The Darren by Baum: An adjustable bike for finding the ideal gravel geometry

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In the following article we take a look at the unique frame that Darren Baum built as part of his research and development on gravel bike design. “The Darren” has an adjustable head angle, bottom bracket height, and chainstay length, which allowed Baum to methodically test the importance of each when the bike was fitted with different tyres.

He learnt a lot from riding The Darren, including a clear understanding of why a gravel bike must be designed to suit the width of the tyres.

Darren Baum started building bike frames when he was a teenager and has been selling them since 1996. In that time, he has designed frames for the road, track, touring, mountain-biking, and cyclocross, and while he has had the benefit of feedback from champion riders in each discipline, more often than not, the design of each model has been driven by his direct experience. In short, Baum rides what he builds to ensure that each model in his catalogue meets his exacting standards.

Those that are familiar with Baum’s custom-made bikes will already be aware of his exquisite paintwork and presentation, but the Geelong-based framebuilder devotes just as much effort to the engineering, construction, and handling of each frame he builds. For example, he spent two years designing the Orbis, his road-disc chassis, obsessing over every detail so those details could be translated to every possible frame size that he might be asked to build.

Once the Orbis was complete, it seemed inevitable that Baum would move onto developing a gravel bike, especially given his experience with mountain-biking and cyclocross. There was certainly a willing market for such a bike, but once again, Baum was in no hurry to come up with a design. From the outset, he had a hunch it was going to take some time to do properly.

Baum gravel bike with adjustable frame geometry

“To me, it’s a lot more complicated than just taking a cyclocross bike and putting bigger tyres on it,” said Baum. “Once you go past a 35mm-ish tyre, rubber [or pneumatic] trail has a huge impact, but I just didn’t know what the effect of that was actually going to be.

“The peak corner speeds on a gravel bike can be very high, too. If you take a cyclocross bike [with a high bottom bracket] up to that speed, you’ll understeer all the way through the corner. Same thing for a mountain bike. The speed difference is a lot, and so the handling, and how quick the bike needs to tip, has to change a lot.

“The other thing that we started talking about was adventure versus misadventure. Like when you’re heading downhill on loose gravel with the natural acceleration of the slope and having to lift the front wheel up to bunny-hop something. That’s scary for even very good riders, and that’s when adventure can turn into misadventure, real quick.”

As an engineer, Baum refuses to leave anything to chance, so he undertook a lengthy phase of research and development to eliminate any uncertainty about the final product. “We dived down deep because people have been laying down real money with us, and I didn’t want to build them something that might create problems,” he said. “It’s the first time that I’ve felt a little bit scared, that adventure/misadventure thing is a true concern for us. It’s something that we haven’t had here before.”

Baum gravel bike with adjustable frame geometry

Baum was not bereft of ideas and strategies, and while he could see that the market was quickly evolving around certain trends, what he really wanted was some reliable data on how well any of them really worked for a gravel bike. Rather than build a series of frames that he could compare (as he might have in the past), Baum started thinking he might be able to achieve the same thing with a single frame that had adjustable geometry.

Say hello to The Darren

A bike with adjustable geometry might sound a little fantastical, however Baum was able to take advantage of three established products to build The Darren. From the outset, he suspected that he wouldn’t need a huge range of adjustment, nor would there be any need to make those adjustments on the fly. Rather, he simply needed fittings for the headset, bottom bracket, and rear dropouts that afforded a relatively minor amount of adjustment.

Cane Creek created the AngleSet headset several years ago with the express goal of offering riders the ability to modify the effective head angle of their bikes. It was a product developed specifically for the MTB market, comprising one concentric bearing cup and another eccentric cup that can be rotated 180° to increase or decrease the head angle by 0.5°, 1.0°, or 1.5°, as dictated by the eccentricity of the cup.

Baum gravel bike with adjustable head tube geometry

All that is required is an oversized head tube with the appropriate internal diameters to fit each cup. That Baum had already invested in the equipment needed to machine tapered head tubes to the millimetre (from titanium, no less) for the Orbis meant that the only thing that he needed to work out was the length of the head tube for the frame.

