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I did a back to back test a while ago with a pair of hardtail Ragley Mmmbop frames (M and L sizes, 440mm and 460mm reach, 425 stays) I'm abble to ride both confortably and confidently, but the medium sized one feels better while climbing (less prone to wheelie in step sections, even if I ended adding more setback to the saddle) and was easier to turn (it felt to me that the wheels paths in the large frame were drastically diferent, sometimes it even feel like riding a shopping trolley)
At the end of the experiment, (this is anecdotal evidence and personal conclusions obviously) what I learned was that sizing, reach, fc:rc ratio is mostly a matter of personal preference and that anybody has a huge range of bicycle geometries that will feel right. for reference, i'm 5'9" tall and, willing to test against my own theories, I ended up building one of those On-one hello dave frames (rebranded sick bicycles) with 480mm reach, 460mm stays, 78° seattube angle and a headtube so slack you can see the headbadge while you are seated pedaling... And this was totally fine too
Now that I remember...during that same time I test another Ragley frame that has steeper headtube and slacker seat tube. I picked a size L based on the reach figures and WB but the bike ended up being totally unrideable on step terrain (in contrast, it was one of the best bikes I have ever ridden on flat corners). At around 655mm, the ETT was probably way to long for me, even if it was comfy to pedal around.
I think RAD and RAD angle are as pertinent as ever. Ran the numbers on my bikes that I ride the most and I found the numbers to be strikingly similar between a trail, enduro, and park bike, despite different fc/rc ratios, stack heights, stem length, etc.
The calculator I used in case anyone else is interested: RAD Calculator - MAD SCIENTIST MTB
We sound like conspiracy theorists
That is so sick man, thanks for reporting on how that is. I am 6ft tall and have a L spire as well and have been trying to get my hands on an XL rear end but I wasn't sure if it was a longer seat stay or chain stay, could you confirm which it is for me perhaps?
Do you mind if I ask how you got that XL rear end? I have tried to ask Transition if I could buy one from them and they didn't even respond back to me about it lol. I bet that they are coming out with a new Spire in the next year or so but if I can just get my hands on a longer chain stay I think I wouldn't need a new one anytime soon. Thanks again!
Hey happy to help! I was lucky to get a full rear end from a buddy who warrantied a frame. I just swapped the whole rear end so can’t confirm if it’s just cs or ss. You should absolutely do it if you can get your hands on one.
Thanks man, I already got a +1 degree angleset for it, I will keep my eyes peeled for an XL frame that I can slap on there. Maybe I'll get lucky on the PB buy/sell
Something I've always been curious about, many companies that advertise "Size specific chainstays" only grow the chainstay like, 2mm between sizes. Are they just moving the pivot locations to increase the chainstay length? Because that would make such small increments make sense rather than a whole new rear triangle thats 2mm larger lol
Yes, that is what many (most) are doing. One of the things that's not often discussed by the long chainstay gang (i'm included in this, I'm 70% all in on long stays), is the change in leverage ratio and travel. While yes, the FC/RC ratio can be the same as a smaller size with proportional chainstays, the LR changes enough that shock tunes and spring rates would be different. Not saying that is insurmountable or a bad thing even, but it would then be on the oem to pay attention to this and test/tune accordingly. Some are better at that then others.
depends on the suspension design. 4 bar is nice because you can just make a couple of different seat stays that have more or less distance between axle and pivot. you don't really want to change any of the pivot locations relative to each other, but you can move the whole rear triangle forward or backward, I think this is what you're saying? I hadn't really considered the swingarm bikes (because I have never owned one or seriously looked at buying one) but now these comedically small differences between sizes makes more sense: they only have so much room to push the rear end in or out (that's what she said.)
It really depends on the linkage that they use, and how they try to preserve the kinematics. A lot of the time that is how it is done, since it reduces the number of parts that they need to make. You already need to make 4 different front triangles for an S-XL size run, why make 4 different size rear triangles or 4 different size chainstays and seatstays? Keeps inventory organization easier, fewer skus etc and is just practical. You can also keep the kinematics more or less the same between sizes, since all the moving parts of the linkage will be in the same place relative to each other.
