Caliper

I was talking to a LBS employee recently about the Michigan coast to coast race and they made the observation that because of some long stretches of sand, a fat bike was the ride of choice. I thought that a fat bike would tip the rolling resistance too high outside of really bad conditions, but they of course mentioned how wide tires don't have as much resistance as we often think. I've actually never been on a fat bike, so was wondering just what is the rolling resistance like on flat dirt/gravel roads? Is it ok, or are salespeople being salespeople? All this has me thinking of a drop bar fat bike or trying to cram fatter tires on my drop bar 29er.

aggiegrads

Metieval

well, if you are riding with other people and their average is going to be 8-10 mph.......because they are on fat bikes, you might as well ride a fatbike.

Other wise....... go as big / smooth as you can on 29er drop bar bike. like the 2.35 Schwalbe G'0ne. or the 2.0

Spoonrobot 

The issues here are that fat bike tires absolutely have more resistance than narrower MTB tires. Number 2 is that wider tires need to specifically constructed to minimize rolling resistance, just increasing the width does not lower rolling resistance. AFAIK fat bikes tires almost always have multiple considerations ahead of rolling resistance.

If you know the sand stretch lengths and can spitball average speed differences you could just math it out. My best guess riding with fat bikes in races is that it would have to be a serious amount of loose sand to make up the loses on regular gravel or pavement. To say nothing of pushing the weight up the climbs.

I'm wrangling a similar problem now with a gravel race that has a ton of climbing and descending. Is a 45mm tired cross bike that's 5 pounds lighter going to be faster overall than a 2.3 tired drop bar hardtail? I'd lose minutes per descent to make up the same minutes per climb + dozens of very small incremental accelerations that are easier on the light tires and bike. It's a fun problem.

Personally I think that for most things considered a "gravel road" anything over ~47ish mm is going to start slowing you down, even on the descents. It's possible to have tires that exceed the frame's ability.

rumrunn6

Marcus_Ti

I know people that do it, and have a blast. They're not breaking speed records--but they're having fun.

When 40mm tires aren't enough you want 29er and much wider. IMHO. Sand. Muddy MMR. Deep pea gravel.

FlashBazbo

Unless the race is predominantly deep sand, I wouldn't go with a fat bike.

Over the past week, I have experimented with gravel bike tire width. I've ridden the 40mm Mavic Allroad and a 35mm Panaracer Gravel King SK. I also had a set of Challenge Gravel Grinder 42mm in reserve. Realizing that these are not apples-to-apples comparisons -- the tires are all different designs -- I found the 40mm tires slowed me down considerably. More accurately, they required a lot more power on the same surface to give me the same speed as the 35mm Panaracers.

In the past, I've found I could ride just about anything (including occasional deep sand washes) really well on a 38mm wide Challenge Gravel Grinder. I only experimented with wider tires to see if there was a significant comfort difference. There is a small comfort difference -- but a much bigger speed difference. I will be staying with 35mm to 38mm tires from here on out.

Spoonrobot 

On gravel or pavement?

I've been progressively faster on our "scattered gravel, hardpack dirt" going to incrementally wider knobbie tires 35->40->45 that seem to be about as supple as each other with sorta the same tread patterns. I haven't been able to suss out that they're slower on pavement but I feel like they are. I already know that 50mm tires are slower for me on gravel and pavement.

Caliper

No, the event in question was coast-to-coast across Michigan. I didn't hear anything about beach riding, the comments were from a number of sections in the latter half apparently that went down some very sandy roads. It's Michigan, even our paved roads are barely roads at times...

I'd kinda had this impression of fat bikes as well. Just too much tire and they've fallen off the other end of the "wider tires roll better" curve. Or, next they'll tell us we're all missing out and need fat gravel bikes!

I do have a drop bar 29er that should be able to clear about 2.5" tires if I got the rear wheel true again. TBH, when they started talking about fatbikes, my thoughts did end up with switching the running gear from the 29er to a different frame that would clear the "29 plus" 3" wide tires out there now. Kinda like a Salsa Cutthroat on steroids. I think a bike like that would look killer, but afraid it would be just too draggy?

I've had this experience as well, but haven't been able to experiment above 38mm yet with a comparable frame. There's enough different about my 29er that it's hard for me to draw great comparisons. So far, all of the rolling studies on bumpy roads seem to stop around a 40-45mm tire probably due to current frames. Obviously at some point that curve has to come back down as the mass of the tire/rim increases, but it would be interesting to know where.