At the other end of the frame, Baum made use of Paragon Machine Works’ sliding dropouts to provide 20mm of adjustment for the length of the chainstay. It’s a part that Baum has used on many occasions, primarily for MTB frames (as well as a one-off custom BMX frame), so it was easily incorporated into The Darren.

Baum gravel bike with sliding dropouts

Eccentric bottom brackets were originally developed for adjusting the tension of the synchronising chain on a tandem, but Baum could see that one could be used for adjusting the effective position of the cranks. In the case of The Darren, he wanted to explore the effect of the height of the cranks as well as changing the reach of the frame so that different stem lengths could be tested. He selected Bushnell’s eccentric bottom bracket for The Darren based on his previous experience building a tandem, which provides 13.2mm of adjustment for the crank axle.

Unsurprisingly, Baum used titanium tubing to build the Darren, which was then finished with one of his hallmark schemes using matte black and silver paint. The frame was then paired with an Enve Cross Disc fork (with 47mm of rake) and a variety of parts. “The Darren has had like, six different handlebars on it,” explained Baum, “two or three groupsets, two … three wheelsets, and three or four stems. We even played with the q-factor for the cranks.”

Baum gravel bike with eccentric bottom bracket

One parameter that went unchanged was fork rake, which might seem like an oversight, but there was a very good reason for that. “We asked ourselves, ‘Do we try to work out the complete solution, or do we work out the solution for a fork and fork rake that we can actually have at the moment?’ The difference between 50 and 55mm of rake is like changing the head angle 0.5°,” he said. “It’s not really significant, so we settled on the second option, but it’s definitely something that we will be coming back to test.”

Putting The Darren to work

Once The Darren was ready to ride, Baum devised a fairly simple test strategy. “The first thing that I did, I just went and rode lots of different tyre sizes,” he explained. “Basically, I had the frame settings that I thought would work, and rode 34, 40, and 45mm tyres. And the thing that took to the longest to tease out was actually the tyre pressures. There was more playing around with changing tyres and tyre pressures than actual geometry changes.”

Baum’s test rides comprised stretches of both paved and unpaved terrain, so finding tyre pressures that worked well for both was always going to be a challenge. “I really tried to set a pressure, leave the house and ride all the terrain without changing it. I didn’t try to change it out on the trail unless I thought it was unsafe because it was under-inflated.

Baum gravel bike with adjustable frame geometry

“The difference between the two [paved and unpaved riding] is roughly 5psi for me. I’m very sensitive to the feel of the bars pushing back in the hand when I’m on the road, so I go up 5psi, which reduces the rubber trail just a bit. It gives a little bit more rolling resistance when you’re on gravel, but it feels better on the hands on the road because you haven’t got as much friction. The extra 5psi handles square-edge bumps better, too.”

Baum eventually settled on a front tyre pressure of 41psi for a 34mm tyre, 30psi for a 40mm tyre, and 25psi for a 45mm tyre (to suit a total weight, rider and bike, of 90kg). From there, he was able to start analysing the behaviour of the bike and making adjustments to the geometry.

The pros and cons of extra grip

Unsurprisingly, the most obvious difference between the three tyre sizes that Baum tested on The Darren was the amount of grip. That extra grip from the wider tyres was always welcome when riding on unpaved surfaces, especially when cornering, but it had its drawbacks, too. “When you put the bike back on the road, the grip goes up, the rubber trail also shoots through the roof, and it takes more torque to turn the handlebars.

Baum gravel bike with adjustable frame geometry

“So what we did is we just went up a size for the handlebar for the 40mm tyre,” Baum said. “A bigger lever made manoeuvring all the extra force from the rubber trail feel comfortable again. When we went to the 45mm tyre, that extra bar width wasn’t enough, so we went up another bar size again, but it still wasn’t enough.”

The 45mm tyre not only made steering more difficult on the road, it also altered the response of the bike. “When the rubber trail goes up, the bike will right itself quicker, and when that happens, sometimes you have to be careful. The bike can snap back very quickly after a sudden change in direction. It’s like having a very long rudder for a boat, it self-corrects and puts a real force back into the hands, and it can be hard to stay on the bike.

