Front disc brake with dropouts

[Brucey]

IMHO that is a very nasty accident waiting to happen.

The wheel looks about half-way out already, and those flimsy things are not going to stop it, they are nowhere near strong enough.

cheers

[foxyrider]

I don't know what part of riding a touring bike includes doing stoppies!

Well maybe not intentionally but on at least one occasion i've been forced to make emergency braking actions. Mark my words, standing a fully loaded bike on the front wheel is not a fun thing to do, especially on a HGV infested German B road (it was a Polish truck which caused the incident). That admittedly was with dual pivot braking.

I'm certainly not trying but to date i've never got a front disc to work that efficiently. OTOH i've had several 'start to lock' moments on steep, damp, greasy Pennine lanes this autumn with DP's.

[Abradable Chin]

My fork doesn't have lawyers' lips. I reckon the ejection force would be about equal to my bodyweight, as a maximum, when allowing for my half my weight bearing down on the axle. The dropout is 9mm thick Al alloy, so I could cut a lip if people here think I should have one. Can it be only on the disc side?

[NATURAL ANKLING]

Hi,
Resultant force of a rim brake on the front fork at axel is approximately between 9 O'clock and 12 O'clock, ( viewed from right) where 9 O'clock is max force with a back wheel lifting, and 12 O'clock is weight on axel (body plus bike in balance with rear axel ).
A range of 90 degrees all into the fork eye, and little or no force trying to eject axel................until you are going down hill or pulling a stoppie, lifting rear axel means the resultant will move to before 9 O'clock (bike not ground) and retension due to weight of bike and body will diminish

Disc front brake-
Resultant force range will go from 12 O'clock to before 9 O'clock ...................the moment you apply the brake the resultant force is attempting to unload weight (part to total body, plus part to total bike) from fork eye.
At some point depending on weight on axel and rate of retardation, maybe before a stoppie the resultant force has moved to before 9 O'clock!
Pulling a stoppie only exasperates the problem.

Hence on a disc front axel the force's from braking is attempting to move the resultant force over a larger range than rim brake and starts to unload weight the moment you brake.
So a good retention clamp is needed, the lawyer lips do a job in part to preventing axel creeping leading to loosening and ultimately out of the fork eye

Remember also that as you use front brake, the balance of total weight body and bike moves to all on front axel,
This is the force which is acting against the resultant braking force and keeping axel in eye.
All this total is available with no diminishing up to pulling a wheelie or going down hill on rim brake ......but starts to diminish the moment you apply a disc front brake.
That is not to say the force keeping axel in eye does not increase on a disc brake it does the moment you apply brake but is countered also at same time by the resultant brake force This is the difference between the two brake systems.
Due to the positioning of the brake in front or behind the fork.
Going down hill on both systems just moves the force from weight of body and bike from 12 O'clock towards 9 O'clock equal to - 12 O'clock minus angle of hill. This will ultimately diminish axel retention force which on a rim brake is not a problem until a wheelie
The axel skewer clamping has to exceed any ejection from braking resultant with a safety factor.

[CJ]

I think if the 0.5G thing were true then the weight distribution that causes that would also mean that the slightest use of the brakes on a ~25 degree slope would result in you going over the bars.

It's true alright, and equates to a 30 degree slope in fact. That's a 1 in 2 hill as roads were historically measured, along the slope, ie the sine of the angle, or 1 in 1.7 when described the modern mathematical gradient way, ie the tangent.

Have you ever been on a genuine 1 in 2 hill? I have. In my youth a bunch of us strayed into JCB's test facility near Ashbourne and found three neatly labelled concrete hills: 1:3, 1:2.5 and 1:2. We had a go on them of course, a few of us even rode down the 1:2, but none of us dared to so much as touch the brakes until we were on the flatter run-out. As this was a modern facility however, the '1:2' sign probably gave the mathematical rather than historical road gradient. In that case it would have been 'only' a 27 degree slope, equal to braking at 0.45g. So it should nevertheless have been possible to ride down it under control of the front brake. But as I said, none of us dared touch the brakes.

