That sounds like high praise to me.

I need to rephrase that...

"you need to hit a pretty big bump to feel the suspension move at all"

In other words, it's not going to do anything going over a washboard-type surface or even take the edge off a shallow pothole. You need to hit something substantial while seated to get the rear suspension to move. It's a leaf-spring design by nature so it's going to be progressive and since it's carbon it's going to be fairly efficient in terms of energy return so I would have made the chainstays thinner to increase the initial compliance even if the risk was introducing a bit of pedal bob.

i think the good people at OPEN said something to the effect of "a gravel bike with suspension is the worst of both worlds."

there's something to this. i used to drive a toyota hilux with low-range 4WD on a separate transmission. it only went up to 4th gear, with the rationale that if you need 5th gear, you don't need 4WD. similarly, in biking, depending on how much suspension you need, you maybe need a different vehicle altogether. i know there have been various attempts at road suspension over the years, and certain some compliance (or even just soft contact points) are a good idea on gravel, but if you're putting on front and rear travel, you start adding weight, changing the handling and geometry, and etc.

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https://lshtm.academia.edu/MikeCallaghan

http://howtobeswiss.blogspot.ch/

Interesting. There was a guy riding one at Crusher that I talked to. Even on the asphalt rollout I could see the rear end flexing/bobbing as he pedaled. I wasn’t sure what to think when I saw that.

Even if it rode well, I don’t think I could do it.

I think most bikes look pretty good, but this one is not even close to a looker...

I haven't ridden or lab tested this bike yet, but I'd wager that it's doing a whole lot more than you think. I'd also wager that if you had paid for it, you'd feel it doing a lot more than it's probably doing.. our brains and perceptions aren't nearly as good at discerning things like this as we think they are.

The challenge with a bike design like this is that the rider will start to feel pedal induced bobbing below a certain stiffness for various components that will make the bike feel awkward and slow on smooth surfaces.. so just like with your tires, you need to have the bike above some level of stiffness to maintain more traditional pedaling dynamics and feel.

Typically we've seen bike stiffness (dropout to seat post measurements) in the 200-300N/mm range and recently some of the most comfortable of the current crop of race bikes is in the 120-140N/mm range. In my experience (pro rider body masses) the riders can be cool to about 60-80N/mm of tire or seat post stiffness (which will drive low system stiffnesses), if you feel yourself bouncing on the tire on flat ground, you'r probably down below 70N/mm. So if I had to guess on this one, I'd bet they are probably in this range for rear end stiffness with a design that allows for a higher degree of displacement in the frame.. which could be a big leap forward for this type of riding.

Since springs in series add like this: 1/a+1/b=1/c softer springs still dominate but bringing the stiffer ones down can also make big differences.

This is also what makes tires so special and unique, the spring rate of the tire is dependent on the shape of the thing is it sitting on or being impacted by, so that 28mm tire at 6bar has a stiffness of ~150N/mm pressed against perfectly flat road, 86N/mm hitting a large cobble with 8cm radius, and 37N/mm when hitting an 8mm radius.. source: https://blog.silca.cc/...r-is-stiffer/harsher

So back to our original example, say the old bike had stiffness of 140N/mm and your tire stiffness hitting a bump is 80N/mm, the system stiffness adds up to be 1/(1/140+1/80)=51N/mm and if they've set the rear end at say 80N/mm then the stiffness on that same impact would be 1/(1/80+1/80)=40N/mm which is more than 20% softer

Take that a step further, for a 600N impact force the system will result in 3 additional mm of travel for the softer frame (15 vs 12mm) which over time becomes a big deal.

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http://www.SILCA.cc
Check out my podcast, inside stories from more than 20 years of product and tech innovation from inside the Pro Peloton and Pro Triathlon worlds!
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Thanks as always for chiming in. Interesting as always. What’s your back of the envelope calc on a Topstone with 700x38 tires vs a rigid frame with 650x55tires?

So ideally your tires will be run at a given spring rate for a given surface that is equal regardless of size. Wider tires are stiffer at the same pressures, so you run them lower to achieve equivalent stiffness, as they then also offer more deflection capabilities because they are taller.

So it isn't that wider tires are more comfortable exactly as much as it is that narrower tires often must be run at higher than optimal pressures to avoid pinch flatting or bottoming out the tire on rough surfaces.

So a good pressure for fast/moderate gravel for a 35mm tire might be 44psi to achieve a vertical stiffness in the tire of ~55N/mm on our 8cm radius bump, moving to a 55mm tire at identical stiffness would require tire pressure of 29.5psi giving the same bump performance but with ~18mm additional travel before bottoming out against the rim. Ultimately the wider tire is just offering much more range to tune the stiffness without risk of damage, so if the optimal tire stiffness is 40N/mm for a given rough surface the 35mm tire may not be capable of being run at low enough pressure without damage risk to achieve this.

Adding a little suspension to the bike is super interesting as it actually allows for narrower tires to be run a bit lower than they would be on a more rigid bike at the same pinch flat resistance as the softer bike reduces system stiffness and allows for slightly more travel which requires more displacement to ultimately bottom out the tire and pinch flat against the rim. Similarly suspension also allows you to run slightly higher than optional pressure with penalty as the system stiffness is lower and impedance losses are reduced.

