Inserts – do you change pressure?

Pondering fitting a rimpact on the rear of the mega for an upcoming Alps trip. I’m swithering as I’ve had limited time on them on bigger bikes and haven’t really liked how they feel in the corners. The rear tyre is an assegai, forget the exact type but it’s not a DH casing.

Do you find rear tyre pressure needs changing with an insert, either up or down? Wondering if there is an adjustment period for most folk or if you just liked / disliked them from day one and that’s it.

I’d say that rather than ,”needing reducing” it “allows reducing” ie that’s why I use an insert.

The insert facilitates the end goal of lower pressure rather than the other way around

Slightly lower pressure I think.

Which inserts did you try before?

Rimpacts are quite unobtrusive in feel IMO. I often forget whether I have them in or not.

The Pro version are an excellent Alps choice – as they drastically lower the chances of ride-ending tyre or rim damage.

I run a pair of Rimpact Pros because the last 2 tyres I destroyed were on the front while running just a rear insert.

A pair of Rimpacts weighs the same as a 29er innertube anyway and gives you so much protection that it’s not worth depriving yourself.

Not rimpacts, but with Cushcore I run much lower pressures. One of their benefits IMO.

I’d say that rather than ,”needing reducing” it “allows reducing” ie that’s why I use an insert.

This, cushcores allowed me to go from 20f and 25r to 15f and 20r and even lower, couple times I popped the pump on and saw 12f and 16r!

But, if I was going to go to a smooth jumpy place like BMCC (think its changed names), I’d put the pressure back up 20f and 25r. BPW wouldn’t touch em as the grounds so rough.

I think if you are going to run it just in the rear it limits you in how low you can go.

Front and rear you can pretty much go nuts.  I used to run 23psi/25psi front/rear.  After installing CushCore I’m now going 13/15 and in the wet it’s closer to 11/13.

There was a guy came 2nd I think talking about tyre choice for unbound gravel race on an escape collective podcast and pretty sure he was suggesting that though inserts allow lower pressure running the lower available volume means should take pressure up? Confused?

I’d say that rather than ,”needing reducing” it “allows reducing”

IME it was needed. Maxxis EXO up front felt terrible (no grip, harsh) at the usual 23psi after fitting a Rimpact Original, reduce to 20psi and it’s much better than no insert and the original pressure.

Not sure if it’s already been mentioned……

The same pressure will feel lower once you install an insert. Due to the decreased volume.

So I’ve generally gone roughly the same pressure, but the ‘perceived’ pressure is lower. Therefore, there is more grip.

The same pressure will feel lower once you install an insert. Due to the decreased volume.

How does that work? Same pressure in a decreased volume should feel harder as the ramp up due to compression is greater.

I know I’m a larger lad, but I’m amazed at how low a pressure many people run.
If I ran 20-25psi with or without inserts, my tyre would be squirming and folding on corners.

I know I’m a larger lad, but I’m amazed at how low a pressure many people run.
If I ran 20-25psi with or without inserts, my tyre would be squirming and folding on corners.

I think most people are really surprised how low they can actually go with inserts front and rear.  And how much faster they can end up going:

Certainly run lower pressure with Procore, although obviously it’s a different system to foam inserts and very much stops the tyre squirming, depending on tyre casing.
15-20 psi front and 20-25 rear. (Approximately).

Without inserts probably 5-10 psi higher.

I know I’m a larger lad, but I’m amazed at how low a pressure many people run.
If I ran 20-25psi with or without inserts, my tyre would be squirming and folding on corners.

Same here. Anything under 25 and I’d be clanging the rims on rocks all day long and burping on every corner.

With the Rimpacts I run 20-25 no problem.

I have some nuke proof ard inserts. Had them for years. I drop the rear to about 20psi. I only run them in the rear as that gets smashed around more. Big difference I noted was support to the sidewall and not  deforming in corners as much.

How does that work? Same pressure in a decreased volume should feel harder as the ramp up due to compression is greater.

Pump up a road tyre, a 2.1″ 26er and a 2.5″ 29er to 20psi, and then compare the perceived pressure of each (squeeze test). The extremely low volume road tyre will barely feel like it has any pressure at all. The 26er will feel a bit soft, and the 29er will feel nice.

I think you are mistaking casing construction with tyre pressure there.

Boyles law, or something.
As volume goes up pressure goes down.
A bigger tyre requires more air to inflate correctly but will reach a lower pressure.

I think Pembo’s basic idea is correct here, but I’m not sure how relevant it is. I think of inserts like volume spacers for your tyres for bottom-out prevention, but I haven’t heard anyone claim that volume spacers in your fork affect the initial part of the travel (which would be the corollary of pembo’s theory). Perhaps they do a bit.

That said, I don’t know how much difference all this actually makes to tyres. I still feel the tyre ‘bottoming out’ occasionally, but it does so against a bit of squidgy foam not a horrid hard clunk against metal.

