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Why do DH casings suck so much to pedal?

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#21 ·
jeremy3220 said:
This idea has been passed around a lot the last few years but I think it's over stated. Otherwise the difference in rolling resistance wouldn't be so noticable on pavement also. There's also the fact that you can run less pressure to make up for the stiffer tire to a degree. An XC tire pumped up to 40 psi isn't going to roll slower than a DH tire at 20 psi for example.
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It's my understanding that even though pavement feels smooth to us, at the tire level it's somewhat bumpy. Which is why the modern road bikers have been moving to lower tire pressures on wider tires.

The good thing is that I'm the Princess and the Pea and don't need to be theoretical as I can just feel RR pretty easily after a short ride.

The Durameter of the rubber is certainly a factor, but since many tires are available with the same compound but a different carcass we've been able to isolate the effects of rubber duameter vs. carcass stiffness. To a certain extent there are even heavier tires that have a softer casing, such as Vittoria's, which allows us to roughly (roughly cause they are different brands) separate out tread pattern & durameter vs. carcass stiffness alone.

Two tires that are very similar in weight/ size/ compound/ tread, the Assagai vs. the Mezzer, yet the Mezzer rolls notably better. That's in the cotton casing imo.

I'll tell you another thing about a stiff tire casing: they tend to skip and chatter across the trail much easier than a soft casing tire.
 
#22 ·
Suns_PSD said:
It's 80% the fact that the stiff carcass doesn't flex out of the way over the many small bumps, instead lifting and slowing down the entire rider & bike.
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While the lack of compliance is a factor in the way you are describing, I do not think it is the most important factor in why these casings are slower. If it was, then the DH casing would not feel slower than thinner casings on smooth ground or pavement.... but they in fact DO feel slower on pavement.

The biggest reason they are slower is because unless a tire is pumped up really hard, the casing is constantly deflecting where it meets the trail simply due to the weight of the rider+bike on it. As it rolls, this deflection moves all around the tire. The damping of the casing (hysteresis losses something, something) is constantly sucking energy. The DH casings have more damping, thus more energy loss. This is just as true on smooth pavement as on a rough trail.

This is all really apparent when comparing slick road/gravel tires on smooth pavement where bump absorption is not a significant factor. RR is primarily about how supple the casing is.
 
#24 ·
kapusta said:
While the lack of compliance is a factor in the way you are describing, I do not think it is the most important factor in why these casings are slower. If it was, then the DH casing would not feel slower than thinner casings on smooth ground or pavement.... but they in fact DO feel slower on pavement.

The biggest reason they are slower is because unless a tire is pumped up really hard, the casing is constantly deflecting where it meets the trail simply due to the weight of the rider+bike on it. As it rolls, this deflection moves all around the tire. The damping of the casing (hysteresis losses something, something) is constantly sucking energy. The DH casings have more damping, thus more energy loss. This is just as true on smooth pavement as on a rough trail.

This is all really apparent when comparing slick road/gravel tires on smooth pavement where bump absorption is not a significant factor. RR is primarily about how supple the casing is.
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To just take this to extremes and make the concept more obvious: Ride with 40psi, then ride with 2psi. Same tire, same weight, same compound, but the deformation of the tire "flat" is sucking a huge amount of energy.

When we talk road tires, wider tires AVERAGE out to be faster because you are trading rolling resistance for comfort and stability, which allows you to put more power to the ground for longer. The 28C tires are NOT lower resistance, they just work out to be better on typical city roads. In a velodrome you would not wan to use a 28. And of course there is a balance point, but it depends on road types etc. 28c is turning out to be a good all rounder for general roads, 32 for very rough paved roads strikes a better balance, and pros will stick with 25c when the roads are quite smooth. 70c is great for a rock garden because the other times simply wont get through it at all.

The dh casings take more watts to deform as they roll than a regular compliant casing tire. When aimed down hill on something that barely registers as a trail, the extra resistance is not outweighing the fact that the beefy tires are allowing you to ride at all. On a smooth up hill, that resistance becomes a quite large percentage of your power output.
 
#25 ·
Suns_PSD said:
It's my understanding that even though pavement feels smooth to us, at the tire level it's somewhat bumpy. Which is why the modern road bikers have been moving to lower tire pressures on wider tires.
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On pavement rolling resistance comes from hysteretic losses due to bulk deformation. Road asperities are a consideration for designing rubber compound properties (especially friction) but they don't significantly affect carcass deformation.

"Two tires that are very similar in weight/ size/ compound/ tread, the Assagai vs. the Mezzer, yet the Mezzer rolls notably better. That's in the cotton casing imo."

There is a lot more going on that you can't just ride a couple tires and say the compound is similar. The tread compound may have similar shore A hardness but very different tan delta curves. There's also casing compounds, sidewall compounds, etc that will affect rolling resistance.