Another thing about this is that, generally speaking, if you lengthen your RC by making the chainstay longer without moving pivot locations, you increase your starting and over all leverage ratio. Since the people generally asking for longer RCs are taller (and therefore generally heavier) that might not be the best thing. A higher starting leverage means higher spring rates or shock pressures, which isn't always the best. For heavier riders, I would try to have a lower starting leverage, too keep the shock pressures lower and put less force through the shock, where lighter riders can have higher leverages to overcome friction easier. Size specific kinematics are expensive and take a lot of work, and you won't be able to account for all riders since weight can vary so much between people on the same size frame. There's a lot of personal preference and design philosophy in here, I'm not a big fan of high leverage bikes since it makes tuning the shock just a bit more annoying in my experience, especially at the end of the rebound.
I can confirm that even forbidden used the method of moving the pivot locations and using one rear triangle for their first gen bikes.
Why make 4 separate rear ends? To have the bikes ride properly.
Why even make 4 different front ends if inventory drives the story.
And kinematics are very much dependant on centre of gravity or the height of it. To have the same antisquat performance (or antirise), a taller rider NEEDS different kinematics than a smaller rider.
I have talked to him a few times online. Unfortunately, I live in the French Alps so it's a bit far to get a test ride on his prototypes. They are very interesting though. Enjoyed Brian Cahal's test ride on the DH version last year.
How are we calculating F/R ratio? I assume it's (WB-CS)/CS = F/R ratio?
If that's the case- I may have accidentally ended up with 2 bikes with the exact same F/R ratio (S4 stumpy Evo and an old S2 Status). Could be an interesting comparison... Have only been jumping the status, but might need to set it up for some trail riding too.
That or FC/RC which is basically the same.
Yep. I have always done (WB-CS)/CS = F/R
It's not the whole story when it comes to handling, but it does determine where the reach and stack should be to compliment the f/r ratio, and ultimately determine riding position.
Personally, in size larges, I've found that 1.8 is the happy number, up to 1.84. Past that cornering performance starts to suffer unless you're riding that front end like a unicycle (longer reach, lower stack).
Above 1.85 you're driving the bike with your hands and shoulders as much as your feet. Below that and you are progressively driving the bike with your feet and hips more.
My s4 stumpy is pretty short in terms of reach but with the addition of the longer CS I have a nice balance on the trail. Just having the same ratio between two bikes won’t translate to their riding the same. Reach and stack is a huge piece of what makes a bike ride well for a person of a given size.
Another big missing piece in our conversation ls here here is CoG which depends on the rider’s height and build. I think bigger riders experience bigger swings in CoG than smaller riders. They have a larger mass up top and so longer reach bikes, especially with lower stacks, will shift their CoG even farther forward. I think this does two things: one, it makes designers assume that there will be more weight on the front, and for some riders this is true. Two, it makes smaller riders not understand the plight of the large/extra large frame riders. Smaller riders are already on a balanced bike and don’t understand what it’s like having a bike that throws your CoG around as much as the long and low bikes do for say a 6 foot tall rider.
for your S2 status, you’re going to have a lot of mass getting thrown out the front of the bike when you’re in big compressions and heavy braking (edit: and less leverage to help brace yourself with your arms…) Yeah the ratio will be the same but your body mass will be interacting with it very differently. But please try it out and tell me what your experience is!!!
Yeah- I just got curious after this Jack Moir video: https://youtu.be/Kj7WuJ6d_vs?si=paQbx0AjeF3yviJc
Mostly want to test short bike vs long bike. When I do the testing- will try to normalize tires and swap the fork/front wheel between bikes to keep everything as similar as possible.
Here are the numbers:
It looks like they kept the sizing proportional.. The bigger question is will you be comfortable given the sizing differences?
I am- or at least close enough. I have a 35mm stem on the Stumpy and a 50 on the Status. Reach difference including stems is like 15mm.
Curious about the 48mm difference in wheelbase, the tiny chainstays and the mullet vs 29". Should be agile vs stable.