Marcus_Ti

They're only "too much tire", for pavement if you're wanting to haul as part of a paceline. But for unpacked country/MMR dirt/pea gravel roads--they're exactly the right tool for the job to enjoy yourself. IMHO.

Caliper

Being the cyclocross/gravel biking forum, I kinda assumed we weren't talking pavement. I forget that others have to ride to get to gravel... Obviously a fatbike tire is the only choice in the really soft sand and so forth, but at what point between sand and other loose or very bumpy stuff and pavement does a 26x4 or a 29x3 lose out to a 700x50 or a 700x40?

I will say that my roads are much like Spoonrobot's. That is, hard packed dirt with a sprinkling of gravel bits on top. When dry, 28-32mm tires actually work very well on many roads and don't require any special attention while riding.

JonathanGennick 

I see plenty of normal-ish tire widths when I watch the video on the race website,. There might be some sandy sections. ATV trails where I live can be quite sandy, and I'd expect similar in the LP.

Try a Google image search on "michigan coast to coast bike race". Most of the photos show fairly normal looking doubletrack and gravel.

bobwysiwyg

How true.

Marcus_Ti

Yea my thinking is that while organised gravel rides often plan out routes to ride unpaved routes entirely....my normal solo riding either has a 50/50 mix for commuting or due to the lay of near paths/infrastructure.

IME, if 35s-40ish tires are not comfortable for riding/control on the surface....usually I find I'd want 2-3"+ width. At least around these parts.

Caliper

I've definitely hiked along some very sandy ATV trails in the northern LP. Would probably wish for a fat bike to ride them!

My initial thought was based off the secondhand commentary about the race, but I also saw a review of the route over in the MI board of the Riding Gravel forums that cursed the sand (for some reason I can't get the page to load at the moment). If fat tires wouldn't be a penalty on the dirt roads, then a drop bar fat bike would be an intriguing option for the situation where you'll inevitably hit some of everything. Plus I think I'm developing a bit of a personal fetish with fat tires on something that is otherwise very much road bike. Maybe part of this is wanting to justify buying a new frame to upgrade my 29er to 29plus and pair those 29x3" tires with my woodchippers. But, honestly, it would be interesting to see rough road rolling resistance tests taken all the way through to fat tires. If 23->35->45mm tires keep rolling better on a bumpy road, where does that curve come back down? I've got a hill or two in my area with a good runout zone to do some coastdown tests on, maybe it's time for a day of hill repeats to roll down the hill repeatedly for science!

Spoonrobot 

Drop bar fat bike?

Paging @Hypno" "Toad too see if he has any input as I believe he's gone back and forth.

chas58

​​​​​​​For that race I will be using a high pressure 54mm tire in the front, and 40mm in the rear for the first half of the race, then dropping pressure down when we get to the sandy part.

The bike heads into the prevailing wind, so drop bars could be important (depending on the weather).


to your original question:
It depends.

at low speeds (<15mph) its probably not too bad. But at higher speeds, weight makes a HUGE difference. So yeah, the first half of the race where it is flatish, it may be ok. But when it getts hilly - weight is an issue.

Three examples where the answer is no:
1) Ive done spirited mountain bike rides with some people onfat bikes. They are always struggling in the hills.

2) last tuesday I did a gravel ride - I was on light 700c 40mm tires, a friend was on some fat 26" compas tires. He did fine in the flats where there was not much acceleration. We got to a 1/4 mile hill, and were side by side half way up where the effort of those heavier wheels really started to sap him - and I just blew past him on the second part of the hill - all because of weight.

3) I tried my 3 hour loop (rolling hills) on a 30lb flat bar bike with fast 60mm slicks just to see how it would compare with my gravel bike.
I would say the first half of the ride was pretty similar. It was a longish ride, not a high speed ride and the fat slicks rolled well on gravel.
But
The second half of the ride I totally ran out of energy. taking that weight (bike/tires/rims) on all these rolling hills just took a toll on my endurance. I would never use a fat tired flat bar bike on the MI C2C unless I dialded way back on my average speed. It killed my endurance.

I see strong riders going fat at races, but a fat bike isn't going to be fast. A mountain bike on 60mm or so tires can be good if wind resistance isn't an issue, and its a light bike, and you are not going much over 15mph...

