Calling all engineer-y types!

TinyExplosions

Right, I have a query, and I'm wondering if it's possible / a terrible idea / a death trap waiting to happen.

Basically, I'd like to move my rear rack further back on the bike to avoid heel strike issues and fit a larger set of panniers, and I am thinking of some harebrained bodge of getting a triangular piece of stainless, and bolting that to the two eyelets on the frame, and then the other end to the rack mounts, thereby moving it back, and up slightly, and allowing me all the clearance I need!

Now, the questions I have are three fold:

1. Will such a bodge be strong enough to take the weight of a pair of panniers, or am I ruining the carrying capacity of the rack by doing this.
2. What's the best shape to ensure that the force on the rack is still transmitted in the right direction to the frame eyelets, or does this matter so much (or will all the weight be trying to shear off the bolts making this a stupid thing to do)
3. Will moving the rack back a few CM totally ruin the centre of gravity of the bike, putting too much weight behind the rear hub, causing handling problems / something else I haven't thought of?

Like I say, I can live with things as they are, but this would seem (at least in my head) a handy way of getting an extra couple of CM of clearance

seamus

If you can get the bits properly made for you (i.e. welded together and tapped with proper threads), I can't see any major problem. Just remember that your centre of gravity is affected by the distance from the centre of the bike to the end of the rack, so adding a few cm onto the length of the rack is equivalent to having a heavier load, which obviously affects steering and the like.

Assuming that a standard rack is 30cm long, then adding 10cm onto the bike will increase the perceived weight of the load by 30%. I.e. turning a 15kg load into a 20kg one :eek:

Not something to be ignored.

High Nellie

seamus wrote: »

If you can get the bits properly made for you (i.e. welded together and tapped with proper threads), I can't see any major problem. Just remember that your centre of gravity is affected by the distance from the centre of the bike to the end of the rack, so adding a few cm onto the length of the rack is equivalent to having a heavier load, which obviously affects steering and the like.

Assuming that a standard rack is 30cm long, then adding 10cm onto the bike will increase the perceived weight of the load by 30%. I.e. turning a 15kg load into a 20kg one :eek:

Not something to be ignored.

The handling of any bike is going to be changed once you start loading it anyway, so you just have to adjust your riding accordingly.
I've welded bits to an old bike 'carrier' to adjust it for touring etc. There's no major problem as long as someone competent does it. Try and make it rigid as possible though - i.e. less movement from side to side.

TinyExplosions

seamus wrote: »

If you can get the bits properly made for you (i.e. welded together and tapped with proper threads), I can't see any major problem.

My thought was that there wouldn't need to be any welding, and that the 'plate' would essentially just move the rack eyelet back and up a bit (so that the rack mounted to the plate as if it was an eyelet, and the plate mounted to the eyelets in the frame, so it would just need to be tapped correctly?

Good to know re centre of gravity etc -it would only be 2.5 - 3 cm max I'd be moving it.

coastwatch

It seems somebody has this problem before and invented this.
There are plenty of other helpful gadgets to attach racks to bikes on the site.

seamus

Maybe I've understood wrong, but you effectively need three "plates". If you put a 3cm plate at the top of the seat stays, pushing the rack back by 3cm, then the rack pivots by that much, and it may now be pointing slightly downwards towards the ground. So to keep the rack level, you may also have to add a plate to the eyelets at the dropouts.

But if the rack is currently at an angle, the rear pointing upwards, then it might not be a problem.

TinyExplosions

seamus wrote: »

Maybe I've understood wrong, but you effectively need three "plates". If you put a 3cm plate at the top of the seat stays, pushing the rack back by 3cm, then the rack pivots by that much, and it may now be pointing slightly downwards towards the ground. So to keep the rack level, you may also have to add a plate to the eyelets at the dropouts.

But if the rack is currently at an angle, the rear pointing upwards, then it might not be a problem.

Nope, need em at the bottom, by the drop outs to move the bottom 2 mounting points of the rack back. There's more than enough adjustability in the top rails to attach just fine to the rack mounts near the seat tube.

It's literally just required to move the bottom of the rack back a bit more, but that tubus kit could do the job!

doozerie

Perhaps this is something you've already considered, but some pannier bags allow you to adjust the angle the bags hang at so that you can basically rotate the bottom of the bags away from your feet. Some of the Ortlieb bags, for one, allow you to adjust the angle of the "bar" on the bag to which the rack hooks are attached to achieve this.

Edit: Or, yet another alternative is to go for a longer rack which allows you to move the bags back further. As an example, I have an old SKS (as far as I can remember) rack on my commute bike which I think is a bit longer than a typical rack. ...though, having said that, I'm not entirely sure that the back of the rack extends any further out, it may be that all the extra space is towards the front end of the rack.

mvt

What about some sort of a trailer instead?
Some of the ones you see seem to fit well and don't lolok as if they are causing undue hassle.

TinyExplosions

mvt wrote: »

What about some sort of a trailer instead?
Some of the ones you see seem to fit well and don't lolok as if they are causing undue hassle.

It's not a huge issue really, and so my thought was simple along the lines of "if it'll work and there's no glaring engineering/support thing that I've missed, I may give it a go", though in truth, I'll probably just move the panniers back and leave it as it stands. Trailers are getting into a whole other world that I want to avoid at the moment!