I mean yeah the ratios are the same but since you don’t scale with the bikes you’re going to feel completely different on them.
not trying to be the guy to beat a dead drum about something nobody cares about, but here I am - my take is that rad and rad angle are more important than front rear ratio. If you control the rad and rad angle between bikes, the difference in the ratio becomes much less pronounced. (Still preferential but I do find lower ratios better for higher speed sweeping turns, and higher ratios better for squaring off corners). Someone else alluded to it earlier or elsewhere, but bikes approaching a 1.9 ratio typically need longer stems and higher rise bars compared to their lower ratio, lower stack counterparts.
My recent lay experiments have been between a madonna and enduro... controlling the rad helps with the CoG stuff previously mentioned.
I’d argue that these are doing opposite things to achieve a similar end result. How a bike fits you in the sense of RAD or RAD angle is absolutely important, but I don’t think we can say that these numbers in isolation can replace looking at FC/RC balance. They are two different metrics that each have influence on one another.
What you’re referring to is adjusting the riders COG to suit the wheelbase balance of the bike, where as achieving a certain FC/RC balance is adjusting the wheelbase/balance to suit the riders COG in there preferred riding position.
I think the reason this is important to distinguish between is that dynamically these 2 setups will be different. While a riders COG might be balanced between the wheels, the rider themselves also has a balance point (weight distribution between hands and feet). This is where riders have been feeling the benefits of longer chainstays, and shorter reaches/higher stacks, as it aligns closer to their natural balance point while keeping the weight distribution balanced between the wheels. Using the BB as the “fulcrum” point for the riders balance point is fairly reasonable, because this is where your feel are located and you’re essentially rotating around that point with your body.
Riders with shorter legs and longer torsos tend to keep their weight more centered over the bb as they squat down. Riders with longer legs and shorter torso tend to push their hips further backwards as they squat down. The shorter legged rider has an easier time shifting their weight forward ahead of the BB, as their hips are already closer to this fulcrum. Whereas the longer legged rider has to throw some shapes and really shift their hips forward to get the same result. IMO, this is why we see such a polarizing opinion chainstay length. You have to establish where the rider feels most comfortable on the bike fit wise (ie reach, stack, RAD, etc), and the adjust where the front and rear wheels sit in relation to their body proportions and preferences.
I appreciate the detailed response, but I disagree completely.
I’m stating that if you have the same rad and rad angle, the ratio stuff becomes preference. In other words, I think that once a rider knows their preferred rad, they rip on either side of the 1.85 ratio spectrum and feel comfortable doing so.
Sure they might be maintaining the same standing body position on any bike that they've set up to have the same rad and rad angle measurements, and they're equally comfortable standing on either bike, but that does not translate to the bike having the same handling characteristics. You can't disregard the physical geometry of a bike by using Rad, you'll likely walk away with some bad results doing so. Like kapolczer implied, Rad doesn't account for a riders proportions, and that affects where your center of gravity is while riding. Whether or not you follow the Rad system is a matter of personal preference, and it's a very useful tool, but it's not the end all be all.
RAD is ergonomics. Can also effect the weight distribution but ultimately we are looking at how weight applied to the pedals is distributed to the front and rear tires. I am disinterested in systems that require me to weight the bars. The RAD “solution” you’ve outlined does not solve for riding through the feet.
I think what you mean is that where a rider weights the bike to get front wheel traction is a matter of preference. I agree with that.
https://madscientistmtb.com/rad-calc/
Im specifically referencing rad and rad angle from that calculator. I am not referring to the philosophy of fitting bikes with rad. I’m not saying that fc ratio doesn’t affect riding characteristics and not making any statements on weight distribution between the setups.
I am saying that fc ratio is confounded with other changes to bike setup and when controlling for specific bike setup (i.e. rad and rad angle), it’s easy to switch between the “long chain stay gang” and short chain stays.
Aka pick your favorite bike, toss it in the calculator then go ride another bike on the opposite end of the ratio spectrum setup to have the same rad measures. You’ll still be able to rip. Yes, they have a different feel, yes the cornering technique is different. Yes they’re suited to different riding style and terrain.
I agree it’s not the end all be all. My point is that neither is fc ratio. Standardizing rad across your bikes objectively removes one area of variation and lets the rider focus on different nuances from riding bikes at either end of the ratio spectrum.
Has anyone tried matching the rad measurement across bikes on opposite ends of the spectrum?
Post a reply to: Modern Geo Talk: Chainstays, Stack, Reach, and Bitching About It