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kiwisimon

Maximum Pace
Dec 14, 2006
3,765
1,987
I was in a race in Nagano one time coming down a fairly gentle singletrack slope and saw a short cut between a couple or trees, buried stup just the right size for 26" wheel, and I went OTB and down a 10' bank. Lost about 50-100 places. Team captain, an Olympian, was not impressed when I rolled in. We made it up later and one other team member endoed the same spot on the same stump!
 

Chuck

Maximum Pace
Feb 7, 2011
1,307
1,500
I don't ride a lot of gravel, but when I'm on a gravel trail, on a steeper incline and the gravel is loose or chunky, I keep losing traction then end up walking more than I would like. So, I'm thinking of getting a knobbier tire for the rear wheel only and keeping my 700x43 GK SK+ on the front.

For those who have experience on gravel, is it a mistake to go with only the rear tire having knobbier tread and staying with the GK (moderately knobby) on the front or would it be better to have knobbies both front and rear?
 

Half-Fast Mike

Lanterne Rouge-et-vert
May 22, 2007
4,464
3,391
For those who have experience on gravel, is it a mistake to go with only the rear tire having knobbier tread and staying with the GK (moderately knobby) on the front or would it be better to have knobbies both front and rear?
Not answering your question directly, but I wonder if you're pumping your tires up too hard. Lower pressure allows them to deform and thus conform much better to the small patch of terrain you're currently in contact with. It's a balance then between the increased grip and increased risk of a pinch flat... plus the slight faff of pumping them back up to enjoy the reduced rolling resistance when you get back to smooth tarmac. Might be worth experimenting when you have time.
 

Chuck

Maximum Pace
Feb 7, 2011
1,307
1,500
Not answering your question directly, but I wonder if you're pumping your tires up too hard. Lower pressure allows them to deform and thus conform much better to the small patch of terrain you're currently in contact with. It's a balance then between the increased grip and increased risk of a pinch flat... plus the slight faff of pumping them back up to enjoy the reduced rolling resistance when you get back to smooth tarmac. Might be worth experimenting when you have time.

Max PSI is 60. I think I run them at about 55. I'll drop down to 40 or so and see how it goes next time. The wheelset isn't tubeless so probably shouldn't go too much lower I suppose.
 

luka

Maximum Pace
Jan 13, 2015
2,217
2,049
I had some MTB tires on my gravel bike (2'1 or 2'2 or so) and ran them with tubes. when things get rough I'd get them down to 25-27 psi. 40 is still way too high in my opinion. even with the city tires of 47mm, I run them around 30-35 psi in the city, and 28-30 if rough. BTW, never had a flat. just make sure the tires are not too worn out
 

OreoCookie

Maximum Pace
Dec 2, 2017
1,832
1,626
I had some MTB tires on my gravel bike (2'1 or 2'2 or so) and ran them with tubes. when things get rough I'd get them down to 25-27 psi. 40 is still way too high in my opinion.
Agreed, and it can actually be dangerous if you are offroading, because your tires are an important part of your suspension. If they are too hard, you get bucked around quite easily. 55 psi is insane for a gravel tire. For comparison: I run my 28 mm tires on my road bike at about 58–60 psi. (No, that’s not too low, if you go to Enve’s and Zipp’s tire pressure calculators and enter my weight, inner rim width, etc., the recommendation is 55–60 psi depending on weather, etc.)

Your tire pressures sound reasonable, about in line with what I run on my XC mountain bike with old school 2.25“ tires. On the road I might up the pressure to 2.0–2.5 bar to reduce rolling resistance. But then I’d let some air out at the trail head.
 

OreoCookie

Maximum Pace
Dec 2, 2017
1,832
1,626
For those who have experience on gravel, is it a mistake to go with only the rear tire having knobbier tread and staying with the GK (moderately knobby) on the front or would it be better to have knobbies both front and rear?
Yes, because generally if you do a mullet setup (i. e. different configurations front and rear), you want to have the tire with more knobs, i. e. grip in the front and the tire with less knobs and less rolling resistance in the rear. Such setups are super common in the mountain biking world.
 

Chuck

Maximum Pace
Feb 7, 2011
1,307
1,500
... you want to have the tire with more knobs, i. e. grip in the front and the tire with less knobs and less rolling resistance in the rear. Such setups are super common in the mountain biking world. ...
Not disputing what you are saying is true, just doesn't make sense to me from a physics standpoint. It would seem that for the purpose of traction on an uphill with loose gravel, having the knobby tire in the back where you're putting the power would make more sense since that is the tire that is spinning out. It does make sense to me on downhills to have a knobby on the front (and rear) for grip on corners though. Kinda sounds like a mullet option is a compromise and maybe not such a good one?
 

