Smaller dogs and Bike attachments

My mum and I are thinking of buying a bike attachment to exercise Harry with as we are fed up of the long line already, I was going to get one for Cooper when he is older anyway, so we can go halves on it

Harry is about 18 inches (haven't actually measured him yet) and Cooper is 24 inches high.
I've seen the Walkydog attachments, and the Springer attachments, my mum prefers the look of the Walkydog but it looks as though it might be a little high for Harry.

Does anyone use the Walkydog with a smaller dog? Do you use a lead extension with it?

I may not be much help but I always tied the lead to the back wheel iron thingy - well with Angel. Sadie was always running along side me off lead, she knows all sorts of commands like "Whoa" for slowing for a road, "Middle" when we're crossing a main road with a crossing thing and "shift" when she was gunna get under my wheels lol.

I was thinking of getting her the walky dog but she did so well off lead, why bother - she always stays on my right - away from traffic and we'd go for a 20+ mile bike ride and she'd still get home and pick up her tennis ball for me to throw for her.

The Walkydog should be alright for Harry - he's not a tiny springer is he? if he is then you can get one a small splitter lead and attach both ends to his harness/collar and the middle is attached to the clip on the walkydog.

I was actually thinking about getting Louie one as I want to start riding with him but he's getting his strength now and he knows it so he'd end up pulling me over