How fat can I go

NickTB

Veteran
Hi all.
My wife’s pretty much unused Triban hybrid is seeing a new lease of life due to lockdown. From only ever being used on London to Brighton on roads she’s now attempting forest trails. The tyres are 700 x 25 and she’s struggling badly with balance and confidence.
What’s the best option? Here’s an idea of clearances


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5-BA9-C94-D-46-C8-4-D49-A7-EB-224-CBEE04-BB7.jpg
 

Smudge

Über Member
Location
Somerset
Looks like you could get 35 between those forks, maybe 38, but you'd need to check the chain & seat stays. There could be less room between either of those.
 

Ajax Bay

Veteran
Location
East Devon
Maybe: https://www.cyclingweekly.com/reviews/hybrid-bikes/btwin-triban-520-flat-bar
See also (more pics): https://www.cyclechat.net/threads/btwin-triban-520-flat-bar-road-hybrid-bike-large.227178/ offered for sale by @midlife
There are three limiting factors:
1) Clearance between the fork blades (and you/she can measure that) (but @Smudge makes a good point about the rear wheel clearances)
2) (Rim braked wheels) Getting the tyre through the brakes when removing and installing the wheel. Any wider (than 25mm) and she can only get the wheel off (eg for cleaning) by letting the air out, and reinstalling flat before inflating (applies also to dealing with a puncture (reinstallation phase). The brake calipers look like 'long drop' so the other route is to accept that the brake cable clamp bolt will have to be released each time to allow the brakes to open out and let the tyre through. This is fine but two points: need to become competent at resetting the clamping accurately; and repetitive reclamping will result in the cable deteriorating relatively quickly and therefore require regular replacement.
3) Rim width - a fair rule of thumb is no more than twice internal rim width. The images don't allow me to tell, but I suspect the OP wheel rim width is 15mm (622-15 may be in text on the rim), so beyond 32mm width tyres would be pushing it. Many will say it's no problem, but given the OP's stated objective concern is "balance and confidence" you don't want the tyres 'rolling around' as over-width tyres can do.
If I was the OP I'd change the tyres for 28s with some tread to grip off-road, tell the rider these are 'really good tyres and wider and grippier' so they are 'much more stable' and will offer her 'more balance', and let that installation of additional confidence do the rest. Because actually the wider tyres will not make much difference to balance: maybe a little more grip, but it's marginal, except maybe uphill on rough forest trails.
 
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Cycleops

Guru
Location
Accra, Ghana
The rear triangle is where you might meet some trouble So let’s have some shots of that. I should try some 32c which will give a bit more comfort over trails.
 
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YukonBoy

The Monch
Location
Inside my skull
Try 32mm on the front. The front and rear tyres don’t need to be the same width. I run 37mm up front and 32mm rear on my recumbent. It copes just fine on forest trails.
 

raleighnut

Guru
Location
On 3 Wheels
I'd go 28s too, you might get fatter but any slight buckle due to a rock/tree root/pothole and you'd be walking
 

CXRAndy

Guru
Location
Lincs
35-40mm tyres are super comfy at around 45-60 psi Measure gap in rear point behind seat tube to see how big rear will take.
 

Ajax Bay

Veteran
Location
East Devon
I said a 'rule of thumb' because I'm aware that opinions differ. And probably more 'reliable' from tyre widths 1" to 2" and inner rim widths from 14 to 23. Apart from 'fat bike' wheels, 23mm is about the largest 'MTB' rim generally manufactured (happy to be proved wrong (NB 'generally' - bound to be some outliers).
https://sheldonbrown.com/tire-sizing.html
Seizing quotes from Sheldon:
"A general guideline is that the tire width should be between 1.45/2.0 x the inner rim width.
"If the tire . . . . is too wide for the rim, there's an increase risk of sidewall wear from brake shoes, and a greater risk of loss of control in the event of a sudden flat."

"
 
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