Chuck

Maximum Pace
Feb 7, 2011
1,307
1,500
I had some MTB tires on my gravel bike (2'1 or 2'2 or so) and ran them with tubes. when things get rough I'd get them down to 25-27 psi. 40 is still way too high in my opinion. even with the city tires of 47mm, I run them around 30-35 psi in the city, and 28-30 if rough. BTW, never had a flat. just make sure the tires are not too worn out

I will give it a try then. Thanks for the info. Didn't think it was wise to go that low on tubular tires.
 

bloaker

Sincerely A Dick
Nov 14, 2011
3,067
4,604
Not disputing what you are saying is true, just doesn't make sense to me from a physics standpoint. It would seem that for the purpose of traction on an uphill with loose gravel, having the knobby tire in the back where you're putting the power would make more sense since that is the tire that is spinning out. It does make sense to me on downhills to have a knobby on the front (and rear) for grip on corners though. Kinda sounds like a mullet option is a compromise and maybe not such a good one?
I raced XC for years with a Semi Slick 26x1.9" rear tire and a 26x2.2 knobby front.
My enduro bike I ride on DH courses now runs a 29x2.6 front with a 29x2.3 rear.

Braking and Steering require the knobs and girth.
You have not posted enough info for me to know what you are doing - but if you are using the front brake (as you should) on the steep stuff, you are taking weight off the rear anyway - making the contact point in the rear less important on a descent. Body position and braking are going to play a huge role as well. I drag my rear brake for modulating speed, but my front is where the real braking is done.
 

Chuck

Maximum Pace
Feb 7, 2011
1,307
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Braking and Steering require the knobs and girth.
You have not posted enough info for me to know what you are doing - but if you are using the front brake (as you should) on the steep stuff, you are taking weight off the rear anyway - making the contact point in the rear less important on a descent. Body position and braking are going to play a huge role as well. I drag my rear brake for modulating speed, but my front is where the real braking is done.
Thanks. Yeah...I can see that. For an aggressive rider/racer it makes sense, especially on the downhill. As for my gravel riding style, I'm less a racer and more of a 'survivor' (or I try to be) on the trails. Mostly trying to stay on the bike longer on steeper, loose gravel inclines.
 

bloaker

Sincerely A Dick
Nov 14, 2011
3,067
4,604
Thanks. Yeah...I can see that. For an aggressive rider/racer it makes sense, especially on the downhill. As for my gravel riding style, I'm less a racer and more of a 'survivor' (or I try to be) on the trails. Mostly trying to stay on the bike longer on steeper, loose gravel inclines.
But the concept is the same. You need more meat up front than the rear in most scenarios. If you are in looser gravel, you may just need more meat everywhere. I cannot imagine a scenario where the balance is more rear tire than front is preferred. Pressure is most likely going to make your situation much better.

In super loose stuff, this is where my Fargo kills my Sage & Fisticuff. The 700x40(sage) and 700x45(fisticuff) just can't get through what the 29x2.3 Fargo can.
But in packed dirt, the Fargo can't keep up with the other 2.
 

OreoCookie

Maximum Pace
Dec 2, 2017
1,832
1,626
Not disputing what you are saying is true, just doesn't make sense to me from a physics standpoint. It would seem that for the purpose of traction on an uphill with loose gravel, having the knobby tire in the back where you're putting the power would make more sense since that is the tire that is spinning out.
You're asking a physics question to a physicist! 😁
Ask yourself the following two questions: what happens if your rear wheel loses traction? And what happens if your front wheel loses traction?

If your rear wheel loses traction, in many situations you are perfectly fine, just feather your brake until your rear wheel starts rolling again. It's not a big deal if you have the bike handling experience.

If your front wheel loses traction, you are very likely to crash. Catching a sliding a front wheel is much, much more difficult. Not good. So offroad you want to have enough grip in the front. A mullet setup is a compromise between having more grip in the front and a little less rolling resistance in the rear.

Now think about why front and rear wheel typically lose traction offroad: for the front you might be in a corner and hit some mud and slide out. The rear starts sliding, because you apply too much torque to the wheel. (That's why mountain bikers often have a lower cadence, because harder gear at lower rpm = lower torque spikes at the wheel.) That typically happens when you are very slow, i. e. you are safe. When you are at speed, I don't think you slip very often unless you run into sudden changes in terrain (mud, rocks, etc.).

Another reason why a faster rolling tire with less grip in the rear is less of an issue is weight distribution: you have a lot more weight over your rear tire, so you also have a lot more traction on your rear tire. That's why rear tires wear more quickly than front tires. (This is also true for road bikes as I am sure you have noticed.)

Maybe @bloaker can chime in, but I haven't heard of any mountain biker with a “reverse mullet” the way you envision it. I have only seen (and used myself) the setup where I put a grippier tire in the front and a faster rolling tire in the rear.
It does make sense to me on downhills to have a knobby on the front (and rear) for grip on corners though. Kinda sounds like a mullet option is a compromise and maybe not such a good one?
Tire choice is always a compromise, and gravel riding is probably one of the canonical examples. There are gravel races like Belgian Waffle Ride where the hard hitters ride road bikes with slightly wider tires (perhaps 30 mm). And then there are gravel races where you practically need mountain bike tires.

The more specialized a tire, the narrower its range of use. I have Maxxis TreadLite on my rear wheel and this tire is great … if I ride it in the right conditions. If there is a lot of mud or slippery rocks, I either won't be fast or have very little fun that day.

