I have a 4 year old that wants to race like his 9 year old brother. The problem is, on our local track there is one straight that is uphill. It rises about 5 feet over the about 125 foot straight. It has 1 large step-up and 2 large doubles. Right now, he's on a 39/18 gearing and he can just make the step on the step up. He then pushes to the top and inevitably tumbles backwards with his bike back down the following double. I think the 39/18 gearing is still too tall for him. I am considering a 34/18 or even a 34/20 but I'm also thinking that I don't want to go too low either.
Does anyone with a bit more experience setting up bikes for a little guy have any good ideas or things I should look for? Things to avoid? Recomendations?