Estuche

Which has less pedal bob when standing to pedal, Birdy or Moulton?? Any adjustments or part replacement to either bike to make it less pronounced?

Ps: I know rider technique matters as well, but this is one of those "everything else being equal"

jur

I would say the Birdy. But the Moulton is adjustable.

Rick Imby

The design of the Moulton is very old and does not take into account pedal induced BOB. The Birdy does. As the chain pull forward on the rear hub the Moulton will move. The Birdy is designed to have the rear suspension pivot inline with the chain. Nearly no pedal induced BOB. However your pedaling motion while standing will cause both to bob...

The mountain bike world discovered this a long time ago and has moved the rear suspension pivot (on single pivots anyway) inline with the chain.


I have never ridden either of these bikes but I have had over a dozen different full suspension mountain bikes.

berlinonaut

I'd tend to say the Moulton but am sure it depends. The Moulton's rear suspension is pretty stiff, therefor there is not much bob to be discovered there. The front-suspension is a different story - I could imagine that the Birdy could be better here but have not enough experience with the Bridy to judge on that. On the other hand "the Moulton" does not exist as there are various Models (even more over the years) using different suspensions (front AND rear) and furthermore i.e. for the front of the current TSRs there are springs of different strengths available plus the suspension is adjustable (and a lot of people seem unable to adjust it correctly). I am still amazed on how well the suspension on my TSR performs (after proper adjustment) while at the same time it is a very fast and very stiff ride. My last ride on a Birdy was a couple of years ago (so memory may lead me wrong) and while I liked the ride on the Birdy, too I'd consider the Moulton more perfomance-orientated, even with the entry level model TSR. Never rode one of the more expensive models.

jur

The question was bob when standing. I have puzzled and analysed a lot over front suspension bob while standing, and have come to the conclusion for the front at least it doesn't matter much as it doesn't waste energy. In fact I think the opposite, it saves a bit of energy compared to no front suspension. I can elaborate if anyone is interested.

The Moulton front when adjusted very compliant will bob a lot more than the Birdy, but it can be adjusted.

Rear suspension for both is pretty stiff so very little pedal-induced bob, not worth obsessing about. Both have their rear pivots close to the bottom bracket.

Sangetsu

It also depends on your pedals and your technique. You can bounce all over a road bike if you don't know how to pedal in circles. It's hard to avoid when standing on the pedals, but clipless pedals and technique help. It's a bigger problem when riding while seated, at faster speeds and higher cadences.

The Birdy rear suspension is just a piece of elastomer, a plastic/rubber spring, with no compression or rebound damping. If you shop around you can find air or hydraulic suspension for the Birdy with damping, but these are hard to find, and a little expensive, usually about $250. These reduce the bobbing, but at the cost of some added weight. The front suspension on the Birdy is a work of genius, and doesn't do much to contribute to bouncing and bobbing, and there is absolutely no brake dive.

pinholecam

I have the Moulton TSR, but I don't stand to pedal as I can't even stand the bit of bobbing with seated climbing.
The front suspension is adjustable, so it can be optimized somewhat to have less bobbing while giving reasonable compliance for the suspension.
The rear elastomer is not adjustable, but its rather hard, so there is less bobbing from it.
All said, there will be bobbing during climbing unless on easy enough gears.
Easy gears may not be at all possible if riding in a group and trying no be ahead or not getting dropped.


I have only tried the Birdy on a few occasions.
The front and rear elastomers can be changed to very hard ones to almost eliminate bobbing.
However, I found that the bike felt very harsh with such an option and defeats the purpose of using it.

Steamer

While I think that was true in the past, most (all?) current Moulton models have a unified rear triangle which includes the BB. Pedaling forces cannot actuate the rear suspension.

jur

Not directly, no. If you consider an isolated torque on the pedals, then the suspension is not actuated. However since the rider is also connected to the rest of the frame via saddle and handlebars, the pedaling counter-torque on the saddle and handlebars does actuate the suspension. It is an improvement on the older design where the BB was not a part of the moving rear triangle, though.

BruceMetras

My New Series Moulton has an easy to use 'suspension lock out' device for the front end... I never use it .. I like a compliant front end, but it is there just incase the front bobbing while hammering uphills bothers some..

Rick Imby

No. I just looked at all the Moulton images. They all pivot right at the bottom bracket. Even with your weight on the seat when you mash on the pedals the chain has leverage against the resistance bumper.

Mountain bikes originally started with the pivot there but it rapidly migrated to be inline with the pulling chain line. To work with longer travel there have been many adaptations to keep the pull from the chain from having major impact on the suspension.

The pivot should be close to in line with the top line of the chain.

If you notice the pivot on the birdy it is 3/4 up between the bottom bracket and the chainline.

It will not be as noticeable on the Moulton as on a mountain bike because of the very limited travel. With longer travel on Mtn bikes and softer suspension it will drive you nuts on a mtb. Any time the single pivot is not inline with your upper chain, when you accelerate the bike the suspension will move.

The Brompton has the same rear suspension design.

On the other side of the coin the loss of energy probably is minimal or even a positive. Just like the recent discovery of wider tires having less rolling resistance, with minimal pedal induced bob it takes vibration out of your body which decreases rolling resistance. Also the push off effect of suspension on the back side of the bob regains much of the energy.

Rick Imby

Pedal induce BOB test.

With both brakes locked up and the rider sitting on the bike with support so he does not fall over. Accelerate without any up down motion from the rider not allowing the bike to move forward because of the brakes. If the bike moves down from only power added to the cranks you have pedal induced bob. If the chain is not pulling directly inline against the rear suspension Pivot (on a single pivot design) the bike will go down.