Spoonrobot 

How long does it take to remove pressure and then reinflate?

I've often wondered if I could get it down fast enough to make it worth while? Figured <15 seconds for a few blips to drop pressure and maybe a minute to add it back? I'm not sure we have enough rough/sandy terrain to make it worthwhile but I always think about it when feeling sluggish on pavement or sinking in the short sandy/mud sections.

HTupolev

Weight makes a bigger difference for slower riders than faster ones; aero is a proportionally smaller part of their total resistance, so weight plays a proportionally bigger role.

What kind of performance gap are we talking about, and how are you judging that it was all because of the tires' weight? The difference between a Rat Trap Pass and a lightweight 700x40 tire is like half a pound or less, and some of that can be made up for in the rim and spokes if the wheel build quality is comparable. Even someone who thinks that wheel mass has double the impact of static mass would have a hard time arguing that we're looking at a net speed effect bigger than half a percent or so on climb performance for most bike+rider systems.

My experience with Rat Trap Pass ELs is that, cruising along the flats, they're quite a bit faster than most tires that people take to gravel rides. If I was merely "doing fine" in a pack of gravel bikes, I'd assume that many of the other riders were stronger than me, and therefore wouldn't be surprised to get dropped on a climb.

HTupolev

Leave a ready Co2 inflator in a jersey pocket, ride with your presta valves open, and figure out approximate release/fill timing in advance.

Super fast.

chas58

weight is only an issue in acceleration, which includes climbing.

For the OP's race, weight would kill me in the second half. First half not so much. That is pretty flat and steady. Second half is hilly.

I'll do time trials in a velodrome and I don't care about weight. I would gladly give up weigh for aero (i.e. solid disk wheel). there is no acceleration after I get up to speed in a 1 hour time trial.

My GF can do find at 10-15mph on a heavy bike with super efficient 60mm tires. Below 15mph its about rolling resistance. Above 15mph it changes quickly to aero. Weight is for acceleration.

HTupolev

The main reason that heavier bikes climb slower is that the gravitational resistance force on them is stronger.

Micro-accelerations might have some effect on a rider's pedaling, but they don't tangibly affect the actual power needed to ride uphill at some pace. Mass on a wheel does accelerate more slowly than mass on the frame, but this goes both ways: it also decelerates more slowly, including from gravity.

chas58

Well, the acceleration from gravity is 9.8m/s/s. So you are going against that acceleration when climbing.

The weight issue is painfully obvious on a tandem. They are very fast on a flat course (due to power to wind resistance ration I assume), fly like a rocket going down hill, but are a dog trying to climb a hill. Similarly with a loaded touring bike. Climbing is no fun, flat is fine, descending is a blast.

HTupolev

You can describe any resistance in terms of the acceleration that it would cause on the object if it were the only thing acting on it, but we generally talk about these phenomena as forces, not as accelerations.
For instance, a 70kg bike+rider fighting 18N of wind drag (which would cost around 200W while riding at 11m/s) is "going against ~.26m/s/s of air resistance."

9.8m/s/s is the acceleration that Earth's surface gravity imposes upon objects at freefall in a vacuum. In Newtonian terms, it's caused by a force that I referred to as "gravitational resistance."

Not necessarily. Tandems do tend to be much heavier than single bikes at a given quality tier, but that's not necessarily true per-rider. Good high-performance tandems weigh in the 20-something pound range.
If you've got two 70kg riders each doing 5W/kg on a 12kg tandem, they'll fight gravity on a steep hill just as well as a single 70kg rider doing 5W/kg on a 6kg single bike.

Tandems can experience some issues around team coordination. Two riders who pedal similar to each other and are well synchronized on the bike can do pretty well. A sloppy team can see lost performance.

But I think the main reason that people think that tandems climb poorly simply boils down to how well they perform elsewhere. A lot of folks don't realize how much better their bikes perform than a single road bike on flats and downhills, and so when they get to a climb and the roadies that they were comfortably riding with suddenly drop them, they blame the bike.
The same thing definitely happens with recumbents. I know a few guys with Bacchetta stickbikes, and they're faster uphill on their recumbents than on their good road bikes until the grades get upwards of 5%. But their performance elsewhere just totally obliterates road bikes and even TT/Tri bikes.