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Sooooo....
I'm ill advisedly going to try and apply my very basic understanding of suspension dynamics to my initial disappointment at how my tyres feel with new inserts in them.
I was hoping to enjoy the small bump performance of running my 40mm tyres at a lower pressure (35 instead of 40) without fear of pinch flatting the tyre or dinging the rims (both of which I have achieved at closer to 40psi).
First ride out which was a relatively pacey (by my standards) local loop, 50/50 tarmac and loose/gravelly tracks and the tyres just didn't feel quite right. Definitely didn't feel any softer, despite letting more air out, and the cornering felt a little bit sketchier. Thankfully the tyres didn't feel any slower on tarmac so I guess there's scope to drop the pressures more.
Could the smaller air volume due to the insert cause the tyre pressure to 'ramp up' too quickly and just end up feeling the same as a harder tyre?
I'm also wondering if the sketchy cornering is just due to the insert changing the shape of the tyre?
Going from 40 to 35 psi doesn't seem like a very big change to get the improvement you were looking for? I'd try lower.
I'm interested in this as I've been thinking of trying an insert on the rear of my cyclocross, pondering if it will improve the somewhat mixed tubeless experience I've had so far.
I put inserts in mine the other week as it was time for new tyres. This is running Schwalbe G-One 38c on DT Swiss tubeless rims. It took a few rides to play with the pressure and the rear does still feel like it might have a puncture (it doesn't!) but they've been great so far. There's a tiny "ramping up" as the tyre bottoms out on rocks, sort of similar to how a suspension fork ramps up at the end of its travel.
Not noticed any adverse effects on cornering but to be fair, the old tyres were so worn that it could just be the new tyres more than compensating. No complaints so far although it did make the tyres noticeably more difficult to get on the rim. Hoping that I don't need to take them off until they're actually worn out and any punctures in the meantime can be fixed by the sealant and/or by Dynaplugs.
I’m interested in this as I’ve been thinking of trying an insert on the rear of my cyclocross, pondering if it will improve the somewhat mixed tubeless experience I’ve had so far.
Rimpact Pro CX inserts certainly made my 40mm Vittoria Terreno Drys easier to seat, it just took slightly more effort with a normal track pump than inflating a tubed tyre, so big thumbs up from me (it usually required a blast from a reservoir tank as well).
Will go a bit lower with tyre pressures then. I might just be expecting too much as I still enjoy relatively fast rolling on tarmac (one of my local tarmac PBs was set on the same tyres with latex tubes at 40psi, despite some decent efforts on my road bike).
My experience of this tyre size is that it’s never going to be comfy. Not enough air volume to help with that. Lower pressures in that size are all about better traction. Great for a wet muddy cx race but not so good on hard surfaces, as you’ve found. Main advantage of cx inserts is rim protection at lower pressure with a bit of support for the tyre wall, although I’m not convinced this would be as significant as it is on the full size rimpact pro.
Can’t see a 40mm tyre “ramping up” as there’s not a significant reduction in air volume compared to tyre size. More likely you’re just running into the insert which is designed to firm up.
Interested to see how you get on though. Rimpact pro have been a bit of a revelation on my new full suss and was considering them for the gravel bike.
Could the smaller air volume due to the insert cause the tyre pressure to ‘ramp up’ too quickly and just end up feeling the same as a harder tyre?
Basically yes, inserts turn a pneumatic tyre into a semi-pneumatic one. You wouldn't expect the same comfort at the same pressure as before because of that. Could be close, or not - all depends on the material and insert shape used. If you drop the pressure you might get more grip at the same perceived comfort level. To get more comfort you may be at a pressure that's too low for good handling or efficiency since the initial bit of compression is the tyre drop, but further compresssion is the comfort over bumps = the ramp up as you say.
Low pressures in a fairly narrow tyre feels pretty bad for tarmac corners to me. I spent ages trying to get used to 650 x 47s at ~28 PSI (I'm 75kg). Felt very comfortable and quick enough for mixed surface riding but tarmac cornering was poor. 35 PSI makes all the difference there, got back the feeling of having an edge/shoulder to corner with.
Can’t see a 40mm tyre “ramping up” as there’s not a significant reduction in air volume compared to tyre size.
Could be true, dep on the insert shape. It's the % drop in volume that will count and a smaller tyre like a 40mm just doesn't have much volume to begin with. The diff in air volume between a 40mm and 50mm tyre on the same rim is about 64%, so an insert may be 25-50% volume drop? Seems significant enough.
The way I looked at it was the section of the tyre that gets squashed is pretty small compared to the over all circumference of the tyre. Compared that to a suspension fork where it’s shrinking to say a third of its normal volume. Insert will certainly reduce air volume by a decent amount, not sure it’s enough to cause a significant air ramp. Also because the tyre is rolling the impact point is released pretty quickly after impact.
Happy to accept I may be talking bollocks but still reckon what he’s feeling is running into the insert on bigger impacts. The Rimpact pro is designed with an extra layer designed to firm up on impact. Can certainly feel it when casing square edge rocks on my mtb.
The other thing to remember is that with gravel inserts there's much less likelihood that the liner is free floating from the side of the tyre. Any time the foam is acting in place of pneumatic suspension and changing the hysteretic properties of the system, your rolling resistance skyrockets.
This is why when the pro peloton has experimented with them for Paris Roubaix and the like they always use the smallest inserts - basically as runflat protection.
On an MTB, inserts trade flat protection and grip for rolling resistance. Lots of corners and often times on the brakes anyway - like Enduro etc - this is good. Not convinced it's worth it for xc unless it then allows you to run a softer sidewalled tyre and so balance the RR.
Happy to accept I may be talking bollocks
I don't think so... I was wondering about tyre volume generally while back. Tyre deformation over a bump vs total air volume etc, with tyre contact patch coming into it plus variation in tyre pressure when using different size tyres - the maths is beyond me and I think it'd need a test rig to actually measure it.