Size cycle vs fit on a trainer (importance of rocking during bike fit)

While trying to do research on where in MD I should go to get a bike fit, I thought this was interesting:

Notice the talk about how the Size Cycle ideal position doesn’t transfer over to the bike as well as a fit done on a trainer? Notice how the new fit bike he got had machine dampers for feet and a stem that allowed some movement? This makes it seem like getting a fit done on a bike that allows some movement can be a very useful thing.

I was always under the impression a fit bike would give the best fit in that it is very easy to make small changes and swap between positions while riding that would take much more time if adjusting a bike on a trainer. But seems like the fit bike should be mounted in a way that allows movement for it to be best. Guessing not too much movement like a full rocker plate if you are using a tool like retul to motion track. Also thinking that you should rock less and so the measurements will be better the closer to the right fit you are.

Though actually finding a bike fitter to do this with may be very hard. (would think there are enough tech people in MD who like playing with tech toys, but no :frowning: )

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Any fitters here have experience with this?

I have some experience and flagged this for fuller commenting tomorrow.

I used the Retul fit bike during training and found it a pretty cool tool in the general sense and for making changes quickly for instant feedback. That said, neither my boss (who also got the same training experience) nor I felt compelled to get a fit bike of any kind. I’ve even had the chance to play with the fancy electronic Guru setup for a short test.

  • Our biggest gripe with fit bikes is that you are essentially doing more work for a moderate gain. Sure, twisting a few knobs and getting instant feedback is great, but the need to totally match that with the actual bike after the fact is an important requirement. It can be a small to large undertaking to pull all those data points over to the actual bike they intend to use, adjust and/or replace components to hit the “magic fit” that was created on the fit bike.

  • Then add in the fact that even the smallest difference in handlebar shape & reach (between whatever is mounted on the fit bike and the current one on the customers bike) may be hard to replicate or account for in some cases. Even differences in hood shape can lead to notable fit differences. Maybe a non-issue in the old days, but with variation between manufacturers (Shim/SRAM/Campy/Microshift) as well as the styles within a single manufacturer (like mechanical vs electronic shifting, cable vs fluid brakes) can lead to a very different feel and reach at the bars. So either having a massive range of parts available for the fit bike, or accounting for any/all variations from the customer bike need to be handled.

  • Add in the motion & flex aspect and you get even more issues. No surprise I have an aim to bridge the gap between rigid fits and free riding outside with my rocker plate history. But doing motion in a fit scenario has issues that lead me to take a “less is more” approach compared to regular training use. I have implemented thick mats and carpet when using a rear-wheel trainer. And I am now testing with a Minoura fork mount roller setup that actually includes some “flex” at the fork support for comfort. None of it is a replication even close to my home setup, but it’s better than pure rigid setups and what we see in the typical fit bike.

All in all, fit bikes are a cool concept, but far too expensive (in real cost but also time of fitting) that is not justified for our use at least.


Thanks. Makes me think about fitting different. I guess to me it seemed like fit bikes are clearly pretty expensive but if you do enough fits they would pay themselves off. So in that sense when looking for a bike fit finding a fitter with a fit bike means they do more fittings (so have more experience with different issues) and are more invested in doing fittings. And my bias is in wanting to play with more tech toys

To me it seemed like the ability to do A/B testing very easily would be a major advantage (switching back and forth between two positions multiple times without any gap so the person on the bike can directly feel the difference), but guessing you can see from the movement of the cyclist which is better and so don’t really need to depend on their feedback and so would not be switching like that so often. From my perspective its hard to say for sure what is better and if I’m indecisive and annoying the fitter having to switch back and forth more…

My other reason for liking the fit bike was being able to use the data from it to buy a new bike. In that I have an endurance geometry fuji bike now but kind of want to get a new bike that is lower in the front end (thinking Specialized athos, but not not really set on that) So thinking a fit bike that can give arbitrary positions of saddle and handlebar positions to the bottom bracket would make it easier to figure out the size of a frame that is a very different geometry.

When I got a retul fit I liked that I had the data to look at my position but then felt like because I wasn’t supposed to sweat and have the ir admitters more they kept the wattage low. To me this made the position I was being fitted for on the bike felt wrong in that it seemed like as my power went up my position changed. (Going from weight on the saddle vs weight being more on the pedals)

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If you are still looking for a fit in Maryland (Baltimore), I highly recommend Full Sweet Cycling

He’s a bike fitter, physio, masseuse, coach, and good guy. Goes through strength and movement work before getting to the bike stuff. He’s patient and takes his time to make sure you get the proper fit.

