25c Tyres - tight clearance

bigfella

Über Member
Location
Essex
One for the frame builder's perhaps.

I have a Omega steel frame that I purchased second hand. It was originally designed for 23c tyres but the previous owner fitted a carbon front derailleur clamp to get 25c tyres on. I have switched tyres and they offer very near zero clearance on the seat tube. It is rideable, but can only inflate the rear tyre to 85psi. I was wondering if a small light indentation could be made, around 50mm long to accommodate the tyre? Probably only talking a couple of mm.
Thanks

566992


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ignore the residue on the frame, it is adhesive from some protective film that I had installed:
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Not much scope to take some material out of the dropouts:
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Here is a photo with the previous 25c tyres, they were a lot less durable, but offered a few mm clearance:
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Smokin Joe

Legendary Member
I'd revert to a 23mm tyre. I know the larger tyres are said to be better now but there is little difference between those two sizes and we all rode quite happily on 23s for years before we were told it was wrong.

Lovely bike by the way, far too good to start buggering about with the frame.
 

Ajax Bay

Guru
Location
East Devon
When considering whether to attempt this, bear in mind that the seat tube typically has a wall thickness of less than 0.7mm.
Nearly all steel frames have a seat tube of 28.6mm outside diameter. The seat post's diameter is = the inside diameter: 27.2mm. The wall of the tubing is 0.7mm thick at the ends and even thinner in the middle (the Omega looks a nice frame).
And the paint will suffer abrasion if you leave it with minimal clearance (first image) when the tyre picks up grit.
However, I see the new '25s' are Contis. What model? The 4000s come up wide (and tall) and the GP4S come up narrower than spec. Measure those tyres (sample the width at several points and take the largest reading).
Unless you're a lightweight (which presumably (two clues) you're not :laugh: ), 85psi is low for a rear wheel (@25mm).
 
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Sillyoldman

Veteran
Do you know anyone running 25c tubeless tyres? If so you could try their wheel as You can probably run them at 85psi.
I recently switched tubeless and seem to have a little bit more mudguard clearance fo the same tyre size.
Lovely looking bike by the way.
 

monkers

Über Member
I really like your bike. I'm going to guess that frame was made to accommodate tubs and sprints. I had tubs and sprints back in the day on a Bob Jackson titanium frame - that frame couldn't accommodate clincher tyres either.

I hanker after tubs and sprints for my Emonda at the moment, and I'm intending to go down that route in the spring, funds permitting.

If I was lucky enough to own that bike, that's the way I'd be going. The Bontrager R4 320tpi tubs are well-regarded for being fast and smooth riding so that's where I'll be looking. Each to their own, I'm a pretty capable engineer if I say so myself, but I wouldn't try to modify the frame in that way, just doesn't seem right.
 

SkipdiverJohn

Deplorable Brexiteer
Location
London
Go back to a narrower tyre. There's only one way you can gain clearance without distorting the shape of the seat tube, and that is to file a tangential slice off the rear of the tube then weld in a flat oval shaped filler piece to close up the hole. It could be done neatly by someone good at sheet metalwork but even then the heat input will still destroy the paint job.
 
OP
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bigfella

Über Member
Location
Essex
I'd revert to a 23mm tyre. I know the larger tyres are said to be better now but there is little difference between those two sizes and we all rode quite happily on 23s for years before we were told it was wrong.

Lovely bike by the way, far too good to start buggering about with the frame.
Many thanks for the compliment, I do agree that it would be a shame to mess with it to that level. The issue I have is that the Fulcrum 5 rims have a minimum tyre size specification of 25mm. I would rather not have to change wheels as well...

Another tyre I have been reading that comes up with a lower profile is the Vredestein Fortezza Senso All Weather. I could buy a new pair of Veloflex's but I only got a couple of rides out of the originals. Maybe they were old, the tyre wall started to come apart so am not so keen on those, especially as I am riding this bike in the winter months.

Some tyres are wider/taller than others. I'd go back to 23c - that's way too close.
The issue I have is that the Fulcrum 5 rims have a minimum tyre size specification of 25mm. I would rather not have to change wheels as well...

When considering whether to attempt this, bear in mind that the seat tube typically has a wall thickness of less than 0.7mm.
Nearly all steel frames have a seat tube of 28.6mm outside diameter. The seat post's diameter is = the inside diameter: 27.2mm. The wall of the tubing is 0.7mm thick at the ends and even thinner in the middle (the Omega looks a nice frame).
And the paint will suffer abrasion if you leave it with minimal clearance (first image) when the tyre picks up grit.
However, I see the new '25s' are Contis. What model? The 4000s come up wide (and tall) and the GP4s come up narrower than spec. Measure those tyres (sample the width at several points and take the largest reading).
Unless you're a lightweight (which presumably (two clues) you're not :laugh: ), 85psi is low for a rear wheel (@25mm).
That is really helpful thank you, I knew the frame tubing wall would be thin indeed. I wonder if a "fillet" could be welded in, that was my thinking, but it is a lot of work for someone.

