We’ve reached a point where bike geometry is pretty dialed for most rider sizes. Reach and chainstay lengths adjust, and brands like Cannondale even implement size-specific kinematics. The next frontier in frame chassis design seems to be ensuring that both large and small riders experience the right frame compliance for their weight and, beyond that, that every rider has a bike that behaves to their preference. Some riders want to plow, others need the chatter to be calmed down, and some just want to pop and play and have an exhilarating ride on their local blue trail, like a track car.
On a carbon frame, you can adjust front triangle tube size and layup per frame size to modify stiffness. Since each frame size already requires a new mold, it’s not a huge additional expense to tweak the tube sizes. Road bikes have been doing this for years.
Aluminum frames, however, present a different challenge. If you’ve invested in custom hydroformed tubes, making additional tube diameters to fine-tune stiffness across frame sizes isn’t cost-effective. Typically, manufacturers cut the same down tube and top tube to different lengths for various front triangle sizes to keep costs reasonable.
For production bikes, a carbon frame has a stiffness tunability advantage over aluminum.
So while the front triangle can be tuned per size, what about the rear triangle? What about the fork? I cringe when I see a 100 lb rider on a 38mm chassis fork. That can’t feel good, right? I’m 190 lbs (86 kg), and a Fox 40 beats the crap out ofme. The reality is that many riders, particularly at the book-ends of body size, haven’t experienced good compliance, just like when tall and short riders had never ridden good geo. That 6’4” friend who rode an XL with 435mm chainstays never realized how bad their front-end traction was until they got on a bike with a real chainstay length and had that “ah ha” moment.
All that said, there may be some simple wins in compliance tuning. Handlebars, forks, and rear triangles offer opportunities for optimization. Of course, there are practical challenges. Brands can’t just create a slightly different fork chassis for every frame size. The tooling would cost too much and it would create a SKU nightmare. But for those looking for fine-tuned comfort and control, choosing a fork chassis size based on body weight might make a real difference.
The Case for Bolt-On Bridges
As a frame guy, I see a potentially easy win in the form of removable or size-specific rear triangle bridges. This concept isn’t new in racing, but it might have merit for production bikes. As in real-world trail and enduro bikes, not just DH rigs or custom nerd bikes (like a lot of us have). The issue in mass production is that brands often use the same rear triangle across all frame sizes to keep costs down. You can vary the front triangle stiffness by molding different carbon tube sizes, but opening multiple rear triangle molds doesn’t make a lot of business sense.
So, how do you solve the problem?
Bolt-on bridges could be it.
You might start seeing more of these on production bikes, especially in the trail and enduro segments. That said, additional tuning elements introduce complexity. Most riders don’t want to spend weeks dialing in their setup. They just want to ride. However, a manufacturer could pre-install the correct bridge based on the rider’s weight and preference (plow or playful) when you order your bike.
To explore this concept further, let’s take a look at my buddy Rob’s latest bike. Vermont local Rob Galloway (@Robbonthecob) has built several cool bikes, starting with a replacement steel front triangle for his old Santa Cruz Megatower, then moving on to a mid-pivot 3D-printed lugged steel frame with a machined aluminum rear end.
Rob's custom steel front end for his old Santa Cruz Megatower
Rob’s “Blueberry” bike. Mid-pivot idler with a steel front and aluminum rear.
Rob is now working on a 3D-printed lugged aluminum front triangle with a more refined machined aluminum rear end—and this bike has all the bolt-on bridges. Rob reached out to me while developing this frame, and we saw an opportunity to learn and share with the Vital fam.
Rob’s new bike. Mid pivot full aluminum. Rear end is pocketed from the bottom. I don’t know what berry it will be named after yet.
First Question: Is It Stiff Enough?
We started by asking whether the new frame was stiff enough. Rob’s previous “Blueberry” bike tended to wind up in corners. If you plan on removing bridges, the base frame needs to start out stiff enough. Otherwise, each removed bridge just makes an already flexy bike even worse.