"I'll tell you another thing about a stiff tire casing: they tend to skip and chatter across the trail much easier than a soft casing tire."

I've found the opposite. If I'm running the appropriate pressures a heavier stiffer carcass feels much more planted than a thin casing tire that I have to run high pressure in.
 
#28 ·
kapusta said:
While the lack of compliance is a factor in the way you are describing, I do not think it is the most important factor in why these casings are slower. If it was, then the DH casing would not feel slower than thinner casings on smooth ground or pavement.... but they in fact DO feel slower on pavement.

The biggest reason they are slower is because unless a tire is pumped up really hard, the casing is constantly deflecting where it meets the trail simply due to the weight of the rider+bike on it. As it rolls, this deflection moves all around the tire. The damping of the casing (hysteresis losses something, something) is constantly sucking energy. The DH casings have more damping, thus more energy loss. This is just as true on smooth pavement as on a rough trail.

This is all really apparent when comparing slick road/gravel tires on smooth pavement where bump absorption is not a significant factor. RR is primarily about how supple the casing is.
Click to expand...
This makes a lot of since. Thanks for explaining it so concisely.

Learned something new today.
 
#29 ·
jeremy3220 said:
I've found the opposite. If I'm running the appropriate pressures a heavier stiffer carcass feels much more planted than a thin casing tire that I have to run high pressure in.
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Performance-wise there is something to be said for running a DH casing in the front for the better damping and choosing a better rolling tire in the rear. Most of the weight is on the rear, and uphill even more so. Up to the point where resistance to flats becomes an issue, which is dependent on rider weight and riding style.
 
#30 ·
Kenda make the Helldiver pro SS which you can get in the DH casing, I compared it to a Minion SS which wasn't a DH casing and to be honest the difference was marginal. however, the tread on the Kenda started pealing off which was not so good.

weight, tread type and how soft the compound will make a difference, but how much? without a scientific method its all personal opinions
 
#31 ·
dave_f said:
Performance-wise there is something to be said for running a DH casing in the front for the better damping and choosing a better rolling tire in the rear. Most of the weight is on the rear, and uphill even more so. Up to the point where resistance to flats becomes an issue, which is dependent on rider weight and riding style.
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Hopping onto this thread instead of starting yet another: I’m extremely sensitive to rolling resistance but like a lot of traction up front - by the principle you’re describing here, would you consider it reasonable to run a specialized GRID casing tire in the rear with a GRID trail up front? In theory, the T9 Butcher up front with a T7 Ground control in back is what I want for my new build, but they don’t make that combo with matching casings.
 
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#32 ·
snow snakes said:
Hopping onto this thread instead of starting yet another: I’m extremely sensitive to rolling resistance but like a lot of traction up front - by the principle you’re describing here, would you consider it reasonable to run a specialized GRID casing tire in the rear with a GRID trail up front? In theory, the T9 Butcher up front with a T7 Ground control in back is what I want for my new build, but they don’t make that combo with matching casings.
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Depends where you ride. I’d flat the rear in a heartbeat.
 
#34 ·
snow snakes said:
Might just have to stick with the eliminator in the back, but a grid trail GC would just be fabulous.
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Specialized now makes a Purgatory Grid Trail T7 which should roll faster than an Eliminator. Alternatively the Slaughter Grid Trail T7 is a semi slick with big side knobs and should roll pretty fast but still corner when leaning.
 
#35 ·
nobody special said:
Specialized now makes a Purgatory Grid Trail T7 which should roll faster than an Eliminator. Alternatively the Slaughter Grid Trail T7 is a semi slick with big side knobs and should roll pretty fast but still corner when leaning.
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Embarrassingly, part of my problem here is that I would strongly prefer tan walls. However, function comes first - at the very least I will be picking up a GT purg for the back of my hardtail.
 
#36 ·
snow snakes said:
In theory, the T9 Butcher up front with a T7 Ground control in back is what I want for my new build, but they don’t make that combo with matching casings.
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This is the most annoying thing about their tire lineup. T9 gets the heavy casings, and I want T9 in the front, with T7 or T5 in the back... but those only come in the lighter casings.
 
#37 ·
plummet said:
Rotating weight, combined with soft compound.

Rotating weight is huge.

I have 2 slayers, one with light carbon wheelset and one with heavy alloy wheelset. Tires are the same. The difference is about 1kg of Rotating weight. The bikes are night and day different in feel.
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Nah, weight is just a flywheel effect. Slower to spin up but maintains speed just fine. Rolling resistance is how much energy is lost in tyre tread and casing deformation.
 
#38 ·
flgfish said:
This is the most annoying thing about their tire lineup. T9 gets the heavy casings, and I want T9 in the front, with T7 or T5 in the back... but those only come in the lighter casings.
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Yeah, that is frustrating.

One thing I appreciate about Maxxis is that they offer the DHF in Maxgripp and Exo casing which is perfect for me in the front.
 
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