Forward facing dropouts. Problems?

[Trikeyohreilly]

Hi,

I have at last, after a 3 week fight finally recived my Yuba Mundo version 4 frameset. Have built it up, but due to more problems with Yuba and a seat post that wont work have yet to ride it.

Problem is/possibly is the forward facing dropouts. I have had two bikes in the past that have had forward facing dropouts that over time, i.e. when the paint wears away from rear wheel removal/replacement, allow the rear wheel to slip out and forward under acceleration. Neither, could I or the LBS solve.

This is as yet an unridden/new bike. Do you have any ideas as to the likelyhood of this happening again? If so what on earth can I do to solve it?

Apart from the problems I have had with Yuba (review to follow) I am pleased with the bike and want to make it work. Of course it may just work, but experiance tells me that its a bad design.

It might be worth saying that the two prevoius frames were aluminium and this one is steel. Will that help?

Thanks in advance for your advice and please pardon the spelling.

Ed

[Brucey]

the dropouts need to be dead true, and if you are using low gears you need grippy locknuts, a top quality QR, and/or a nutted axle.

Unfortunately the Yuba frame design looks like it makes fitting a chain tug of any kind rather tricky, but I daresay it could be done if necessary.

cheers

[Trikeyohreilly]

Thanks Brucey.

The dropouts had some flex when I tightened the QR (its an XT like the hub). First time I ever felt spring back from a QR. They didn't want to take a 9 speed rear wheel either as the chain rubed on the inside of the frame. I had to use adjusters (from 14mm to 10mm) to make my wheel fit in the first place so I have tried putting one of the adjusters the wrong way around (drive side) opening the dropout spacing buy 1 to 2mm. That makes the nine speed fit, but wonder if it will reduce the friction needed to hold it in place?

Not sure what a chain tug is unless it is what I have seen on horizontal dropouts to hold the axle back which, like you say I don't think will fit.

[Trikeyohreilly]

Forgot to say, I do plan to use low gears.

I have a 22 up front (lowest of the triple) and have just ordered a 36 (largest cog) casette for the rear. That must produce some tork and is far more than the other bikes that prevoiusly failed.

Thanks

[gaz]

An XT hub is 135mm OLN(Over lock nuts). If you've taken 4mm of spacing out then the frame seems to be designed for a 130mm OLN hub (Most road models).

If you have not shortened the axle (grinder) it is now 5mm longer than the frame builder was expecting. It is possible that the q/r is clamping down on the axle ends rather than the frame's forward facing drop outs. If this is happening it would certainly explain the problem.

[Trikeyohreilly]

Firstly thanks for the help.

The frame is designed for an axle width of 14mm where as my rear wheel is a 10mm axle so the adjusters are for the dropout size rather than the OLN.

I'm a bit above my league here technically to be honest but need to get this resolved.

I think they spec'd it 14mm for the carrying capacity of these hubs.

Thanks, Ed

[Brucey]

Not sure what a chain tug is unless it is what I have seen on horizontal dropouts to hold the axle back which, like you say I don't think will fit.

yup, I reckon it'll have to be specially made for this frame, maybe an extra bracket fitted to it....

as to the rest of it, choose the best bits, fit them as well as you can, then just suck it and see...

cheers

[Trikeyohreilly]

You are right, I'm talking about a problem that has not happened yet, at least to this frame. When I finally get the part that will allow me to ride it I doubt I'll be able to resist.

But as it is I could still return it. But I don't want to do that, firstly I really like it, secondly Practical Cycles have worked really hard to overcome Yuba's short comings.

I would be prepared to look at a bespoke and expensive option if I just knew it was possible.

[JohnW]

I do have sympathy with your dilema, although I've always had 'forward facing', horizontal dropouts, and never had this problem - my first serious adult bike was 1956ish! Dropouts were much thinner then.

I have three bikes on the road at present, all with Campag forged horizontal dropouts, all using QR hubs & skewers, and my lowest gear at the moment is 30-28 - quite low I think - and I don't suffer from this problem, and as far as I know, neither have any of my contemporaries.

When you've solved it, let us know how.

My hunch is that it's the hub or the skewer that's the problem.

