Cycling UK Forum

So, cosnidering changing forks with a view to disk brakes. The bike is a nearly 20 year old on-one Pompino, so non-tapered headtube, no integrated whatevers and not modern whatsits.
Wondering what compatibility issues I should keep an eye out for.
And how I work out what the current geometry on my fork is, so that I can ensure that the new fork doesn't result in toe overlap.
(bonus question, should I just get a second bike with disk brakes as it will be about the same price?).

Two measurements are important for replacing like with like:
Axle to crown, exactly what it says, straight line between the center of the dropout to the bottom of the steerer.
Offset, how far in front of a straight line from the headtube the axle is. You might find that listed in a geometry chart, if not it isn't hard to measure - Measure BB to dropout, turn 180 degrees, measure again, minus the second from the first, divide by two, that's it.

Any deviation from those measurements will effect steering geometry. How much you'll notice that is subjective, I don't notice a few mm either way. Unless PX did something abnormal there'll be plenty of options in the same ball park.

Measure the axle to crown height and fork offset - easiest if you drop the front wheel out and clamp an axle in the fork - then look for a disc fork with the same (or close) dimensions. Spa and SJS both sell a range of disc forks,and generally quote the axle to crown and offset dimensions for each fork. Most disc forks with 1-1/8" steerer are steel, though there are a few carbon ones around. The other consideration is do you want a thru axle fork, in which case you will need a thru axle front wheel/hub, or stay with conventional dropouts.

Though if I was you I'd treat myself to a new bike...

Depends how picky you are, I do a lot of fork changes, removing half-ton budget MTB forks and replacing them with solid forks. This messes up all manner of dimensions but rarely affects the handling in a way anyone would notice.

The guru (Mike Burrows) has a similar opinion https://www.youtube.com/watch?v=AZrvLdX7B3E

If you’re after some steel forks you can always get them made to measure. If you want carbon then post your fork measurements on here and somebody might be able to help you track some down. Has anybody mentioned steerer tube diameter yet?

I have changed forks to correct handling issues. I have a 1951 Holdsworth where the original fork is very beautiful but is also very flexible, allowing the brake to rub badly on climbs, and it has too little offset so the bike was "stiff" going into corners.

I fitted a heavier steel fork (sold to me as 531 but I'm not convinced) with more offset, and it has completely transformed the bike.

Take your existing forks to a reputable frame builder and have disc mounts brazed on.

I have an unused pair of pompino forks as I upgraded to carbon forks and stored the originals.

Dedicated disc bike every time,why risk a front/rear end failure?I,ve retrofitted front discs on a mt.bike whose frame was already hugely reinforced,not a road bike though so give it a long,hard thought.Be aware,a front disc will exert a alot more force through the beefed up fork which then transmits that force where?The frame, which,crucially,has not been engineered for it.If you,ve got the room,splash out on a dedicated machine,preferably with mudguard clearances.One of my cycling companions has a titanium gravel bike with discs.Still has issues with disc rub,pain in the butt,just saying.....So,for me,off road......disc every time.Road.....rim brakes.All weather road.....discs...maybe...with a dedicated frame.

cyclop wrote: ↑18 Dec 2022, 8:58amBe aware,a front disc will exert a alot more force through the beefed up fork which then transmits that force where? The frame, which, crucially, has not been engineered for it.

Errrm, you're going to have to explain how, assuming both types of brake are capable of locking the front wheel, the force going into the head tube is any different between the two systems? (Internally, the fork will, of course, see some interesting forces applied at the disc caliper mounts.)

rjb wrote: ↑18 Dec 2022, 8:37am Take your existing forks to a reputable frame builder and have disc mounts brazed on.

Or maybe not:





Not my photos.
I doubt a reputable framebuilder would add disc mounts to a fork blade not intended for one.

I have an unused pair of pompino forks as I upgraded to carbon forks and stored the originals.

Do you know the A-C and offset for the OP?
I'm assuming if you found a carbon fork, they're pretty standard 395 - 400 A-C and 43 - 46mm offset? If so, there's loads of options, in steel and carbon, several Surly forks will fit fine and are easily available.

