crashBangWallop

Hi

I recently converted an old steel frame to front disc to make a steel city bike (rear 3-speed internal hub and roller brake) with a bit of stopping power. I used to have front roller brakes but the nexus hubs internal anti-lock mechanism meant stopping quickly in Dublin traffic was no use.

I low temperature silver brazed an iso disc tab onto the fork and added a metal fin to strengthen the fork against the bending forces that the brake will put on the fork. Ultimately it might not handle the stresses but worth the experiment.
The front rack has thru-hole axle mounts and so this was secure with the quick release (maybe a mistake!).

All good so far until under heavy braking the wheel came out of the fork ends and I went superman through the air and into the stone kerb. Luckily my helmet smashed and saved my head the trouble.


So the question is - is the idea just too faulty or perhaps did I just have something set up wrong?
Some thoughts -

The quick release is allen-key type - was planning to use a pentagon key one for bike theft safety.
The fork ends are totally flat and since the frame was powder coated relatively smooth - maybe this was a contributory factor.
Would safety tab washers work to prevent the wheel ejecting? Would a aerated safety washer do the job as well?
Could I make a safety washer that bolts into the mudguard eyelet to retain the wheel in emergency?


Thanks


Oh - I can't post pictures as I'm a new user

finished bike

crash

Andrew R Stewart 

You can do whatever you like, already did some.

Front axle ejection from a downward facing dropout is a well known possible safety issue. So much so that all kinds of preventative methods are used by those who have big liability insurance costs

Dropouts with axle retention tabs (lawyer lips) is perhaps the most common, as they already are pretty much standard for QR skewered secured front wheels. Axle washers with an "engaging into the dropout" tab in slot is also pretty common, often found on the least expensive bikes for a reason.

What type of QR cam design does your QR have? Open cam riding on a plastic curved "washer"? Or an internal cam with only metal parts? Guess which has a greater clamping ability?

As long as the fork's design (specifically the dropouts) has the dropout slot pointing in the direction that the brake reaction force is acting on the axle the chance of axle slipping within the dropouts and sliding completely out is a real possibility. The best solution is a fork that (among other aspects for hub brakes) has a dropout design/slot direction that points nearly forward. This way that reactive force is applied against the side of the slot, not to the open end. Andy

Crankycrank

Generally, a bad idea. To be safe you would need to get a properly made fork with a mount that can handle the extra stress of the disc brake. No matter how much you add bracing to your current fork there are still twisting forces that can overcome just a fin brazed on. The twisting also contributes to your quick release not holding especially when a rack stay is clamped between the quick release nut and fork ends and as Andrew mentioned above, your fork ends are facing the wrong direction for discs. I like to experiment with lots of things too but a crash like yours would have put an end to that experiment for me. Be safe.

zacster

If you want to use that bike, replace the fork with one that has the mount built in and thru axle. There is a reason for thru axle and you found out the hard way.

And while doing that you'll start to find out why you can't just replace that, then your lever won't be right, and then a matching lever for the rear, then the cable, and on and on and on. I did an upgrade to an old MTB to make it a better city bike and I ended up replacing almost every moving component.

Barry2 

crashBangWallop ,
Welcome to Bike Forums.
Lots of information available here, hopefully pre-tinkering in future.

Glad you walked away.

Barry

13ollocks

When I saw the title I assumed that you were going with a new disk-compatible fork.......but you went full "home-made" - I admire your enterprise, but jesus.....
Given that the fork already used a roller brake (ie, a hub-based brake), it should be all able to handle the forces exerted by a front disk. Since you're already up with brazing, could you replace the fork ends with something with a more positive retention? FWIW I ran an MTB with QR skewers and disks for years without issue - the hated "lawyer lips" were the key feature

Andrew R Stewart 

Lawer lips and tabbed retention washers are like having an alarm system for your store. It won't stop the problem (axle slipping in the dropout) but just reduce the severity of the results. keeping the axle secured is the best answer and a through axle design does best here. If you must keep a QR skewered axle use the best skewer you can get.

I've only made a few disk braked forks and while my home made dropouts do have a recessed clamping surface (lawyer lips of sort) I use quality QR skewers (enclosed cam, steel) and haven't yet had any slipping axle issues. Andy

GamblerGORD53

You just bought a Roller brake 3 speed?? LOL. Who told they were brakes?? They are disgusting BOAT ANCHORS. Biggest mistake in your life. LOL.
Sturmey Archer XL DRUMS are the king of brakes. So strong it broke my fork in Vietnam. Stopping distance from 13 mph is 4 inches. This hub now has 32,000 miles including both tours of 8,200 miles. I've tested this bike from 39 to 0 mph, flawless.
You can get this with a SA 3 speed. The smaller 70 mm drum is far weaker.
Probably needs a lipped fork, both mine have it.

Rick

​​​​​​​A less convoluted solution to keep the wheel from loosening would simply be to make the wheel tighter.

Andrew R Stewart 

​​​​​​​Which is what has been said at least twice here. Andy

grumpus

The axle lock nuts should bite into the inner faces of the fork ends to secure the wheel, without needing the qr to add much grip on the outside - sounds like you just didn't tighten the qr enough, the rack legs probably don't help the clamping force any. But disc-brake wheel ejection is a known phenomenon - the easy way to counter it is to mount the caliper on the front of the RHS fork leg, then the reaction force isn't in the direction of the open fork end (but manufacturers instead went with through-axles). Tab washers don't address the actual problem, but I guess ok as a short term solution.

base2 

In addition to lawyer lips, a lot of forks have the drop out pointed forward so that the torque from the brake application at the very least levers the axle into the dropout side. Levering the axle deeper into the drop out is best, like rear brakes.