“One of the tests that I did was to come down a fast road descent, one that I knew was clear and would get me up to 60-70km/h, and then I’d give the bike a violent input … swerving as if a kangaroo had just jumped out in front of me. When I did that with a 40mm tyre with the wider bar, it would nearly behave like a road bike … but if I did the same thing with the bigger [45mm] tyre, the bike would upright itself quicker than I could change speed, so I was literally falling off the bike. The bike was doing too good a job and the organic matter wasn’t keeping up.”

In that moment, Baum realised he needed to start experimenting with a higher bottom bracket rather than changing the head angle or trail. “When you get back to basics of what is happening … the front of the bike was nearly too stable, and the bike was tilting too quick. You always want the steering force to be greater than the tipping force, so that you can lean on your hands more than you can lean on the seat, but this was too much.

Baum gravel bike with adjustable frame geometry

“If we think about it the other way, you can’t put a high bottom bracket with quick steering because what happens is your hands are lighter than your bum force, and that’s really unnatural to ride. Actually, that’s the worst bike you can build because it doesn’t want to tip into a corner. You have to throw your body weight around and be very light on your hands.

“So, there were two options: slow down the tilt-speed of the bike, or speed up the steering. The fork rake was close to the maximum that the market had to offer, so there wasn’t much I could do to speed up the steering. But I could slow down the tilt of the bike, because the higher the bottom bracket goes, the slower that bike wants to change in its tilt axis.

“Everyone has kind of been quizzing me about head angle changes, but to be honest, we didn’t actually do a lot of head angle changes, we kind of nailed that pretty easily. It was the bottom bracket height that we were changing to adjust the tipping speed of the bike on asphalt.”

Baum gravel bike with adjustable frame geometry

Adjusting the front end of the bike

As Baum experimented with the geometry of The Darren, he kept one eye on the handling of the bike, and the other on the biomechanical fit. So when he fitted wider bars to improve the handling of the bike with wider tyres, he also noticed how it affected his fit on the bike.

“As you go wider with your hands, you have to lean further forward,” Baum said. “We wanted to hold the same back angle, and so as we went wider, the bars needed to come back. There is no hard set of rules, because they will change slightly for each individual, but wider bars generally mean they need to come back because your reach is diminished.”

Baum was able to adjust the bottom bracket to reduce the reach of the frame to suit the wider bars, but he found that a shorter stem worked better. “With handlebar width and stem length, we didn’t look at it through the lens of cyclocross or road, we looked at it from more the lens of a mountain bike,” he said. “As we get more grip, to take advantage of that grip, you put a bigger lever on top of it, but as that lever becomes longer, you don’t want a bigger arc. That’s why we started pairing a shorter stem with the wider bars.”

Baum gravel bike with adjustable frame geometry

There was another important consideration — namely, weight distribution — that Baum was concerned about as he adjusted the front end of The Darren. “If you look at the weight distribution for a modern road bike and compare that with a cross-country mountain bike, the seating position is almost identical, but the front end position is very different, because the purpose of what each is trying to achieve is very different.

“With a road bike, you normally want the weight to be as far over the centre steering axis. The more forward and lower it is, nearly the better the bike handles. If you do the same thing on a mountain bike, not fun. On your cross-country bike, it’s all about front-end traction and not going over the handlebars. You’re trying to get the centre-of-gravity and your vector nearly behind the front wheel axle. So that gave us two end-stops for what we could do with the front end.

“We already had to come back with the handlebar on The Darren, and that worked well to send my weight distribution backwards, to put more weight behind the front axle. It didn’t transform the bike into a cross-country bike, but for small drop offs and little, very minor things, like bunny-hopping rocks and ruts, it made riding gravel easier.”

After multiple rounds of testing, Baum found that, relative to his road position, he needed to raise his handlebars by 5mm and reduce the reach by 10mm for the The Darren. “On rough terrain, you probably want to drop your saddle height ever so slightly to stay comfortable, but that will close down your hip angle a bit. So, you need to raise the bar, ever so slightly, to keep your hip angle open as well.”