Yes, I know there's a hill by Harlech Castle that calls itself 1:2, but surely isn't. British road signs are notorious for exaggerating gradients, probably to make caravanners not even think about it!

[andrew_s]

Yes, I know there's a hill by Harlech Castle that calls itself 1:2, but surely isn't. British road signs are notorious for exaggerating gradients, probably to make caravanners not even think about it!

Ffordd Pen Llech. It used to say 1 :2.5 at the top, and 40% now, and No Entry at the bottom.

My understanding is that UK signs relate to max gradients over a short distance (20 m or so), and usually only apply to the inside of the steepest bend.

[LittleGreyCat]

I ended up at this very useful discussion after seeing the term "lawyer's lips" in the instructions for my SON 28 disc hub dynamo.

I thought it was a hilarious mistranslation from German but decided to check just in case.

This explains why friends have had issues removing front wheels from disc brakes bikes, so I will share the information.

I'm still amused by the term (which doesn't appear to be used in France according to the multilingual instructions). It looks to me to be more suited to an insult.
"Sir, you have lawyer's lips! How very dare you!!!!".

It does make me wonder if a through axle should be specified with front disk brakes, though.

Edited to add that my 90s mountain bike has a sort of thumb clip on one front dropout which has to be pressed to release the front wheel once the QR has been released but I haven't seen this on other bikes. Presumably a safety feature if you don't tighten the QR but not man enough to cope with disc brakes.

[Cyckelgalen]

Thread resurrection.

I have a front axel that will move about 1 mm when braking. It is a Genesis fork and thankfully the dropouts are front facing, so no real risk of ejection, but the wheel will move and fall out of alignment. I have tried a quick release and and a skewer with Allen head, tightened far more than I normally would and cannot get the axel to stay put.

I fear that over-tightening will shorten the life of the bearing by compressing the hollow axel and adding too much preload. The hub is a SP dyno hub with cartridge bearings, so i cannot adjust it to compensate for any unusual amount of torque.

Any idea as to how to go about to fix the axel without over-tightening? Removing the paint perhaps? It would have to be replaced by some corrosion protector though.

[Brucey]

I would suggest checking the tightening torque on the security skewer using a torque wrench. 'Tighter than normal' could mean anything.

cheers

[reohn2]

Surely if the axle ìs fully engaged in the dropouts up against the d/out stops and the the d/outs are forward facing I don't see how it can move,unless I'm misunderstanding the OP's problem.

[Brucey]

FWIW I just measured an SP dynamo and the axle stubs are undersized at 8.7mm; maybe they can be modified with tube shims so that they are a better fit in the dropouts?

cheers

[Cyckelgalen]

I had thought of the possibility of shimming the axel, getting it to fit snugly in the dropouts would prevent the movement.

By tightening more than I normally would I mean tightening the QR so that I can just about close it with my bare hand, that is tightening it as much as I possibly can without using any leverage. I will check the torque with the Allen nut.

By the way Brucey, what a reasonable or rather the maximum torque for the wheel locknut?

Thanks

[Cyckelgalen]

Surely if the axle ìs fully engaged in the dropouts up against the d/out stops and the the d/outs are forward facing I don't see how it can move,unless I'm misunderstanding the OP's problem.

Well, it doesn't move out of the dropouts, it moves laterally. If the axel undersized as Brucey points out, then there is enough play for some lateral movement.

8.7 mm, that is 0.3 mm thinner that it should be, is it? Somehow, it seems more to me that it moves more than that.

[Brucey]

the dropouts will have been 9mm clearance to start with. Sometimes they make them to fit 3/8" axles (which are used in some front hubs). In any event if the wheel has been moving around the dropout will probably have worn to a larger size. What size does the dropout measure?

cheers

[Cyckelgalen]

Just over 9 mm measured with a manual calliper. I haven't got a precise digital vernier calliper at hand.