On the other hand, you have tires that are generalists. Examples are the Gravelkings in their various incarnations, Schwalbe's G-One line of tires and WTB Byways. The Byways take a slightly different approach, you have less tread but more width, but they are generalists which can get you over a lot of surfaces. If you are planning to ride on- and offroad alike, these are great choices. Depending on where you ride more, you can e. g. opt for the G-One Speeds when you are doing more onroad riding or the G-One Bite if you are mostly offroad and you just ride on tarmac to get from trailhead to trailhead. So from that perspective, the Gravelkings you picked are a great choice.
 

OreoCookie

Maximum Pace
Dec 2, 2017
1,832
1,626
As for my gravel riding style, I'm less a racer and more of a 'survivor' (or I try to be) on the trails. Mostly trying to stay on the bike longer on steeper, loose gravel inclines.
Then I suggest getting a wider tire and err on the side of having more knobs.
 

Chuck

Maximum Pace
Feb 7, 2011
1,307
1,500
Then I suggest getting a wider tire and err on the side of having more knobs.
I'm limited on increasing tire width by my rim internal width (19mm) and because my bike is not disc brake (Surly Cross Check). The GK SK+ are 43. Might be able to get a 2.0 on the rims but not much more than that, I figure.
 

OreoCookie

Maximum Pace
Dec 2, 2017
1,832
1,626
I'm limited on increasing tire width by my rim internal width (19mm) and because my bike is not disc brake (Surly Cross Check). The GK SK+ are 43. Might be able to get a 2.0 on the rims but not much more than that, I figure.
43 mm is already quite wide for a 700c gravel tire. If you had a disc brake bike, you could easily swap in a 650b wheel and get a wider tire. (A much larger tire size choice is an inherent advantage of disc brake bikes that has nothing to do with braking performance.)

By the way, out of curiosity, I entered the data you have given to me into Zipp’s tire pressure calculator. I didn’t know your weight and bike weight, so I just entered 75 kg and 10 kg, respectively. Zipp’s recommendation is 36 psi in the front and 38 psi in the rear. In the wet that is lowered to 32 psi and 34 psi. These pressures are for gravel riding. If you enter road riding as the purpose, the wet pressures for wet road conditions are essentially the same as for dry gravel conditions.

As usual, don’t take these values as gospel, but rather as starting points and ballpark figures.

One more thing: the trend is for tire pressures to ever lower values. Part of that is because tires are typically run tubeless and with tire inserts, you don’t have to worry as much about your rim getting damaged.
 

bloaker

Sincerely A Dick
Nov 14, 2011
3,067
4,604
I'm limited on increasing tire width by my rim internal width (19mm) and because my bike is not disc brake (Surly Cross Check). The GK SK+ are 43. Might be able to get a 2.0 on the rims but not much more than that, I figure.
Come to the dark side Chuck!
Get a MTB! Come play off road with the cool kids.......
 

joewein

Maximum Pace
Oct 25, 2011
3,254
2,966
@Chuck, I agree with the numerous comments above regarding tire pressure: 55 psi is far too high for a 43 mm tire (unless you were seriously obese).

I run 3 bar (42 psi) on my 42 mm 650B tires. I have had zero pinch flats in almost 40,000 km. I could run them lower than that to improve traction off-road, but then they can get a bit squirmy while braking and cornering at the same time on fast road descents. If you go for knobbies you may get some of that effect anyway in these conditions so that may not even be that much of a consideration.

I see absolutely no reason to go any higher than the pressure I'm running for this kind of tire width. You're only giving up comfort and traction without gaining rolling resistance benefits. You just get more road buzz that dissipates energy shaking your body parts.
 

Chuck

Maximum Pace
Feb 7, 2011
1,307
1,500
43 mm is already quite wide for a 700c gravel tire. If you had a disc brake bike, you could easily swap in a 650b wheel and get a wider tire. (A much larger tire size choice is an inherent advantage of disc brake bikes that has nothing to do with braking performance.)

By the way, out of curiosity, I entered the data you have given to me into Zipp’s tire pressure calculator. I didn’t know your weight and bike weight, so I just entered 75 kg and 10 kg, respectively. Zipp’s recommendation is 36 psi in the front and 38 psi in the rear. In the wet that is lowered to 32 psi and 34 psi. These pressures are for gravel riding. If you enter road riding as the purpose, the wet pressures for wet road conditions are essentially the same as for dry gravel conditions.

As usual, don’t take these values as gospel, but rather as starting points and ballpark figures.

One more thing: the trend is for tire pressures to ever lower values. Part of that is because tires are typically run tubeless and with tire inserts, you don’t have to worry as much about your rim getting damaged.
Thanks, Mr. Cookie. Good info.

So, looks like the first thing to do is lower my air pressure quite a bit and then see how that works for me. I'm looking forward to giving it a try. But probably won't be till next year that I hit a trail with much gravel.
 

Chuck

Maximum Pace
Feb 7, 2011
1,307
1,500
Come to the dark side Chuck!
Get a MTB! Come play off road with the cool kids.......
Thanks, but I like my bones to remain in one piece. As we grow older, we grow wiser....in theory.

(But it does look like a lot of fun.)
 
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