We ran FEA (Finite Element Analysis), similar to the Danny Hart Fury analysis, to compare the new design to the Blueberry. Initially, the new frame was less stiff. Not ideal. To address this, we increased the height of the rear triangles seatstays and chainstays, re-ran the FEA, and confirmed that the new design was stiffer than the Blueberry and a good chassis to proceed with.
Rob increased the heights of the rear triangle members and widened the chainstay as much as he could near the main pivot. You can see where new material was added in blue, it doesn't look like much but it made a notable difference. This resulted in the stiffness we needed to move forward with the frame design.
Measuring Compliance: Bolt-On Bridge Modifications
In our last discussion, we analyzed the modifications made to Danny Hart’s GT Fury frame. Now, we’re doing the same with Rob’s bike. But instead of modifying an existing frame, Rob can design from the ground up. Rob’s frame features removable seatstay, chainstay, and link bridges. Hell yeah.
We ran a simple FEA setup at 50% travel to simulate high-load scenarios. This setup provides a useful starting point. The goal is to see how different bridge configurations affect stiffness.
Computer magic. Red shows where the most displacement happens when the frame is loaded at the rear wheels’ contact patch with 1000N. With all bridges installed, the rear wheel contact patch moves 11.8mm relative to the center plane. Note these displacement images are exaggerated by a scale of 4X to allow you to see the movement more easily so take them with a grain of salt.
Rear view of the loaded frame, showing the changes in compliance.
Top view of the loaded frame, showing the changes in compliance.
Key Findings from FEA:
Removing just the chainstay bridge had the biggest impact on compliance. Surprisingly, the rear end did not exhibit as much wag as I expected, which was a valuable takeaway. I thought it would have been more noticeable. One thing to watch for is whether excessive chainstay compliance makes the bike feel noodly when pedaling hard. To achieve a stiffness level between no chainstay bridge and a full bridge, a smaller bridge could be used to fine tune the balance.
Rob’s design allows for personalized tuning. It will be interesting to check in midseason and see what he has learned.
Bridge Configurations for Rider Size and Terrain
Bridge setup can dramatically change how a bike feels and performs.
A bridgeless setup is perfect for slippery, chunky terrain where grip is the top priority. The rear wheel moves out of the way of obstacles more easily, adapting to the terrain and maintaining traction. The downside is that the rear end can twist unpredictably at higher speeds, especially when hitting berms, making handling less precise. Lighter riders may not flex the frame enough to notice this effect and could benefit from the added compliance.
A fully bridged setup makes the bike feel precise, and it feels like it carries its speed through corners really well. It is perfect for jumping and flow trails, where quick response and stiffness are key. The tradeoff is that it transmits more trail feedback, making rough terrain harsher and more fatiguing. Larger riders benefit from the stiffest bridges, while smaller riders can achieve a similar feel with thinner bridges.
A balanced setup offers versatility. Riders can mix small and large chainstay bridges with or without a seatstay bridge to fine-tune stiffness:
- Lighter riders may prefer a smaller chainstay bridge with no seatstay bridge for added compliance.
- Midweight riders have plenty of room to adjust both stiffness and compliance.
- Larger riders may prefer a large chainstay bridge with a seatstay bridge for maximum stiffness, but can fine-tune compliance by removing the seatstay bridge.
The best setup depends on weight, terrain, and riding style. Bridgeless setups excel in technical, low grip conditions. Fully bridged setups shine on jump and flow trails. A mixed setup offers adjustability for different preferences.
The Future of Tunable Stiffness
Could tunable stiffness bridges make it to mass-market bikes? Bolted-on solutions seem clunky, but they allow riders to adjust stiffness based on weight and preference. The challenge is simplifying the system. Instead of removable bridges, what if stiffness could be adjusted by loosening or tightening bolts? Running bridge bolts or linkage bolts slightly loose can increase compliance, similar to how a bolted connection with some play allows for flex. Even using rubber bushings at bolted connections, like in cars, could reduce harshness.
Would riders notice? Would they care? Maybe it would follow ride quality refinements in cars—small, unadvertised changes that improve feel without extra complexity. The real challenge is making it work without adding cost or setup headaches.