The new tyres are Continental GatorSkins, I have another bike I could use those on, so they won't be wasted. Interesting to read that the GP4s come up smaller than spec. Assume that GP4 is referring to "Continental Grand Prix 4 Season"? They sound like a definite contender to solve the issue, as well as being durable. It is a pain to keep buying tyres to trial and error, but is necessary, I could always use them on another bike I have. As above, another tyre that I read has good all weather performance, puncture protection and come up slightly smaller than spec is the Vredestein Fortezza Senso All Weather - have you heard of these?


(and yes, I am not a lightweight unfortunately ;-) )

Do you know anyone running 25c tubeless tyres? If so you could try their wheel as You can probably run them at 85psi.
I recently switched tubeless and seem to have a little bit more mudguard clearance fo the same tyre size.
Lovely looking bike by the way.
Thank you for the compliment there.

I don't know anyone running tubeless unfortunately, but that is an option for sure. I would need to check if the Fulcrum 5s can run tubeless. Interesting, out of the box take, on the problem. Thanks.

I really like your bike. I'm going to guess that frame was made to accommodate tubs and sprints. I had tubs and sprints back in the day on a Bob Jackson titanium frame - that frame couldn't accommodate clincher tyres either.

I hanker after tubs and sprints for my Emonda at the moment, and I'm intending to go down that route in the spring, funds permitting.

If I was lucky enough to own that bike, that's the way I'd be going. The Bontrager R4 320tpi tubs are well-regarded for being fast and smooth riding so that's where I'll be looking. Each to their own, I'm a pretty capable engineer if I say so myself, but I wouldn't try to modify the frame in that way, just doesn't seem right.
Cheers for that, it is a lovely bike.

Seems a shame to ride it in the winter, but it was my size and represented good value to money so why not! I know what you mean about modifying the frame - it does not sit that well with me to be be truthful. But somehow I would like to get the frame, wheel and 25mm tyre combo working. I think really the answer is to find some 25mm tyres that come up with a slightly lower profile, as the rim specs state 25mm minimum.

Go back to a narrower tyre. There's only one way you can gain clearance without distorting the shape of the seat tube, and that is to file a tangential slice off the rear of the tube then weld in a flat oval shaped filler piece to close up the hole. It could be done neatly by someone good at sheet metalwork but even then the heat input will still destroy the paint job.
Thanks, this is the sort of thing I was thinking. I am aware that the paint would be damaged, but not sure to what extent. I did think that the section that has had work could be masked off and painted black - hopefully not that noticeable if partially hidden by the tyre.
 

vickster

Legendary Member
The issue I have is that the Fulcrum 5 rims have a minimum tyre size specification of 25mm

Really? I have 23mm Vittoria Rubino tyres on my carbon with 11 speed Fulcrum 5s (and did on a previous 10 speed pair of the same wheels which looked just like yours)
 
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monkers

Über Member
The nub of the question is how to turn this beautiful race frame into a winter bike frame. It must remain on the same wheel type, and accept durable, grippy, and presumably puncture resistant winter tyres. Given that tyres in that category, including the ones mentioned, all tend to have a sidewall height of 24 to 25 mm, there's little scope there without compromising on the tyre characteristics that you are looking for. There are tyres at around 22mm, but they otherwise don't quite fit in with what you are seeking.

I think that you've reached the right conclusion that it can't be done without modifying the frame. My problem, and I'll own that because it's hardly your's, is that I'd hate to see that being done and not turning out well.

What you can do though is go back to the chap (Mark Reilly) who originally designed and built this frame and get his advice. In case you don't know where to find him, I've given a link below.

https://www.reillycycleworks.com/
 

Ajax Bay

Guru
Location
East Devon
the Fulcrum 5 rims have a minimum tyre size specification of 25mm
You need to consider the limitation you place on yourself wrt the rims. Consider the extent of the jeopardy of running narrower tyres (imo a lot less than 'bending/denting' a steel seat tube). The rims have an internal rim width of 17mm. Sheldon offers the guideline of tyres best between a multiple of 1.45 and 2 times internal rim width. So minimum (Sheldon) is 24.6mm tyre width/height on a 17mm inner width rim.

Sheldon: "A general guideline is that the tire width should be between 1.45/2.0 x the inner rim width. If your tire is too narrow for the rim, there's an increased risk of tire/rim damage from road hazards."

With that in mind, then consider fitting a tyre that (we 'know') comes in over spec (4000) in 23 flavour to give that extra mm of clearance.
This site bicyclerollingresistance.com gives measured/spec comparisons in a table. Their measurements of 25-622 tyres are " All size measurements are taken at an air pressure of 100 psi / 6.9 bars on a 17.8 mm inner width rim."
HTH
Plenty of 4000 23s on the bay - are selling at a discount (4000s have been discontinued).
Or maybe some 24mm Vittorias?
https://www.ebay.co.uk/i/133619394339?
 

Ajax Bay

Guru
Location
East Devon
Well wiggleman (trying to sell wheels/tyres) is correct: they physically 'can' be fitted. But the Fulcrum site says from 25mm to 50mm - of course the latter dimension is 'barking' - far too wide and prone to rolling around and associated instability in corners/ on bends.
 
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