep the chain in the more middleish part of the cassette (because, remember that the smaller the cog the more rotation you'll get from each mm of chain growth). and ride steep fast descents where you are not pedaling, then I think you'll find that pedal kick is a non issue, on all but the most extreme chain growth frames.
If you have a I9 Hydra hub with 690 points of engagement, and shift to the 10 tooth cog on every descent, regardless you'll be pedaling or not, then you will find that pedal kick may suddenly become something to consider. Even with a lower engagement hub, if your descents are more gradual and require a lot of pedaling even on rough sections, then, again, it can become a problem.
As gillish said, if the conditions do not engage the freehub, then you can have as much "theoretical" pedal kick in the frame design as you want and it won't hamper the suspension or ride experience in the slightest other than the added drag of the derailleur clutch.
Which shock/bike combo has you running your compression damping fully open?
I'm 200lbs, race at a pretty high level still, and design shocks/tuning so I'm pretty picky, especially regarding spikes in HSC. Aside from a few newer gen shocks/forks(2017+ Lyrik, GRIP2 Fox, 2018+ DHX2/X2), previous to that I think it's pretty common knowledge that most people racing at higher levels, end up with the compression basically fully off, as there was just too much HSC built into the flow/valving design.
Ive always found the x2 grossly lacking in damping support. Basically bottoms out if you sneeze while riding, even running minimal rear sag.
I really dont get mtb riders obsession with eliminating "harshness". If i wanted a ride like i was in a limo id get in a limo. If i want to have a bike whos handling is predictable from one moment to the next, ill wind on some damping.
It be very informative to readers if you wouldchoose bikes that aren't like the rest; ie "the GT is just like Specialized".
Generate some discussion by going to extremes.
IIRC with the fezzari (was looking at buying one recently), the IC stays mostly inside the rear triangle, whereas that stumpjumper was like a meter if front of the bike.
Your two cents on Cotic Rocket Max and Bird Aeris AM9 would be appreciated as well. Thanks
No shit? Most non single pivot bikes do this. And when it happens you’re well deep into the travel, a place where you would never be pedaling. This is not a negative at all.
He is looking for a good rear shock to replace his stock one but he may want the one from my force if I upgrade.
For myself, I prioritize all day technical climbing and dh performance. So, for example, I rode a 6" travel vpp bike for a few years and decided that using chain tension to counteract suspension compression really did not work for me: a) traction was noticeably less under power in the rough, and b) I thought that over a long day it drained some of my power on trail chatter and was therefore less efficient. Sprinted over smooth "buff" trail super awesome though, and maybe that's great in California but not so much where I ride.
Also, pedal kickback is not an issue for me. Maybe that's inconsistent, but I mean deeper in the travel - it's like getting a free body weighted pedal stroke through the rough dips. I do care about brake jack on the descents alot, so definitely prefer designs with independent braking characteristics. Not totally sold on progressivity without being able to also adjust the end stroke damping. For example I added some tokens to my Pike and found the increased spring rate at the end overpowered the rebound damping and got too springy. Had to slow down the rebound and then the early stroke was overdamped and rough. That's all just preference.
So there, if I have a rough idea what I like then I can read some balanced analysis and apply it to my situation. I would also be very interested to read about why the author would prefer something else, and learn how certain characteristics work in concert with others based on author's experience with more bikes than I have time/money to ride - maybe they would work for me and I could get hyped to try.
Asking realistically could we build martins bike with available parts?
Anti Rise is a strange measurement at the best of times, varying with the height/weight/style of the rider. Your body position during braking has a big effect on anti rise numbers, so analyzing exact percentages in the article seems a bit silly. High anti-rise deep deep in the stroke seems a negative trait to me, while the article makes out the opposite. I agree with some of the opinions in the article, but some seem very strange. Strange enough that I would go as far as saying I think they are wrong.
Not to mention the fact that the anti-rise figures don't consider any forces coming from front brake application, so in that regard, it doesn't represent a lot of real world scenarios.
High pedal feedback numbers on a long, fast, washboardy dh run will absolutely tire out your legs faster than low pedal feedback numbers. This can be felt daily during weeks spent at bikeparks in the summer.