❓Would you want tunable rear triangle stiffness on your bike? Share your thoughts in the comments! ❓
Bonus: Machined Chainstay vs. Tubed Chainstay Stiffness and Weight
Here’s a quick comparison of weight and stiffness between a fully machined and pocketed chainstay versus a tubed chainstay. Neither design has been fully optimized, but it’s fun to see the differences. The big takeaway is that making a machined chainstay structurally efficient is tough (compared to a tubed chainstay) but it’s a great way to prototype. Compared to welding, bending, aligning, and heat treating separate frame bits, machining a prototype chainstay can be cheap(er) and easy(er).
For this comparison, we’ve got a machined 6061-T6 aluminum chainstay with a 3mm wall and a tubed chainstay with a 2mm wall. The FEA results show how stress moves through each. The tubed chainstay distributes strain over a greater length, meaning stress isn’t concentrated in just a few areas so that part can be made stronger for its weight.
The stress on the tubed chainstay is spread out pretty nicely meaning it’s less likely to break.
The welded tubed chainstays weak point will be the welded connection between the machined end bits and the tube. The machined chainstay, on the other hand, sees stress focused around the edges of just a few of its pockets.
The stress on the machined chainstay is concentrated on the forward pockets. These could be beefed up. The rear part of the chainstay has very little stress meaning these could be made thinner! But, it can be hard to make them thinner.
The left half of the chainstay barely sees any load, so it could be thinner—but at 3mm already, going any thinner becomes tricky for manufacturing. You could make the same argument for butting the tubed chainstay, but again, manufacturability becomes the limit.
Here’s how they stack up in weight and stiffness, measured by how much the frame’s contact patch moves when installed on Rob’s bike.
Does this mean a machined chainstay can’t be stiff enough? No, it just ends up heavier for the same stiffness. Does this mean a tube will always be stiffer than a machined chainstay? Also no, you can make a tubed chainstay that’s just as flexy. It all depends on the design goals. If you’re making a lightweight, competitive bike, tubes are the way to go, but switching from machining to tubes and forging only makes sense once you hit a certain production volume. Machining every chainstay gets super expensive at scale. When does a fully machined chainstay make sense? When you’re prototyping and when sales volumes are too low to justify switching to tubes. Machined frame bits are kinda more fun though IMHO 
Bike Tech With Burney articles:
I think stiffness-tuning makes sense on MTBs but I think for some companies it would come down to the question of what the purpose of these are.
For an aluminum-only company like Commencal it makes sense: your competitors are essentially offering tuning via carbon models, and with removable bridges you can do that as easily (or easier) than the flip chips everyone is using. This would also work very well for small brands and custom builders as they get to offer more tuning/compatibility for the customer.
For a larger company like Giant/Specialized/Trek, you are probably offering the aluminum model as a budget/price-point option. If the bridges add too much cost to the bike, it may not be worth the bonus or potential draw from certain customers. Another way they may think about it is: Is someone buying a CUES/Deore bike with budget suspension and no-name wheels really going to use the feature?
Separately, it seems wild that the chainstay bridge is pulling most of the weight here. Is this unique to the scenario of a machined chainstay, or would the same trend be seen from a tube stay? Part of me thinks the answer would be 'similar but not the same' as the tubed rear end already has an increase in stiffness (and this is presumably in all directions).
I would also think that removing the link bridge could be increasing the movement at the shock mount as well which could lead to issues there depending on the rear shock being used (e.g. air vs coil). I suppose there is some balance to achieve between having just the rear end move the way you want and the middle of the bike (shock, bb/crank) being stiff in a particular dimension.
I've learned that I prefer a narrower rear rim than the standard 30mm. I started playing around with rim width after running 32mm internal rims that felt harsh and unforgiving. Depending on type of MTB somewhere between 25-28mm feels best at my 195-ish lbs, with the standard 30mm up front.
My guess was always that my preference for narrower rear rim was because I like rounder tire profiles. After reading your excellent articles, I'm now thinking these narrower rear rims add beneficial lateral compliance for the tech riding I love and the tire profile wasn't the primary benefit.