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http://www.SILCA.cc
Check out my podcast, inside stories from more than 20 years of product and tech innovation from inside the Pro Peloton and Pro Triathlon worlds!
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Let me reframe my prior comments about the Topstone with the following:



That's a picture of where I ride, the Lake Apopka gravel loop in central Florida. It's not the best picture as it's actually very flattering when it comes to the condition of the trail. Most of the trail is composed of the coarser, chunkier lose gravel you see in the center of the trail in that picture. There's also a ton of "potholes" ruts, etc. throughout the trail. Not that I'm complaining! I love riding it! But I also like to be comfy :)

The first time I rode it I was on 38c Specialized Sawtooth tires on a rigid bike and, tbh, that was sketchy in sections in addition to being uncomfortable throughout. Switched to 45c WTB Riddlers and comfort improved (dropped pressure when adjusted for casing size) and traction was a non issue. The next time I rode it I was on my Cannondale F-Si (100mm XC hardtail with 2.4" front 2.2" rear tires). Comfort improved drastically and, tbh, I wasn't much slower. Suspension really took the edge off the front of the bike (hands and upper body fatigue) and my F-Si became my go-to bike for that trail. Earlier this year I bought a Trek Fuel Ex (130/130 trail bike) and for grins I took it out on the Lake Apopka gravel trail. I was noticeably slower than on my F-Si or gravel bike but man was it comfy. Specifically, my ass wasn't sore at the end of the ride.

So with all that in mind I think you can see why I was underwhelmed with the Topstone but that doesn't necessarily mean that suspension doesn't have a place on gravel (or road!) bikes.

In the context of what you stated previously, the new system from HiRide that's being used on Pinarello's bikes is interesting as the ability to lock and unlock the suspension on the fly allows them to run a lower spring rate when its unlocked (right?). Given that some of us ride on poor quality roads, do you see a place for suspension on road and possibly even TT bikes in the future? Also, in the context of how you characterize bumps by their radius (which is sensible and easy to visualize, I basically just think of a speed bump with different radii) how would you characterize a road surface like chipseal? (maybe a subject for a podcast)

Ha! I've ridden there. Really nice area for riding.

I think people discount hardtail mountain bikes for gravel riding because they are so capable that they become "boring" to ride. You can ride singletrack, so why ride gravel? Around here, the older mountain bike guidebooks are filled with doubletracks that nobody rides anymore because they aren't singletrack, or maintained gravel roads with sieved aggregate.

Just to share some perspective for "who buys gravel bikes," I'll be picking up either a Topstone (non suspension) or Jamis Renegade Expat here soon. I'm moving to a place where I can bike commute (18mi round trip) and also has some decently smooth single track. I don't want to use my road bike (carbon, no fender/rack mounts, won't fit above 23s, rim brakes, etc) so at the very least I'd have to buy a beater roadie. Well instead of having a good road bike and a crap road bike, why not get a bike that'll also let me hit up the dirt trails that are literally across the street from my new place. Yes, I'll take a speed compromise vs proper road tires, but a bunch of mounts, disc brakes, and softer tires will fill my commute needs and expand my biking possibilities. I know it's no hardtail or full suspension, but it's more than I've got now.

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JustinDoesTriathlon

Owner, FuelRodz Endurance.

Thanks for that picture, I need to check out that area. I'm in Tampa and just ordered a Topstone 105 (the 2019 alloy version without suspension) and need some places around here to ride it. I'll have to research how far away that is and the distance of the trail. If you have any other recommendations in the area, I'm all ears as I planned to primarily use this bike as a road bike in Florida and a gravel bike when I travel to North Carolina.

I'm too lazy to post pics but here's a Strava ride I did with a few pictures of the trail, quite rocky. I'd love to try the Topstone. A lot of the trails I ride or want to explore are a ways out, so I wouldn't enjoy the slow ride out to them using a hardtail mtb.

https://www.strava.com/activities/2518506921

Thanks for the picture and descriptions, really good stuff in there! I think your experience would have been different moving from a 38mm of one tire to 45 of same construction.. from testing I've done, different sizes of similar construction behave rather similarly when inflated to same stiffness.. of course, you are right that the wider tire has the advantage of being able to recruit casing more easily over fine bumps and something we know, but is very hard to measure is the superiority of wider tires on surfaces that have high percentages of negative space. In this case, the Sawtooth is essentially a thick treaded tire with negative sipes whereas the Riddler is a thin high performance casing/tread with some protrusions.. the Riddler is considerably less stiff in the tread area, so your equivalent tire stiffness is going to be at a higher pressure in the Riddler. Similarly, thicker tread tires have slower response and higher damping in rebound, so even when they are inflated to similar static stiffness values they will ride much more harshly as the have higher dynamic stiffness.

For your second point about suspension, yes I'm very familiar and have worked with Sky on these bikes on a few occasions. I see great potential in this for events like Roubaix and for mixed surface events (which is essentially was Roubaix is..). The suspension throws a real curve ball for the engineers as it offers a whole new range of tuning with options in both directions. So for Roubaix, you could theoretically run higher tire pressure for faster rolling on the pavement and in the sprint on the velodrome, OR you could run slightly lower pressure on the cobbles which will be both faster than what they can run currently, the suspension will reduce the damage risk, AND the softer setup is less fatiguing to the riders. I can't tell you what they chose, but very interesting when you start going down the rabbit hole on this one as the added tuning complexity opens entirely new doors in terms of strategy.

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http://www.SILCA.cc
Check out my podcast, inside stories from more than 20 years of product and tech innovation from inside the Pro Peloton and Pro Triathlon worlds!
http://www.marginalgainspodcast.cc

I demo'd the etap and mechanical ultegra version and swapped between laps with my cross bike (cannondale hi mod). We rode gravel, a non technical single track section, and a short road section on the demo laps.

I did enjoy the ride on the topstone. I had the same concern as rob_bell with a 'bobbing' feeling of rear suspension but could not feel it on the demo.

My current gravel short list:

topstone
warbird
lynskey ti
new cervelo gravel bike