I would start here… This seems way too low. I know low pressure is lovely, but going just a PSI too low will lead to squirm, and perhaps thats what you’re mistaking as the drifting…
You’ve got about 30lbs on me, and I’ve never once comfortably ran less than 15psi without feeling the tire a bit lost.
I’ll add to this - with bike, gear and water, I weigh about 195. I used to ride at 13 psi front, 15 psi rear.
I recently switched to 19 psi front, 20 psi rear and have noticed a real increase in speed and confidence on trails.
I run specialized Fast Traks on a Blur TR, if it’s useful. On 29mm Roval rims.
190lbs here before bike, gear and water. I ride 22 front / 23 rear on 2.4 Aspens. BCBR type trails. I know that’s higher than a lot of people, but anything less and I get the skitterish feeling you are talking about.
Since tire pressure has come up… 
Should we account for airliners? Like start at normal then drop until it feels dangerous and go back up?
Has BRR tested a tire with and without liners?
Tested them on the steel drum and it made no difference? I can believe that. What about impedance?
Would imagine it would help riders achieve the low recommended pressures per the Silca Tire Pressure calculator. At least for myself, using inserts means that I can go as “low as I want” with air pressure, and not suffer any consequences like tire roll in corners, rim strikes and so on.
I have an insert in my rear tire and got a pinch flat on a ride this week. I think inserts lower the risk, but don’t eliminate it. My Dynaplug fixed the hole at the tire bead (+1 for Dynaplug).
I thought Vittorias marketing on this was interesting - I really don’t believe the claim that they shrink any appreciable amount and can confirm that at normal riding pressures they absolutely provide sidewalk support and definitely change the behavior of the tire.
Edit: This is for Tubolight Evo SL, maybe the actual Vittoria acts completely differently, though I doubt it. If 15-20 psi is enough pressure to completely shrink it, I don’t think it would provide enough support to actually ride on.
I’d also argue that if Vittorias idea that the insert shrinks while the tire is inflated, it would provide very minimal protection to the rim because unless the rim strike occurs only while there is no pressure in the tire, it would be too small and not yet spreading out.
I can’t seem to find a BRR review on the MTB inserts, could you link to it? That’d be interesting. I did see their gravel liner review and they didn’t make any claims on whether it contacts the sidewall or not, though they do mention that it doesn’t significantly increase rolling resistance until very low pressures where the insert is assumed to be actually directly supporting the wheel.
As I mentioned, I have not run Vittoria inserts, only Tubolight. It’s possible that the Vittoria are a completely different material/insert, I can’t speak to that but I’ll say their weight, appearance, and specs are very similar to one another. With the winged style insert of the Evo SL (the Vittoria looks to have a slightly different shape so as mentioned maybe it’s totally different but I suspect it’s similar) you can feel the insert supporting the 1/2” or so of sidewall outside the lip of the rim, even with the tire inflated.
I have ridden on a wrecked tire with an insert and I agree with your experience/explanation - it provides a small amount of added support protection to the rim and does a decent job of keeping the tire from peeling off the rim but little beyond that.
Same here and very frustrating, especially when trying to get tires done in a hurry/time crunch.
I should clarify, I definitely can’t feel them through the tread, just down by the bead, on 2.4 Aspen/Aspen ST
I can say from experience that both Tubolight SL and Vittoria Airliner Light / XC definitely press into the bead of a 2.2 race king on a 25mm IW wheel. Breaking the bead on that set of wheels for me is a pain in the rear and I’ve even taken to carrying a different set of tire levers because of it that gives me better leverage. I don’t think they press much into the tire much above that, but will take a closer look at the set I have mounted up downstairs with Vittoria’s over the coming days. (Tubolights in 2.35 Ray/Ralph on 30IW, and Vittoria in 2.2 Race King on 25 IW)
I think I prefer the Vittoria to the Tubolight, but not by a large margin No center “channel” to hold sealant, never had an issue with the join, I can’t even remember if there is one, although there must be? The outer surface of the vittoria does start to degrade a little faster than the tubolight though, gets a little fuzzier, maybe absorbs more sealant? But, the Tubolight appears more porous and rougher at first…
Yeah what is up with that? It’s like you can’t put sealant in the easy way with those or maybe I’m missing something.