Hope you find a good fit wherever you end up!


I am sure that some shops do enough fits to justify a size bike and or motion capture, but definitely not all. I would bet that some places do as many fits in a week that I do all year. We are in a black hole market where even our fair $125 charge for a 1.5 hour fit is a hard sell for some. I think we deserve more, but had low takeup rate when we tried ranges between $150-200. All that to say that making large investments in equipment are not worthwhile in terms of ROI for our case at least.

I think all these wizbang tools are cool, but don’t take them as some sign of a good fit. As I mentioned in another related thread, the most important tool to a fitter is their head. They need skills & knowledge, along with the ability to offer and request feedback from the rider. I don’t care what a screen tells me if the rider is notably out of form or uncomfortable despite being “in range” of any “standard” measurement.

Dr. Pruett told me once that every rule in fitting is meant to be broken. The challenge lies in knowing when, why and how much to break that rule.

We do use a quick adjust Retul stem that makes those front end experiments super quick, and it was a decently affordable investment that speeds the test/feedback cycle. Outside of that, most other changes in things like saddle position take mere moments to do with the rider simply standing for 30 seconds or so. I know the fit bikes can handle that and more in a faster method, but the delta there seems not to be worth the cost IMO.

Add in the fact that I see more than a few people commenting on issues with stuff like the Retul Vantage app, devices and other tech issues on the support site. I have a hard time thinking I’m missing much vs my “simple” fits vs the complication that the tech also adds. I’d play with them to try long term, but the pricing is far from a small investment. Your mention of effort is an important one, as I have people test a range of powers that relate to their needs, but usually have them at that higher middle range if they are more performance oriented riders. Mix in sprints or hard pulls as needed, along with easier stuff if that is someone’s more common use.

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I am only 10 minutes into this podcast, but he hits on some key points that parallel what I mentioned here and in other fit discussions (about tech in fits specifically).

I reserve the option to recall my approval once I finish the podcast, but this is starting off well IMO.

Seems to be the bike fit guy on this channel:

Yup, that’s him. As I mentioned in the other post where I shared the podcast, I don’t agree with all of his views, but he has a very good grasp on this world and has some great info to share.

The hard part is figuring out how knowledgeable and skilled the fitter is. They don’t all post in forums so we can tell they are knowledgeable. There seems to be no useful certification (useful meaning if they pass it they can give a good fit vs they had a day of training and passed some easy test) or review site to read how good a fitter is. And when Fits are in the $300+ range around here… So then try to judge by things that you think may correlate which can go poorly.

Back to the original post look at this video of Neill Stanbury (bike fitter):

ok, not the subject of the video but look at the equipment used. Rock and roll Kinetic bike trainer so he clearly likes a bit of movement on the bike doing a fit. Doesn’t seem to use any advanced tech.

There are some “cert” places, but they largely align with the fit methodology and teaching connected in parallel. Nothing independent or far reaching, unfortunately.

But not much different than things like bike mechanics, there is a need to do some research (find prior customers and such) and potential discussion with the fitter as pre-interview. There is no clear or easy pass/fail, good/bad identifier here.

Checking out the vid now.

Bikes either work or don’t. A fit being bad can be hard to tell. Are you more comfortable right after? Maybe you need time to adjust and then things will be better. Did the fit make things worse but only slowly…

Slow twitch only has 10 people in all of the US and Canada ranked. ibfi only lists one person in MD and bike lists no one. Just frustrating

Yup, I am aware we are talking humans vs machines here in my mech vs fitter example. Just trying to make a point that there are few if any direct endorsements or guarantees like it seems you are looking to find.

I get the frustration and fall into that category a bit on my own. I considered connecting to the institute, but I don’t even do the minimum of fits they require, so it’s a pointless for me to dig into that one.

We are talking about a niche industry within a niche industry. Sucks to be sure, but we have what we have. Fitting is a small industry that plenty of people question as valid or worthwhile in the first place. Those that have either taken the risk freely, or been driven that direction from issues may have found relief, but we all know cases where that didn’t happen either.

So it takes some research and a bit of faith to be sure. I essentially guarantee my services as I will refund if the person is wholly dissatisfied with my work. But considering the relative challenge that lies here, it’s tough to do more than that. Perhaps the better parallel is coaching, where much relies on the coach and their abilities, but also the interaction between coach and athlete. Fitting is a bit like that too.