There have been some designs in MTB where the pivot was above the chain and the bike would actually go up when you accelerate with the brakes locked up. Moving the chain up or down on the front chainrings will impact the pivot alignment also.

Steamer

Nope. On at least some of the current Moultons (including my TSR) both sets of rear stays are all part of the rear triangle that also INCLUDES the BB. It's all rigid. The whole rigid rear triangle assembly has two tabs that are welded onto the BB shell. The end of these tabs is where the rear frame rotates about the rear suspension pivot mount which is affixed to the main (center) part of the frame.


So imagine that when you hit a bump, the BB moves. Although you don't feel it because it moves so little. One, it's a short travel suspension, and two, the length of those tabs from the BB shell to the pivot is only about an inch or so.

I will post a pic tonight.


EDITED TO ADD: here is a pic (not my bike):

Steamer

Ok, I revise my statement to 'pedal forces do not actuate the rear suspension to a significant degree.'


The only things that seem to actuate the suspension on the Moulton are changes in location of center of gravity and hitting bumps in the road.


My Moulton feels as solid in and rigid the rear of the bike as any of my non suspended bikes under heavy pedaling. Actually even moreso than my old Volare with Reynolds 531c tubing.


Front suspension bob when standing up is an entirely different story, though. (an oscillating COG)

berlinonaut

And then, if you look at the Birdy's position of the bottom-bracket and the hinge of the rear frame in comparison to the moulton it is a different story as the bb is not part of the rearframe here (and probably more affected by bob on the rear end, especially with the simple elastomer suspension).



Picture source: https://en.wikipedia.org/wiki/Birdy_(bicycle)

berlinonaut

Not really. On the Brompton the BB is in front of the hinge plus it is NOT part of the rear frame. On the Moulton the bb is below the hinge and the bb is part of the rear frame.

recent as in "decades ago"? Schwalbe have this discovery on their webpage for ages https://www.schwalbetires.com/tech_i...ing_resistance and it was brought into practice in the 1920ies and discovered even earlier: https://hadland.wordpress.com/2012/0...dult-bicycles/

Rick Imby

Berlin---If you look at the single rear pivot on the Birdy it is barely below the chainline. They moved it up significantly from the BB. to cut down on pedal induced bob.

You are right---The road bike world just "discovered" that narrow 150 psi tires have more resistance.

Even with the Moulton design having a unified rear triangle ---the second version of Trek's full suspension MTBs were this design----You still get a lot of pedal induced bob----this is very noticeable when you have significant travel. I would totally agree with you all that it probably does not matter and it probably does not slow you down. I would also believe it adds significantly to the comfort of the ride you get from 20" wheels.

Whoever posted the new Moulton bike Porn----you got me drooling. Sweet bike.

Rick Imby

Really nice looking Birdy too.

smallwheeler

interesting discussion.

berlinonaut

Unfortunately no one can see the pictures that you posted as you are using photobucket. Which is a shame as they seem to show a beautiful Moulton. If you want to embed your pictures here you better go for a different hosting service for your pictures: https://www.theverge.com/2017/7/4/15...sy-paid-update

Jipe

Maybe some more information of the current Moulton models rear suspension.

There are two different design.

The AM models with 17" wheels (AM GT, AM 22, AM Speed) do not have the unified rear triangle, the bottom bracket is part of the main frame, the pivot of the rear triangle is behind it, similar to the Brompton.

The other models TSR, SST, Jubilee, Speed and New Series models have the unified rear triangle.

Even if all those models have the unified rear triangle, the actual design and behavior of the high end models Speed and NS is different.

The pivot of this triangle on the NS and Speed was recently modified because its design wasn't compatible (too wide) with the new four arms cranks first introduced by Shimano, then by Campagnolo.

Having ridden the Birdy, Moulton Speed and NS Speed, I can say that the Moulton are much more efficient than the Birdy, with the Moulton you can sprint like with a road race bike what you cannot really do with the Birdy.

fietsbob

Both have suspension.. [Bike Fridays don't. ]

Rick Imby

The unified rear triangle doesn't make the difference with pedal bob. The pivot location is what is critical. The URT may help with frame stiffness. The other issue in comparing Bob with the birdy and Moulton is the difference in Travel. The significantly longer travel of the birdy suspension will add to the bob.

Pedal induced bob is created when the chain is not pulling directly into the pivot of the rear suspension. The larger the angle the larger the pedal induced bob.

The other factor in Bob is the rider induced bob which comes from the up and down movement of the rider----this adds significantly to pedal induced bob especially with longer travel suspensions.

Schwinnsta

Please do, I am interested. It is counter intuitive to me.

jur

Basically when you pedal out of the saddle the entire body gets a vertical up down motion which takes extra energy. But with front suspension, with the latter part of the down stroke of the pedal, the downward motion of the body which is normally arrested by muscles, is at least partially absorbed by the front suspension spring, compressing it. So that stores energy.

In a different part of the pedal stroke, that energy is released again. I have observed that to be about just after the body is lifted up for forcing the opposite pedal down, maybe 2 o'clock. At that moment the spring relaxes again, lifting the frame+body up a bit more, giving a substantial part of the stored energy back.

The overall result is the up down motion is not as sapping as compared to a stiff frame. With some practice you become accustomed to this assistance and you notice it smooths out the pedal action substantially. It's a bit like jogging with sprung shoes.