Anyway, the TLDR is I can run substantially less pressure when I use inserts, which I like. Also, I haven’t had a pinch flat in years, which is good.

A bigger tyre requires more air to inflate correctly but will reach a lower pressure.

Yep, exactly. Much better explanation than my poor attempt.

So if you take that bigger tyre and reduce the volume by adding a tyre insert, you will need more pressure to achieve the same perceived pressure. Or, if you pump it to the same pressure, the tyre will feel softer.

It’s not the increased volume of air that makes a larger tyre feel harder at a given pressure, it’s the increased area of the casing. You can see this if you draw out the force diagrams with force per unit area (ie pressure). So adding an insert doesn’t make a tyre feel softer.

The insert acts like a volume spacer in a shock, as well as like a bottom out bumper in a shock. And some support the sidewalls against side loads too.

I run slightly lower pressures now but my switch to inserts has also gone in sync with slightly bigger tyres and slightly heavier casings. Maybe 10-15% lower. Hardtail is at 20/22 F/R on 29×2.4, and maybe a couple of psi lower if it’s wet enough. I really hate tyres squirming though!

That’s with old Rimpact front and Rimpact Pro rear.

It’s not really the same as a suspension fork with volume spacers.

You aren’t really changing the volume of a tyre when you squish it, either with your finger or by sitting on the bike.  You are deforming the tyre to change the shape so that a larger area is resisting the force.  If you apply 100 pounds of force to a tyre with 20 pounds per square inch pressure then the contact patch will deform until the area of the contact patch is 5 square inches but the overall volume of the tyre won’t change significantly.

Of course, you are changing the volume when you put an insert in but the tyre volume itself will still not change dynamically and the tyre will still need to deform until the contact patch is 5 square inches.

As the tyre deforms is experiences various levels of hysteresis depending on the tyre construction which is where the feeling of drag comes from (apart from the rubber sticking to the road/trail surface).  Air itself doesn’t absorb much energy but rubber and carcass changing shape does.

An issue I had with the Rimpact inserts and Oko tubeless sealant was it created two air chambers.

The only way I could get correct air pressure to the outside edge of the tyre was by drilling a few holes in the insert.

Possibly because the 30mm rim and the 2.5 assigai was too tight a combination.

So if you take that bigger tyre and reduce the volume by adding a tyre insert, you will need more pressure to achieve the same perceived pressure. Or, if you pump it to the same pressure, the tyre will feel softer.

Makes sense. So why would my tyre feel harsh and lacking grip at the same pressure after fitting an insert? Then after dropping 3psi it feels better – better than no insert at all.

“ It’s not really the same as a suspension fork with volume spacers.”

Good point, well explained!

So basically it’s a sidewall stiffener/damper plus a bottom-out bumper.

“Makes sense.”

It doesn’t, see above!

When I put a cushcore in and left my tyre pressures the same it felt like I was riding with flat tyres. I could feel the valve as it rotated through the contact patch. I ended up increasing the pressure which is the exact opposite of what is suggested

“Makes sense.”

It doesn’t, see above!

Ah, I didn’t realise who was disagreeing or agreeing with who!

Question remains though, why was it less comfortable at the same pressure after fitting an insert? Because it impedes the carcass changing shape and absorbing energy?

Fitting an insert doesn’t alter the overall volume of the tyre, yes the volume of air required will be less but the insert will generally be less compressible (if that’s the word) than the rest of the contents of the tyre. This will vary by type of insert and change how hard the tyre feels for a given pressure.

Thats how I understand it. Might be bollocks.

Question remains though, why was it less comfortable at the same pressure after fitting an insert?

Honestly, I think it’s probably one of those things that’s best filed under, ‘just because’.

I think it’s best to think of inserts as changing to a new system rather than altering an existing system. There are so many variables that change it’s kind of like changing fork model and then wondering why it feels different even though you are using the same pressure.

There are so many new factors to take into account like the extra weight, possible changes in the way the tyre deforms, and the change in overall air volume.  Too many new dynamics.

What I found after I started running inserts was that I dropped the tyre pressure way down and then gradually started raising the pressures on my front and rear suspension.  Then I found I had to alter my handlebars as the geometry of the bike was different due to the stiffer suspension.

It really is amazing how much one simple component can completely change your entire bike.

What I found after I started running inserts was that I dropped the tyre pressure way down and then gradually started raising the pressures on my front and rear suspension.

A similar thing here. Less pressure in my tyres make them more initially compliant, so I can run slightly higher pressure in my suspension giving more support.

I drop 2 psi when I fit inserts, using the same tyres.

Front shorty exo, 19psi with rimpact original

Rear specialized butcher / hillbilly, black diamond / grid trail casing, 23/24 psi with rimpact original