Good stuff, thanks for this!
"Is this unique to the scenario of a machined chainstay, or would the same trend be seen from a tube stay?" @Masjo - Absolutely it's significant on tube/welded structures. About 15 years there was a trend to make hardtales and road bikes without chainstay bridges, it made bikes worse regarding stiffness, strength, weight, having a place to mount a fender, and ease of manufacture (chain stays are often joined by the bridge as a sub'assy, then welded to the bottom bracket); the advantages were mostly fashion and sometimes mud clearance.
Great points! Tunable bridges is probably too complexity and not enough benefit for for most bikes and most bikers. For the premium production space it may be neat and we may see it but probably won't be as prevalent as flip chips. This is mostly a thought experiment and was fun to investigate.
If you wanted to introduce compliance and make sure the shock doesn't see too much load you can keep that link bridge and make sure your link and link to front triangle connection are plenty stiff. This would keep the flex on the rear triangle and not allow link/shock flex.
Thank you for the kind words! Rim width can have an effect on wheel flex! It's important to remember it's not the only factor that drives wheel stiffness, just like how carbon and aluminum frames can both be too stiff or too flexy, rims can be the same and it all depends on the rim design, wheel layout, and material and how its applied.
You're right in that that narrower rim can feel better and its cool to hear how you've found a good setup for your riding style -well done. I think thats why we see top WC DH racers still running the enduro DT alloy rims? But not sure. Wheel's are a great way to tune compliance. After this article we will depart from frame compliance and chat about some other components and not just compliance. Wheels may be a part of that and will be fun to investigate and test.
Just like bridges were added and removed back then, here we are discussing it all over again! Haha
I messed with seat stay stiffness on a merida one sixty Carbon Recently(it has bolt holes for a mud guard)
A guy at work made me several different inserts from 6 series, 7 series and a steel one, all of different thickness.
it 100% makes a difference in feel, For testing i ran a ARC30 rim laced to a Vault hub with Very high spoke tension that was destressed.
I dont have the ability or time to Convert findings to actual technical wording but side loading from pushing into small features or on flat leaned over corners The rear of the bike was quite different, in basic the way of general thinking, no Stiffener definitely had more grip but Also felt like it was not as controlled/predictable when it let go.
I put a Turbine pre built rear on, Turbine rim/vault hub 28H, The differences was immediate from pedalling stiffness and Side loading.
Not going to lie, I like the feeling of a stiff rear end and currently running the 2.5mm 7 series insert, not sure how it'll hold up though.
Hopefully ill remember to edit this in future if it Bends or something drastic happens haha
A while back I proposed some magazine editors do a test to compare an aluminum-frame-with-carbon-wheels vs a carbon-frame-with-aluminum-wheels.
I think they made some progress but changed jobs before concluding/writing/publishing.
great read, thanks.
also, 69% (Nice)
+1 for rim width. Spoke no. Could also be a size specific option. I have no problem running ex471 28 spokes over 32 ex511’s at 90kg I had 29” roval 28/20 spoke carbon rims on my enduro and they actually held up really well. I think the narrow rear thing is actually a tyre profile/roll thing as much as a stiffness thing, like there’s 20g in the difference between an ex471 and 511
Absolutely fantastic article, thanks a lot! Such interesting findings and very well written 👌
I'm really surprised how well the cnc chainstay holds up against the tubed version. What's your prediction, would one of the solutions have more potential / have more room for optimization?
Great work Ryan, Keep the good stuff coming!
I’m all in for bracing coming on bikes, it would saving me from taking angle grinders and cutting my frames 😇.
There’s no way the frame should be the same stiffness for a pro racer, 190lb novice and 140lb kid.
Replying to @AvgMTBEnjoyer re:Would CNC lugs bonded to drawn tubes be the best compromise between the two?
Typically no. Bonded overlap joints require a double thickness of material, so they're heavier than a welded joint. The edges of lap joints can suffer stress risers where the overall thickness transitions abruptly from one layer to two. Being thicker, the lug and lapped areas are typically much stiffer than the tube, that localized stiffening can increase the stress/strain on the tube spans.