Baum gravel bike with adjustable frame geometry

The limits of grip strength for drop bars and road brake levers

As Baum started applying more and more thinking from cross-country MTB to The Darren, he uncovered a disturbing paradox. “We were getting to tyre sizes that I had raced on a 29inch mountain bike before, but I couldn’t go anywhere near as fast,” he said. “Why couldn’t I ride on 45mm tyres as fast as the super-skinny hard-tailed mountain bike that I’ve had in the past? It wasn’t that I was running out of grip on the front tyre; it wasn’t that I wasn’t sure where the tyres were; and it wasn’t that I couldn’t get the weight back enough so I wouldn’t go over the handlebars.

“It took me a while to really work out what the limiting factor was … and it was grip strength. My hand [on the brake hoods] was actually facing the wrong way to take the impacts and I physically couldn’t get a handle on the grips. It’s a natural position for riding on the road, but it’s an unnatural position for maximum grip strength. We had been trying to work out what is the crossover point between a drop-bar bike and a mountain bike, and I think the limiting factor between those two bikes is actual grip strength.

“Some might ask why don’t we just keep going to bigger and bigger tyres for a gravel bike? Well the point is, we might have more grip but what are you gonna do with it? We’ve still got the same brake lever on there and you can’t hang onto the stupid thing. Your hand force just isn’t strong enough, and that’s why we don’t have drop-bars on mountain bikes.”

Baum gravel bike with adjustable frame geometry

Unravelling that paradox proved to be a turning point for Baum because he could see how a gravel bike slotted into the spectrum of possibilities. “That’s what the Darren was all about: categorising the differences and finding the crossover points, working out where the lines were in the sand.”

Not one, but three new gravel bikes

In an era where the appeal of a gravel bike seems to depend, at least in part, upon its versatility, Baum is challenging this paradigm. “For me, there’s actual true windows of handling, of what works and what doesn’t work, that are related to the width of the tyre,” he said. “I’m not a believer [in multiple tyres sizes for one bike] … just because it fits doesn’t mean it’s actually appropriate or it will make a good bike.

“Some of these new bikes are amazing, well-manufactured, well thought-out Swiss Army knives. I’ve got a Swiss Army knife, and it’s got a bottle opener and all the rest, and it’s great for camping … but when I’m at home and I’ve got all my knives there, I’ve never thought about grabbing my Swiss Army knife.

Baum gravel bike with adjustable frame geometry

“Our aim is to make the best-riding bike for that person for the conditions they’re going to ride it in. We’re not just about what blade … it’s like, what style is your chopping style? Are you left- or right-handed? That’s where we’re at.”

To this end, Baum believes gravel bikes can be sorted into at least three purpose-built sub-categories that can be distinguished in various ways. Tyre width is perhaps the most obvious, but Baum can see that terrain, gearing, the rider’s mindset, footwear, and even clothing are all just as useful for discerning each sub-category.

“From our perspective, if you’re coming from road and you’re looking for gravel, you’re really about that 40mm tyre,” he said. “Speeds are high, but there is no need for being able to pull up the front-end except for very straight up-and-down sort of bunny-hopping. You’ll be wearing road-oriented clothing with road shoes because you don’t need to walk. You can ride everywhere and you can choose where you’re going to stop.

Baum gravel bike with adjustable frame geometry

“A 40mm tyre will work with a slightly wider handlebar, and the weight distribution … it’s a very road-going bike that can handle wide, hard-packed gravel roads. A 40mm tyre will give out grip for most people at a one-to-one gear ratio, so a modern road groupset will work well with this tyre size, though you’ll need compact chainrings to compensate for the larger wheel diameter.

“If you’re coming from mountain biking, you’re actually looking for a 45mm tyre, because you’re going to be more adventurous … you’re going to want to go to some short singletrack and ride over some small logs, which is why a higher bottom bracket makes sense. It’s not like it’s a mountain bike but it does tap into that kind of agility. This tyre size provides enough grip for hard-packed terrain that you can actually control with a hooded lever.