[BigG]

Forgot to say, I do plan to use low gears.

I have a 22 up front (lowest of the triple) and have just ordered a 36 (largest cog) casette for the rear. That must produce some tork and is far more than the other bikes that prevoiusly failed.

Thanks

The chain tension is what controls the forward force on the rear axle. This is a product of your pedalling force, and at maximum your weight, multiplied by the ratio between the crank length and the radius of your smallest front ring. The size of the rear sprocket does not affect this.

[cycleruk]

See Sheldon - scroll down to the photo of the "over-locknut dimension.
http://sheldonbrown.com/gloss_n-o.html#old

The section of exposed thread that sticks out of each side of the axle must be shorter than the thickness of the drop-out.
If it's not then you will not get a grip when tightening the axle.
I have found that the quick release had to be very tight to stop my wheels slipping when under extra pressure, such as when climbing or setting off at a road junction (just when you don't want the wheel to slip).

Also have a read of this on Sheldon:-
http://sheldonbrown.com/skewers.html

[Brucey]

The chain tension is what controls the forward force on the rear axle. This is a product of your pedalling force, and at maximum your weight, multiplied by the ratio between the crank length and the radius of your smallest front ring. The size of the rear sprocket does not affect this.

with my pedantic head on, I'd argue that it does, albeit weakly. Whenever the torque on the rear wheel goes up (for a constant chain tension) there is an increase in the reaction force from the road to the wheel; this load is also borne by the axle and adds to the the chain tension force. In practice this might add 15% or so for a big sprocket vs a tiny one, so it isn't a huge effect. Wheel diameter also affects the reaction force (for constant chain tension) similarly....

cheers

[BigG]

The chain tension is what controls the forward force on the rear axle. This is a product of your pedalling force, and at maximum your weight, multiplied by the ratio between the crank length and the radius of your smallest front ring. The size of the rear sprocket does not affect this.

with my pedantic head on, I'd argue that it does, albeit weakly. Whenever the torque on the rear wheel goes up (for a constant chain tension) there is an increase in the reaction force from the road to the wheel; this load is also borne by the axle and adds to the the chain tension force. In practice this might add 15% or so for a big sprocket vs a tiny one, so it isn't a huge effect. Wheel diameter also affects the reaction force (for constant chain tension) similarly....

cheers

I still disagree! The ONLY force pulling the wheel forward is the chain tension (actually including the negligible tension on the bottom chain run). Chain tension is provided only by the rider with the mechanical advantage of the ratio between crank length and chainring radius (again ignoring the contribution of the lower chain run which is controlled by the derailleur spring).

[kevbo]

The reaction force of the road against the tyre is also trying to slide the axle forward . This would be the case even if you had an electric or hydraulic drive system that did not involve a chain. On a normal bike, this would only be a fraction of the chain force before the tyre would spin. But on a loaded cargo bike with the load adding to the traction, it might exceed what you get from the chain.

I have a commuter with a nexus-i8 IGH. I walk a fine line between having the 3/8-26 axle nuts tight enough not to slip and stripping them...have to carry spares. I also have had a Mundo since about mid January. It has the stock 14mm nutted axle. I have had no slippage issues, and I don't have to tighten that one overly much...though I don't think I have yet used the granny ring.

I'd worry over a QR axle. Use a shimmano internal eccentric lever skewer for the best hope. Some sort of long tug that clamped around the rearward extension of the seat stays might be needed.

[BigG]

The reaction force of the road against the tyre is also trying to slide the axle forward . This would be the case even if you had an electric or hydraulic drive system that did not involve a chain. On a normal bike, this would only be a fraction of the chain force before the tyre would spin. But on a loaded cargo bike with the load adding to the traction, it might exceed what you get from the chain.

The reaction force of the road against the tyre is just that, a reaction force. It provides the reaction torque to the torque provided by the chain tension on the rear sprocket. The only force tending to shorten the distance between the rear axle and the bottom bracket axle is the force delivered by the chain tension. This pulls the rear axle forward (and creates the original problem) and pulls the bb axle backwards. This is rigidly fixed and will not move. The road/tyre force acts to propel the bicycle, but has no effect on squeezing the above mentioned two axles together.