Also, CNC parts are typically structurally inferior to forged parts (including drawn tubes). Machining is most suitable when production numbers are small, or where small/detail features need to be added to a forged blank.
There may be of course be atypical situations, where a frame designer wants to mate disimilar materials, needs localised stifness/mass, or high precision, or another joining method isn't possible, or they want a pretty CNC feature, or to be more lead by the design side than the engineering side. Bonded fork CSU's are the most common example I can think of.
@Ryan Burney thanks for the write up! Excited to get going with the build, will be sure to write an update once I get to test everything over the summer.
For context, I weigh 220lbs and generally prefer a stiffer bike. Im curious to see what a 10% vs ~100% difference in compliance feels like on trail. Eventually I’d like to have a set of replaceable stays to adjust frame compliance for particular tracks/conditions which would be a more minor change. Then if I need to make a significant adjustment (if I were to build a frame for someone much lighter than me) I would change the width/height of the stays. Love the flexibility of using machined stays but the weight penalty definitely hurts.
Great info!
I think this is a good example of how preference can come into play. Some people will want it stiff and some will want it compliant.
If you remember, please do give us an update.
and great work on these bikes, Rob. It's fun to see you getting after it and making very modern mountain bikes on your own. And thank you for letting us learn from your latest rig. Can't wait for the initial learnings once ride season kicks off.
Also, how much does the shape of the rear triangle make a frame prone to flex or not ?
I mean, if you compare for instance a Giant Glory to an Intense M1 (moreover the high pivot one) or a V10, doesn't the shape of those 2 bikes is more prone to flex because the triangle is way more compact tha nthe glory ?
To explain better, let's compare a similar set up, that of a Santa Cruz Hightower for instance (or most SC), and that of a Scor. They're quite similar, both are VPP, except the SC has the upper link pointing upward and fixed to the top tube, while the Scor has the link pointing downward and fixed to the downtube.
Considering all things equal, wouldn't the Scor flex more because both fixation points are close and pretty much aligned ? Or if it's not flex, maybe it's about bearing life ?
At what point are you going to increase failures at the welds by increasing flex in a welded aluminum structure? I think that's why they use rivets instead of welds on planes and boat hulls.
IIRC aluminum plane fuselages are riveted because they’re too big to fit in a heat treat oven, because riveting requires less skill (more repeatable, cheaper), and rivets can be easily drilled out if repairs are needed.
Many tubular structures under the aircraft skin are welded, brazed, bonded, bolted, etc depending on the attributes required.
The shape of the rear triangle has a big impact on stiffness. The most efficient design is one with a large, well-defined triangle, like what you see on a road bike. It works like a truss bridge, where tall supports on the sides make the structure stronger without just making the base thicker. Adding more material at the base would make it heavier without adding much strength. A well-designed rear triangle keeps the bike stiff and efficient without unnecessary weight.
Frame and pivot layout also play a huge role in stiffness, making things more complex. The main takeaway is this: the more linkages and pivots you add, and the smaller or more compact the rear triangle is, the less structurally efficient it becomes. This means that for the same weight, the frame will be more flexible, or to maintain stiffness, you’ll need to add more material, making it heavier. A superlight road bike frame is often the most structurally efficient way to connect all the parts of a bicycle. The further you move away from that layout, the more material is required to maintain stiffness, reducing overall efficiency. But sometimes thats intentional to add more flex.
Regarding your comparison between the Scor and Santa Cruz, my bet is that the Santa Cruz will be stiffer at the same weight. So to answer your question "Wouldn't the Scor flex more because both fixation points are close and pretty much aligned? Or if it's not flex, maybe it's about bearing life?" yes, I think it would be more flexible! Great observation.
As for bearing life, that can be improved by using stronger bearings, but that usually comes with a weight penalty. It all comes down to balance: is the suspension design so good that it's worth adding weight to the frame? If the answer is yes, then the trade-off makes sense. Some riders think it's worth it, others don’t. That’s why we see more idler bikes and complex suspension designs on downhill bikes, the benefits of better suspension performance can outweigh the added weight and complexity.
