Question on dualie sizing for a growing child

[Syncr0s]

Hi all,

I'm in a bit of a quandary and was hoping to lean on the experience of those more learned than I.

My son and I have been riding hard tails and he wants to advance to a dualie, and has found a second hand Spesh Stumpjumper S3 ('20 or '21 model) for a fair price. He has been doing some jumping (fairly adventurous kid) and wants to push a bit more.

He is a slim boy ~145cm, and my view is that an S3 (medium in other brands) is the wrong size for him for his size and build, especially given he is starting out with dual suspension. When he hopped on the bike for a test ride he barely troubled the suspension.

I think he'd be better with a smaller dualie, which is more responsive to give him more confidence, or stick with his existing bike (Giant ATX) for longer.

Has anyone been in a similar situation and can offer advice to he and I?

TIA.

Syncr0s

[bychosis]

Kids grow. It’s hard to manage bikes around that without an unlimited budget. However, having something with air suspension means it’s adjustable. You can lower the pressure in the shock and fork to increase the responsiveness at lighter weights.

Having a smaller MTB tends to be better IMO. More room to move around the bike, but it’ll be grown out of quicker. I now have a second small sized dually that I picked up cheap (in need of lots of TLC). One is already been outgrown and sold. Next kid is just about to hop onto the second at similar size to yours. Lent it to a nephew at Christmas time and he loved the dually compared to the hardtail he’d left back home. I think I’ve set it up OK for a light weight kid bit will fine tune once it starts getting ridden properly.

Don’t forget much of a bikes handling characteristic is defined by the geometry so if he want a to ride jumps etc will probably not enjoy a dually with a XC bias. Make sure you pick something that has a trail geometry.

[Syncr0s]

Hi Bychosis,

Thanks for taking time with the detailed reply. I read it the other day, but wanted to go through it with my son to make sure he understood and to see whether he had any questions.

From my side, I'm beginning to understand more about geometry, and it backs up what I've been advised about getting a good second hand bike to ensure you get the most of modern geometry changes.

The Stump Jumper at least has trail geometry, so we are on the right track, and really just making sure we size it right. You are right about the money pit not being bottomless, which is one reason why we are tending toward a quality second hand bike now, and he can progress to a new bike once his growth completes.

My son certainly understands this.

We are going to keep a look out, but probably go to some shops and try bikes for sizes, so if an opportunity presents we can jump on it with more confidence.

Happy trails.

Syncr0s

[bychosis]

Good stuff. Having an idea of size is the best start. I didn’t get the best bikes for my first kid as I was looking for bargains first.

On one ride we swapped bikes for a valid reason I can’t recall. He got my quality dually and I hopped on his budget hardtail, it was horrid to ride and was sold and replaced with a better geometry soon after. Learned a lesson from that.

[Duck!]

Duallie sizing isn't really any different from any other type of bike, it's still generally based on frame height from the centre of the bottom bracket, with the upper measurement point somewhere in space at the manufacturer's discretion.... However to allow for suspension travel, unloaded duallies will typically stand taller from the ground than an equivalently sized & set up hardtail or rigid bike, and that can make mounting & dismounting more difficult for someone on the small side for a given frame size....

145cm is a fair way under the generally recommended height range for a "medium" frame in any manufacturer's interpetation, so while it allows growing room, the setup in the short term cam be pretty compromised.

Suspension tune is anoher kettle of worms. Department store BSOs aside, even low-grade duallies up to about 170mm suspension travel will have air springs (longer will still use coils, but that's another discussion), which is great for tuning to rider weight....

To a point......

Air pressure/spring rate is one thing, and easily adjustable for rider weight, but damper tune - how quickly the shock can compress and return is another thing that needs deeper works to fine tune.... OEM frames & shocks are tuned on a pretty vaguely estimated rider height/weight correlation... e.g. a "medium" frame may be calculated to fit riders between 165 & 180cm in height, with a weight range of 70-90kg. So even if you tune the air spring chamber pressure to suit a rider who is a kid & built like a toothpick with the wood scraped off & only weighs 50lg, the damper tune may be too firm, vczusing a rugh ride.

[Syncr0s]

A humorous, but thought provoking post. Many thank Duck!

My toothpick with the wood scraped off is definitely under 50kg

[BettyJohnson]

Thanks for the advice, I will keep it in my mind.