Wobbly Pannier - any ideas for better seat post mount?

[Flatbar Steve]

The Pannier is secured to seat post collar mount - when fitted like this the Pannier is wobbly because the mounting points are closer together. On my other bike with the pannier fixed to seat stay mounting points it is solid.

This is my bike for Touring when I need to travel and is made easier with a folding bike.

As you can see the seat stays are much lower (It's a Montague folding bike and folds at seat tube).
The seat stays don't have the usual pannier mounts - also they are much further away so P-Clips won't work.

How can I get the mounting points further apart?

Solutions:
1.  I thought about getting a seat post mount pannier and modifying it and bolting my pannier to it
2. attaching to an accessory bar somehow like this   https://www.sjscycles.co.uk/accessories/thorn-accessory-bar-mk1-t-shaped-105-mm-extension-222-mm-0-deg-satin-silver/

I'd be grateful for any help, the wobble from the pannier when loaded is not fun!
The woodwork holding my mudguard on is just a temporary fix.

[aidan.f]

The sjs T bar looks like a good idea. ID is 28.6 mm I think you may find the seat post is oversize :-( if your post is 27.2  a  shim would be needed. The T end would need fettling to bolt onto the rack fixing plate.  Drill and bolt through?. V Brake pad washers self align.
If your seat post is too big how about tandem stoker stem? I happen to have some  spare. Will look at size tomorrow.

[LittleWheelsandBig]

Perhaps a different rack might be a better approach. A well-designed, triangulated rack will feel quite rigid when mounted with just the bolts to the mudguard eyelets. If so, the upper fixings then just act to keep the rack top horizontal. I am a fan of tubular steel racks as they are inherently rigid.

[Flatbar Steve]

The sjs T bar looks like a good idea. ID is 28.6 mm I think you may find the seat post is oversize :-( if your post is 27.2  a  shim would be needed. The T end would need fettling to bolt onto the rack fixing plate.  Drill and bolt through?. V Brake pad washers self align.
If your seat post is too big how about tandem stoker stem? I happen to have some  spare. Will look at size tomorrow.

Thanks! Yes actual seatpost is 27mm but short green section of bike frame below is 32mm, I will look at the options. 
Good idea about the v-brake bolts

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[Flatbar Steve]

Perhaps a different rack might be a better approach. A well-designed, triangulated rack will feel quite rigid when mounted with just the bolts to the mudguard eyelets. If so, the upper fixings then just act to keep the rack top horizontal. I am a fan of tubular steel racks as they are inherently rigid.

Thanks? Like the Thorn one? That seems to be made of separate pieces that are welded together.

Or Tubus as well? They are made up of U-shape on one end for an upright

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[LittleWheelsandBig]

Either rack would likely be much stiffer than the current rack but the Thorn should inherently be stiffer. Have a think about which rack would be easier to deal with when folding your bike.

[mzjo]

I thought that rack looked a bit odd so changing it was my first thought - but the paniers sit well behind the rear wheel to my eyes. It might be that just having the paniers that far back is causing a wobble or shimmy in the steering. On a folder moving them forwards might not be possible due to heel strike. I am thinking of getting a Tern Spartan rack to put them over the front wheel on mine. My rear rack isn't good for anything more than a cycle cape!

What's the bike btw? Wheels look a bit big for 20" but that may just be me.

[BFC]

The complex bends in the stay bars can't help the flex at the front of the rack, whatever version/manufacturer of rack is fitted.

For the rear of the rack some triangulation would be an improvement, perhaps an upgrade from bits of wood.

I made (from stainless tube) direct stay bars on my posh bike to replace the aluminium rods and adaptors.

[LittleWheelsandBig]

Montague folding bikes always have big wheels. Smallest I’ve seen has been 26” but often 700C. https://www.montaguebikes.com/ were invented almost as long ago as Brompton.

[Flatbar Steve]

@LittleWheelsandBig @mzjo it's a Montague Paratrooper 26 inch MTB.  It has a clever folding system that doesn't break the frame so causing a weakpoint. It has an extra headset on seat tube.
Makes trips so much easier I can even take it on local busses not fully dismantled, just folded and front wheel off.

The pannier could move a little further forward before fouling my heel. But it's only slightly further away than pannier on my 700c with regular stay mounting points.
The 'woodwork' is my bodge to fit the mudguard temporarily.

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[grams]

I have a Tortec rack with the same stays. It’s perfectly solid. As LWAB says most of the force goes through the lower bolts and the seat stays just stop it rotating.

It may be that the lower stays are too flimsy for the weight and leverage of your luggage.

[Flatbar Steve]

The complex bends in the stay bars can't help the flex at the front of the rack, whatever version/manufacturer of rack is fitted.

For the rear of the rack some triangulation would be an improvement, perhaps an upgrade from bits of wood.

I made (from stainless tube) direct stay bars on my posh bike to replace the aluminium rods and adaptors.

BFC also Known as Ian, made some very nice metal work to stabilise my rear rack, now don't that look neat! Thanks Ian!