Picking My 7-Year Old a New Bike

Me and my partner are currently looking for a suitable bike for our 7-year old son and want to ensure we make the correct purchase, not in regards to the price, we just want something that's going to keep his posture correct, is going to be comfortable for him to ride and that doesn't weigh too much. From the articles I have read so far on posture etc, it is best to have a bike with the correct height of seat and handlebars to suit your child's size and riding style. Our boy does have what you'd probably call an aggressive style with the top part of his body quite forward. Am I right in thinking then that we should get him a bike with lower handlebars to suit his style? I guess then you'd have a lower saddle to compensate for this. The recommended tyre size is something else I'm unsure on, it seems as though the recommendation for this age group is around 16". As I've mentioned, having a bike that is light-weight would be preferable. He is a strong little lad but some of the bikes are heavy for even me to pick up. Can anyone suggest a model of bike that may suit a 7-year old based on some of the things I've mentioned? Again, price isn't the biggest issue here, its more about getting the right bike for him and if that costs more, so be it. It will be a combined birthday and Christmas present anyway. There's a good website that I've come across called http://www.easyprices.com/cars-bikes-parts where you can buy bikes online at cheaper prices. Once I've found a good model, I might check back there to see if they are doing it cheaper, depends on the brand though I suppose. 

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That seems a long way out to me. My youngest is 7 next month and has been on a 24" bike since the spring, when he became too big for his 20" bike, including racing in some closed road events in the summer and cyclocross now. He is fairly tall for his age but I can't think that you will need to go below 20" and 24" will likely be spot on.
Usual suspects for light-weight and good quality bikes are Islabikes (linked above), Frog and Raleigh now have a performance range.
HTH,
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Frog bikes are a worthy alternative: marginally heavier but cheaper. The Frog 62 is probably the right one... Frog do have local dealers so you should be able to try one for size.
Wiggle now do a great range of own-brand Verenti kids bike - http://www.wiggle.co.uk/verenti/kids-bikes/ As above, the 24" is probably best but check the size guide.
(Declaration of Interest: I now work for Wiggle - but I've seen these bikes and they look very good to me... and I'm a kid's mountain bike coach so I see a lot of kids on bikes!)
As to riding position... get the right size bike. If they want a more aggressive position (or less aggressive position) you can usually raise or lower the bars with the spacers, or even swap bars for ones with more/less rise.
Giant Anthem X4 (2010) GT Avalanche 1.0 (2010)
Kingley Vale and QECP Trail Collective - QECP Trail Building
Specialized riprock 20
MTB Scotland youtube channel
Ah ok, seems like the article I was reading was totally off then with sizes for a 7-year old so thanks for recommending the size guide. I'll have a look at the Wiggle & Islabikes websites now and see what they have on offer. The Specialized riprock 20 looks like a nice little machine mind.
Giant Anthem X4 (2010) GT Avalanche 1.0 (2010)
Kingley Vale and QECP Trail Collective - QECP Trail Building
weight is almost pretty irrelavent for most part and 7 year olds generally don't go for long rides either. Its a bike for him to play about on and have fun, not for doing miles of off road. He might not even be interested in going mountain biking either.
MTB Scotland youtube channel
Our argument has always been that suspension may encourage them to ride at speeds beyond their skill level. A rigid frame will teach them the bike handling skills first and enable them to read terrain/surface. Once they've developed such skills and awareness, they'll move on to suspension bikes with the skill necessary to make use of the greater speed they can reach off-road on rough surfaces. They are also much, much lighter. If, as you say, cost isn't a major factor, I'd go for Islabikes - they also have a very useful sizing chart to pick the right bike. they also have an excellent resale value.
http://www.facebook.com/groups/eastbradfordcyclingclub/
http://www.facebook.com/groups/eastbradfordcyclingclub/
The only negative point I could give it is that she found the twist grip gears difficult to operate when she was 7, she is only just getting good at operating them now (easier without gloves on)
Absolutely. Here is my 6-year old (7 next month) on his Beinn 24 on his way to winning the U8 race in a local CX meeting:
</proud dad>
The seat post is probably 2 inches up from its lowest. Of course, he can't touch the ground with both feet while sat on the saddle so it does depend how confident you child is...
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Well 7 yr olds on heavy bikes don't but my kid was doing 20 miles on canal towpaths at 5 1/2 and 20 miles offroad on his first reasonably light 20" on some Blue/ and mainly red at 7 on his lighter 24".
He's not particularly tall.... and at 5 1/2 wasn't fit though he is now and drops plenty of adults on climbs. (As a consequence or added bonus I got fitter) but he wouldn't be climbing those hills on a heavier bike
As it happens his 12" was heavier than the 20" which was heavier than the 24" (after upgrades) ... you have a good idea on what I spent on upgrades
Well you don't know.... I bought the 20" to go on cycle paths and canal towpaths and was frankly surprised when the first "proper" ride after riding 10 miles he wanted to cycle back.. (especially as his mum had come specially to pick him up from the pub) - Within 6-8 weeks and due to local access to Swinley we were doing offroad every weekend ... and by the time he was 6 1/2 he could get on a 24" and benefit from decent suspension....
I bought the 20" new... quite honestly I wish I'd just got a used one and see what worked out .... but that's 20/20 hindsight.
https://www.cube.eu/uk/products/kids/kid-200/cube-kid-200-bluenred-2016/
2014 canyon ultimate cf 9.0 sl
2016 Planet x pro carbon
2017 Scott Spark 730