“When you go to a 45mm tyre, your weight distribution is shifted very much behind the front axle. You have an adventurous mindset so you’ll probably prefer wearing a gravel short and a t-shirt and you’ll need MTB shoes for some walking or lifting the bike over fences or obstacles. You’ll also need some lower gear ratios, what we call a gravel groupset now, like SRAM 1x or Shimano GRX with 0.8 gear ratio and ~1.8m rollout. That’ll get you up 20% climbs, but any lower than that, and your speed starts to fall to a point where you’re likely to fall over.

Baum gravel bike with adjustable frame geometry

“From there, there is another category — we’ll call it 45mm upwards — and that’s not about more grip, handlebar size, or more hand grip. It’s more about … the terrain is quite soft, like snow or soft sand. Or, you’re into packing a load and you’ve upped your weight, so you need disperse that weight over more surface area. I reckon that’s where you’re wearing a flannie, and the shoe is a more walkable mountain bike shoe, because you’re touring and you’re going to get off to be walking somewhere.”

The final products

At this stage, Baum has designed bikes that satisfy two of those subcategories, both of which were recently added to his catalogue of custom-built titanium frames and unveiled at the Handmade Bicycle Show Australia. The Orbis + is designed around 34-40mm wide tyres, while the Orbis X is meant for 40-45mm tyres.

Now, Baum has two new bikes for a little more field testing. “I literally started riding my Orbis + over the weekend and it will probably end up being my training bike, or the bike I ride by myself,” he said. “It won’t have a 40mm tyre, I think I’ll go back to a 38mm tyre with a file tread, and the first thing that I’ve noticed is that it still feels very fast.

Baum gravel bike with adjustable frame geometry

“If I’m riding my road bike down where I am, I’m always chasing the smooth bit of the road. With the Orbis +, there’s no need to bother with that, and it still rolls very fast. Going through a corner, it isn’t as fast as my Orbis, I can’t tip it quite as quick, and it isn’t quite as balanced, so the ultimate speed won’t be the same. But the safety factor going through that corner feels way better, and there’s more room for error.

“So for me, riding around, if it’s a non-bunch ride, it’s my preferred bike. I’m just happy to ride over more gravel and grit and all the rest.”

As for the final geometry of each bike, a comparison of Baum’s personal bikes provides some important context for understanding the magnitude of the changes that he has been working with. In general terms, the geometry of the Orbis and Orbis + is much the same, while it is slackened off, ever so slightly, for the Orbis X. In contrast, there are bigger differences in his position on each bike, as shown in the table below:

A comparison of Baum’s position on his gravel bikes versus his road bike. All measurements are in millimetres. Notes: setback is measured for a Pro Stealth saddle; crank length increases from 172.5mm to 175mm for the Orbis X; a flared handlebar accounts for the extra width at the drops for the Orbis X.

More research and development with The Darren

Baum’s first cycle of research and development on gravel bike design may have come to a close but he is not about to retire The Darren yet. At the forefront of his priorities is the desire to re-visit fork rake with the promise of further refining the steering of gravel bikes. “I think we’ll be lengthening fork rakes,” he said. “We’re not talking an extra 5mm … we need to go back to what they were doing in the ’20s and go out to 80mm of rake.

Baum gravel bike with adjustable frame geometry

“The next big experiment will involve a fork with widely-adjustable rake. I haven’t concentrated on that yet because there’s not a fork available with that much rake. Doing a theoretical experiment to get answers on this is not going to provide a benefit for our customer base for a while.”

Then there is the matter of brake lever design. “From our point of view, none of the new lever shapes are actually right or even suit flared bars,” Baum said. “The interaction with the flare is all wrong. You see even the best riders in the world doing gear changes and their elbow is moving 100mm … fail. I was watching Sam Hill today in the Enduro World Series, and going through the roughest descent, his elbow movement was less than someone I saw on a gravel bike changing gears.

“I think there’s going to be a lot of change. We’re going to go through opinion and opinion and opinion … and what will probably work … let’s say if we had the magic answer for what would work at the moment, I don’t think the market would accept it. So where we’re gonna end up, I don’t think the market is ready for it at the moment.”

Baum gravel bike with adjustable frame geometry

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