Joe
I have no problems with the notion that if it contacts the side walls, it alters the stiffness properties of the carcass. It sounds reasonable that making the carcass stiffer, increases the hysteric losses.
What is less clear, is how large is this loss? Is it negated by ability to run much less pressure, and have smaller impedance losses? I don’t know, and I have not seen any proper testing done on this.
All I can say for myself, is that by using larger Rimpact inserts, that definitely makes contact with the tire side wall, I can run substantially less pressure. And not just because the total volume is changed. You can visibly see how much more the tire is deformed at the contact patch.
The change in ride quality is also night & day. Using the inserts significantly dampens the trail chatter. Especially the high frequency small amplitude chatter when riding fast on rough double track. It also appears to aid in outright braking and cornering grip.
Can’t say if it overall reduces or increases the rolling resistance, but it certainly feels faster, and is less fatiguing. My guess is that for a longer technical marathon XC race, it is definitely faster overall.
No, I am not sure of that at all! That could be it. Although I am guessing that using the Rimpacts in combination with Roval Control wheels, Fast Trak tires at 15 psi does more than just allow for the drop in pressure.
At this pressure, I think the tire is forced into contact with insert at a certain threshold of “square edge hits”. What it does to alter the characteristics of the tire itself, I have no idea!
It is definitely a mixed bag when it comes to gravity focused riding at the sharp end! The tires & rims have gotten so good. On my end of the spectrum, far, from the sharp end, I definitely do not need inserts on my enduro bike, when it is setup with DD casing tires!
So, I’m a BRR pro member subscriber and the first to refer to their results when picking tires or in discussions. However, tire rolling resistance can be a weird thing at times.
I happened to have at hand a pair of Schwalbe Racing Ralph and Ray 2.35 Super Race and a pair of Aspen 2.4 120tpi (the non team ones). I also have a 700m stretch of road next to my house that’s essentially a bowl : a gentle downhill, followed by a flat and then a gentle uphill. So why not do a little tire rollout test.
Protocol :
- using same 30mm id carbon wheels
- the Schwalbes measured 58mm wide, the Aspens 63mm. I forgot to measure the height, but the Aspens were a fair bit taller
- tests done within a 40min window
- tarmac in that road section is mid tier, hardly a velodrome but not completely broken either
- there was barely any wind that day, 18°c
- did 5 runs with the Schwalbes, ran home, switched to the Aspens and another 5 runs with those
- all the tires set at 20psi
- according to BRR there should be a roughly 15w difference in RR between those two sets, so hardly negligible
- I was lazy and didn’t do the math considering the potential energy, an aero drag estimation and what was left for the RR to play a role in those conditions
Results :
There was no difference whatsoever between the two tires sets. Both were doing the same rollout distance, with remarkable consistency (about ±0,5m across the 10 runs).
Hypothesis :
- my test sucks and all of this is completely irrelevant
- the Aspen’s bigger diameter was enough to make it eat better the road irregularities and thus overcoming it’s higher tested RR
- the relatively small (compared to planet earth) diameter of the test drum at BRR inflates the RR differences between different tires
Curious to hear your thoughts
You were effectively running higher pressure in the Aspen due to the bigger volume. Normally the bigger the tire, the less pressure you would run in the real world on the trails.
Not wanting to sound pedantic, but was running the same pressure on both setups. The bigger volume of the Aspen with the same pressure resulted in a stiffer tire. This could result in a higher rolling resistance due to the road irregularities, which would contradict the results
Or it could result in a lower rolling resistance dependent on the surface (e.g. BRR shows decreasing rolling resistance as pressure goes up)
What was the max speed for the test and the acceleration and deceleration experienced? Tom Anhalt has written some about how tire rolldown testing will become ruled by aerodynamics if the speed(s) and acceleration is too high.
Like both sets of runs were done in 40 min total? Was the temp pretty consistent? Sun about the same or really early or late?
I’m surprised. I think we need more runs. I don’t believe it yet but I don’t have a good explanation either.
Joe