PH wrote: ↑18 Dec 2022, 1:41pm

rjb wrote: ↑18 Dec 2022, 8:37am Take your existing forks to a reputable frame builder and have disc mounts brazed on.

Or maybe not:

....and then there's the rear brake, by which time you need a new frame, not just new forks.

belgiangoth wrote: ↑16 Dec 2022, 11:49pm (bonus question, should I just get a second bike with disk brakes as it will be about the same price?).

Yes!

I like messing around with bikes and as a result I found a set up which really works for me (and I had to find out, because the set up I use isn't commercially available) however it's not been without its sacrifices, which has been time and money spent trying other hubs gears, brakes and handlebars.... I reckon I must have spent over four grand on trying different transmissions* (derailleur set ups/ 3 speed hubs, nu-vinci hub) and brakes (shimano roller brakes, v brakes, cantilever brakes) and simply too many weekends bodging it all together.. at the time it felt like fun but looking back now..... that was time I could have spent actually riding the blooming things, or learning to cook, learning to play a musical instrument, finishing some art projects..

You must like your bike to have kept it for so long - so keep it as it is, and get another to ride during the days when the weather ain't so good

(*Annoyingly I toured on 5 speed bikes with, pull back handlebars and 26 inch wheels during the 2021/22 years... if I had stuck with that set up and just moved to using drum brakes, I could have saved at least 2 grand and at least a handful of weekends)

axel_knutt wrote: ↑18 Dec 2022, 4:02pm ....and then there's the rear brake, by which time you need a new frame, not just new forks.

Rear brake is far less important, even less so if the Pompino is fixed.
I like disk brakes, I doubt I'll ever buy another bike without them (Though maybe I'll never buy another bike at all) but I don't think I'd start messing about with a bike I already had and liked.

squeaker wrote: ↑18 Dec 2022, 12:05pm

cyclop wrote: ↑18 Dec 2022, 8:58amBe aware,a front disc will exert a alot more force through the beefed up fork which then transmits that force where? The frame, which, crucially, has not been engineered for it.

Errrm, you're going to have to explain how, assuming both types of brake are capable of locking the front wheel, the force going into the head tube is any different between the two systems? (Internally, the fork will, of course, see some interesting forces applied at the disc caliper mounts.)

So Chris Juden took some time out to explain to me the exact same thing, it took all his patience, some of his sanity and lots of short words and wax crayons and some glitter.. but here goes
(1) the braking force with rim brakes is at the top of the fork where the stresses are easily managed, simply because the fork tubes are thicker at this point
(2) the braking force of the disc brake is located in one of the fork blades only, at a point where the metal of the fork leg isn't that thick (so that's the weight of the rider, plus any luggage, plus the weight of the bike x speed) all that is concentrated at one point, in one of the two fork blades...

So what happens is the brake bites the disc and the holds it, however if the force of braking (newtons=mass (kg) x speed (metric) over comes the tensile strength of the fork blade (remember it's a thin walled tubing at the point where the capiler has been wield to the fork leg) the brake capiller is then carried around with the arc of the disc pulling the fork with it and twisting the other fork blade as the resulting forces are expressed through the front wheel hub

Bicycle forks which are designed for disc brakes use fork blades which have thicker walls to overcome the braking and twistjng forces exerted by the disc brake.
However as a result of this these forks are less compliant over rough surfaces meaning they'll transmit more road and track vibration to the handlebars..

..interestingly Thorn are (I think) the only manufacturer which have produced a bicycle which has rim braking at the front and disc at the rear, for rider comfort

A good,technical, explanation,Cycle tramp,of the forces involved.I also get what you,re saying,Squeeker ,i.e.both brakes will lock up the front wheel and ,therefore,a similar(not equal though) force gets transmitted into the frame.Surely, the disc will exert a greater force over a shorter period before locking up?Force=mass x acceleration,or deceleration in this case.The photos are scary ,further convincing me that a front disc for road bikes just doesn,t feel right.Not scientific ,not technical.