I also find bottom out on my rear shock daily during big alpine holidays, but never find myself complaining of ankle pain induced by bottom out. Only ever from long periods hitting trail chatter at speed. There is something very off about this article...
I would think antisquat matters most up to about 40% into the stroke in low gears to give efficient climbing in crud and keeping momentum in mid gear quicker rooty climbs. In high gears, if a company is going to offer antisquat further into the travel they better offer a higher bb and/or short cranks because that kind of pedalling is frightening (high speed, using more than 1/2 the shock stroke). Just some thoughts...
glad to hear its someone actually trying to measure them.
andrextr: www.youtube.com/playlist?list=PL4tH8eqoJoZ9N-v5D6bW3iHK6hXB2m38Q
Certainly, the poetry should be left out of it: "To be a fly on the wall in the development meetings would provide the reasons as to why they chose this."
I find it really weird that your talking about progressively dampened shocks, separate mentioning volume spacers (which is progressive shock) and overspringing like its an absolute. Yet you make no mention of high speed compression control or a hydraulic or ifp bottom out, which non-coincidentally are out of industry favor (probably cause of build cost and complexity)
The only kinds of progressive damping we see generally is that caused by the spring ramping up, or a hydraulic or ifp bottom out.
Stronger spring air or coil, with less preload / more initial air pressure, will create more support, is supple compared to damping, and returns energy unlike oil damping, which allows for stronger backside pumping and more traction as there will be more ground pressure. With too strong a spring and or too much damping for the leverage&system will cause chassis instability and harshness. A soft spring with more preload to achieve desired sag, or an air shock with lower initial psi and less air volume; will not return energy to the ground, create tractionor hold the rider from diving (creating chassis instability) if you add a bunch of damping to a undersprung setup to reduce top of stoke wallow harshness is likely. Within those extremes there can be a few different spring rates that will provide different viable setups for different riders and terrain.
I feel like your proclamation homogenizes the perceived range of what's good, and will influence what all these numbskulls think they should be buying and so what the industry will make.
"There’s more progression than the Specialized, which is good" like who are you to say that more progression is good?
Personally i find linear linkage provides a good basis from which to add damping affect, to achieve the desired ride characteristic. it's much harder if not impossible to take a progressive linkage ration and create a bike that behaves linearly.
Pinkbike is apparently a marketing agency with capitalist agenda's, a puppet for the bike companies who pay. theres always a slant : (
It's all a balancing act, and Dan is drawing on his years of riding and engineering experience to shed some light on the kinematics of these bikes. There's not one bike out there that's going to be the best for every rider, but there are certain characteristics that are more desirable than others across the board.
I'd agree however that the "capitalist agenda" rant was a bit far fetched. Maybe a case of both sides of the argument getting a little distracted by their personal biases.
Still, my contention is that simply making a linkage or air spring very progressive to avoid bottoming is a fools errand, that it is a cost cutting measure by brands of components and bicycles, promoted by the greased media, to sell us cheaper shit with a better margin. progressive cheap simple shocks and linkage did not come from professional enduro or dh riders & mechanics request nor that of the educated mountain biker, rather it was jammed down our throats by the industry.
The brands, media and bike shops financially benefit from advocating and specifying shocks without adjustable bottom out pressure, oil/ifp volume, low and/or high speed compression, or that don/t require making/stocking shock springs and shop employees that are competent to prescribe the correct spring because these shocks are more expensive to make and setup. air shock with rebound, lsc or lockout and volume spacer setup seems simpler to the layman, and are cheap to spec' they're usually inadequate for optimum performance.
For a long time the preference of educated fast riders has been one that minimizes friction, and maximizes control of forces, by the rider using: spring preload, spring rate, high speed compression, low speed compression, piggyback ifp volume and pressure, hydraulic bottom out and bottom out bumper, as well as the predetermined shock rate.
A linear linkage shock rate with additional control provided by a quality shock will allow the rider/mechanic the widest range of tuning possibilities, avoiding initially mushy or harsh ride, midstroke wallow or bottom out; all while allowing speed sensitive, maximally compliant suspension.... which a highly progressive spring just can't match. Even with a linear linkage rate and a well controlled, feature rich shock, a rider who needs or desires more progressive characteristic could outfit their bike with a progressive coil spring or low volume air shock.