Derbi Terra Adventure - clearance for 18" rim?

[lardshop]

Hi,

I'm in the market for a Derbi Terra Adventure (DTA) but for competition reasons (yes, really) I'll need to fit an 18" rear wheel to allow me to fit (4.00x18) trials tyres.

I reckon the overall increase in diameter will be about 1.5", is there anyone out there with a DTA who could tell me if there looks to be room / clearance to accept a bigger rim / tyre?

Thanks in advance.

[Steve Pickford]

If you're fitting an 18" rear rim, I'm assuming the Derbi has a 17" rear as standard? If you go ahead, you'll need to take the difference in rolling circumference between the two tyres (fitted to their respective wheels & fully inflated) in to account and either fit a smaller front sprocket or a larger rear which in turn could lead to a longer chain being required. The greater circumference of the 18" rear tyre will raise your gearing by a reasonable amount, especially on a 125. If you're competing on the bike, you may to to lower the gearing even more? Measuring the difference between the two will give you a base from which to make your calculations.

If you think the diameter increase will increase by 1.5", then you only need an additional 0.75" of clearance as it's the increase in radius & not the diameter that's the issue.

[lardshop]

Thanks for that.

Yeah, I'd forgot to mention the standard rim is 17" with OE tyre being 130/80 compared to a 400x18 (100/100x18) trials tyre.

Overall, I reckon the increase in radius (you were right, my typo) is about 1.5" with a bit of clearance for mud etc.

I'd planned to lower the gearing anyway as it's all slow speed stuff I have planned but your point about the bigger overall cicumference I'd overlooked. Thanks.

Anyone out there got a Terra Adventure? Some pics of the wheel clearance against the swing arm would be nice...

Thanks

[Steve Pickford]

Not sure where you get an extra 1.5" in radius from? The larger rear will add 0.5" to the radius and I can't see the tyre itself adding a huge amount, offroad tyre sidewalls tend to be a similar size.

In your position, I'd remove the seat & throw a ratchet strap over the bike & attach to the swingarm. Protect anything you don't want marked with rags. Get someone to sit on the bike to compress the suspension & ratchet the bike down until the shock's fully compressed before measuring clearance. Ensure that the 17" tyre on the bike is not worn as this will throw everything out by the depth of the tread. If you've got at least 1", you should be okay. Perhaps get the circumference or diameter measurement from a (new) 18" tyre as a comparison?