There is a very famous picture here on bike forums of a pink bike with home made disc tabs such as yours. The fork leg itself is folded back at the upper brake tab.

I've fielded the home made disc adaption question from a variety of bike enthusiasts. For whatever reason people seem to think anyone with a torch can retrofit anything. I admit it is a tempting proposition. In this case, it's best to be well studied in the forces involved. A new fork is always cheaper than dental work.

icemilkcoffee 

Did you weld the caliper mounts on the rear of the fork? You can try putting the mounts on the front of the fork. That way the torque reaction will be more backwards and less downwards.

70sSanO

Except for thru axles, most retention systems, lawyer lips, recesses, etc., are QR retention, not axle retention. However, as will be beaten over and over, a steel closed cam QR is important. The skewer rod is basically absorbing all the force if the skewer remains in place. This has not been an issue for many years when a high quality skewer is used properly.

What I have not seen, most likely due to cost, is a slide/rotating mechanism on the hub side of a fork that essentially closes the dropout. With disc brakes it has to be made with enough precision to locate the rotor consistently.

The cost of a thru axle setup has to be so much cheaper that it would be foolish for any company to pursue a true QR axle retention system.

Trek has a Thruskew that is designed to partially mimic a thru axle, but ot is really to help mitigate improper use of a QR skewer.

John

13ollocks

I think the OP's trying to avoid using QRs of any kind - bike theft, and bike part theft, is rampant in Dublin (like most cities, I imagine), so he was going with a screw-on skewer with a 5-sided socket head for security. There are a number of security skewer options out there, but I don't know how good they are compared to an old-fashioned internal-cam QR

crashBangWallop

This would have been a good idea.

crashBangWallop

Yes - this would be the plan alright.

70sSanO

If you are using a hex skewer, all you would need to do to retain the skewer is close the dropout on the outside with a half round. Like a closed lawyer lip.

You would need to remove the skewer completely to remove the wheel. And you would have to have access to tighten it. But you wouldn’t need to change the dropout direction.

I’m not a frame builder, so I’ll accept any criticism for suggesting this.

John

bboy314

I envy the person whose biggest mistake in life is this.

Bill Kapaun

For better retention, I'd try a solid axle with good, serrated nuts.

Duragrouch

Yeah, the disc brake with QR issue is a serious one, especially with wheels that large, that generate a lot more torque for the same braking thrust at the pavement, then say, my 20" wheels. Briefly, as you brake the front disc with the caliper behind the axle, the caliper acts as a "pivot point", so with the wheel being pushed back, it wants to rotate about that pivot point and pull the axle down, and hard, there's a big leverage ratio there.

If the caliper were in front of the fork, braking would push the axle up. But that would mean either the brake cable/hydraulic line would exit the caliper on the bottom (unless someone tooled up a new caliper), or if rotated the other way, the disc would need to be on the outside of the fork for the caliper to grab it, impossible. Dahon came up with a brilliant way to solve this without thru-axles on their new Curl D9: They mounted the front caliper on the front of the fork on the RIGHT, and just flipped the wheel around so the disc is on the right too. (And discs are designed to rotate one direction, but the disc can just be reversed to allow this.) This pushes the axle up under braking, and allows use of a standard caliper, and the cable exits up like it should. I've not seen that on any other bike, but a brilliantly simple solution to allow normal (and cheaper) quick-release axles, versus thru-axles.

crashBangWallop

This is an interesting idea.
I could braze a washer or something to the outside of the fork ends. the skwer etc could still go through axially but not come out radially in an emergency.

3alarmer 

...every "security" skewer I've ever seen, uses an alloy rod. You can't tighten them down much without risk of stripping the threads. The whole of the modifications mentioned in the OP strike me as convoluted in approach. I get maybe wanting to stop better on that bike, but this conversion to disc is not the way to do it. For the OP, if you can lock up the brakes on your bike and skid, you don't need discs. If you can't lock up your wheels and skid, try using better shoes/pads on the brakes you have first, and review the mechanicals for any problems that can be fixed. That said, I'm glad you're OK.

If you absolutely have to have a front disc, the suggestion for a new fork with mount and thru axle is appropriate. Here, where I live, it would be easier and probably cheaper to buy a different used bike off Craigslist.

Aubergine 

I swapped a disk fork into an old Peugeot so I could find out how well a disk would work in Seattle's rain. Braking was excellent, but the handling characteristics were pretty bad with the different fork.

if I were going to modify an existing frame for a disk, I'd add the mount to the rear triangle. It'd be easier to stiffen those tubes to handle the stress of the brake, and the dropout issue would be gone.

Duragrouch

Yeah I've tried to think of an easy retrofit for this that didn't involve brazing or welding. The safety tabs on cheap bikes works, but it interfaces into a slot just above the dropouts that you don't want to cut into a fork not designed for it. Also, the tab hook I think is just designed to prevent the wheel dropping out by gravity, and might not handle the pull from disc torque. I'm envisioning a steel washer with extension to just above the dropout flare, clamped around the fork blade, so not relying only on friction, but the interference of the dropout flare keeps it from sliding off.

I think the ThruSkewer on Treks is a smart solution, but does require a decent steel skewer. But I think the best solution I've seen is on the Dahon Curl D9, standard caliper on front right fork blade, with disc also reversed; It also allows them to use a mono-fork, one blade on right, which helps the fold, but also is a brilliant solution to the front disc without thru-axle issue. Rear has long been solved by just putting the caliper inside the rear triangle, or if too tight like on a folder, above the axle, just not aft.