Brilliant write up again Ryan.
One thing I'll add, is that it's interesting that the engineering has become so good, that a lot of pros and frame builders are now looking to make frames more compliant, and that is with fancy high end suspension and light weight bikes. We really are approaching the final frontier of design.
Welds (or the area right next to welds) are often the weakest part of an aluminum frame when it comes to fatigue... But I do not know the answer to this one so I did some googling.
It seems like rivets help avoid aluminum fatiguing to quickly. When aluminum is welded, the area around the weld gets weaker. Heat treatment can bring back most of its strength, but it still might not last as long under repeated stress as the original metal. Planes and boat hulls flex a lot and all the time, and by using rivets instead of welds, they avoid this fatigue life issue and stress concentrations. Als i think it would be too labor intensive to weld a whole plane or boat and it would need a big oven for heat treatment. Great question.
That's all i got though on that one. I'm definitely in no position to be the expert on that. Anyone else know why?
Thanks Steve!
It is cool, kind of like race cars. If you want to, you can build a great one in your garage. The push for more stiffness is over and now we are just fine tuning the small details.
I'll only speak to "boats" but riveted construction is usually limited to small jon boats (12-14' length) using thin gauge sheetmetal (1.5mm or so) construction. Welding requires higher skill and a slower manufacturing speed. Rivets are quick and low skill in comparison. That said, most small boats for rougher water use are welded these days.
The type of aluminum used in the design drives the way parts are manufactured. 5052 used in marine applications has a relatively low tensile strength but is less affected by the heat from welding and has good corrosion resistance. These qualities work well for boats where heat treat of the entire structure would be slow and expensive. 6061 is readily available in bar /tube and is easily welded but requires an involved heat treatment post welding. 7005/7020 is nice for small scale weldments as it only requires a relatively easy artificial aging (8 -12 hrs at ~300f). It is less common and much more expensive than 6061. That leaves 7075 that we are seeing used in machined bike structures (and a lot of aerospace). It is effectively not weldable in our applications so must be bonded, riveted or other mechanical fastening types. It's mechanical properties approach 4130 but at 1/3 the weight.
What alloy were you working with in the most recent anaylsis?
Cheers
I've learned a ton about welding, rivets, and metalurgy from these answers, but I'm still wondering: aren't you gonna increase the failure rate at welds by inducing additional flex in a welded aluminum structure?
It's not that you won't, just that the stresses should be below the fatigue limit at the weldment. A well designed joint is important, a quick transition from thick to thin in an area that sees high forces might be bad if it relies on the weld joint.
And hey @Ryan Burney I looked back and noticed you were working with 6061, 7075 should give a different result.
I still need to find the time and find a donor frame to do my bushing bearing test to isolate rear frame flex.
I'm trying to find an old Santa Cruz to do it on, less bearings so I can isolate rear end wag and roll quite easily. Add rubber bearing grommets for movement and install smaller bearings inside is the plan. Not sure how feasible it is, but won't know until we try making it happen
@rhodefab is spot on it depends on how it is designed. For example you could build a really flexy bike with excellent fatigue life by making sure that flex is nice and evenly spread out along the tubes rather than being focused at the welded joints. You can imagine if you have very stiff tubes and you flex that structure, the welds will see more stress. It is very much an "it depends" situation and it's all about spreading out the stress so you can keep a high fatigue life in your parts.
This is where FEA tools come in. They let you test these assumptions before building the bike ensuring that you are not unintentionally creating stress concentrations that could dramatically shorten the frame's lifespan.
You are also making a great point about the challenges of designing for this balance. One of the challenges with replaceable bridge design is ensuring that all configurations meet lab testing requirements. During the design process you would likely go back and forth between the most flexible and stiffest setups checking stresses as the design evolves to make sure neither configuration experiences excessive stress. Then you'd check all of the configurations at the end, making sure they will all pass testing.
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