15 November 2015

I was just asked about the ride quality of a frame, and having composed a response I thought it might be interesting reading to others:
Ride feel – here is some information: All steel has essentially the same modulus (stiffness), regardless of the alloying used. The different alloys and processing are used to increase the strength, which allows thinner wall tubing to be used. The variables that affect the stiffness of the frame are:
– the tubing wall thickness (thicker tubes get stiffer linearly with the increase)
– the tubing diameter (stiffness increases with the square of the diameter)
– the tubing shape – to some extent; ovalising increases the stiffness in one plane and decreases it the other.

The rear triangle is a very stiff structure – it is the only part of the frame triangulated in two planes; the triangle of seatube/chainstay/seatstay and the triangle of chainstays/rear axle. So the place where you feel the difference in frame stiffness is in the front triangle – this is lateral and torsional stiffness, which determines how well the wheels are held in the same plane. For a road going bicycle, infinite stiffness here would not be a good thing for ride quality – a little bit of give allows the wheels to have a small amount of independent tracking, so they can conform to the imperfections in the road with less chatter. However, if there is insufficient stiffness in the front end, then when pushing hard going into corners the front end will start to feel a bit vague, and the required steering precision will be lost. I experimented with this on early frames, riding them for a while, then cutting them apart to change the tubing sizes; doing this for myself and other sized riders allowed me to get a pretty good sense of what gives the ‘right’ amount of stiffness for different size/weight riders.

With modern bikes we have gotten used to very stiff frames. Back in the days of ‘standard’ lugged frames, toptubes were 1″ (25.4mm) and downtubes were 1-1/8″ (28.6mm), with the most extreme butting being around 0.9/0.6/0.9mm. Although everyone was happy with these bikes, for racing and everything else, it isn’t a very stiff front triangle. I tend to stay away from the really large tubes that are commonly used now (1-1/4″ (31.8mm) toptubes and up to 1.75″ (44.5mm) downtubes), unless I am building for a very tall/heavy rider, as I feel they make the frame too stiff to give good ride quality. But with a full range of high strength steel tubing available in thin wall (butting in the 0.6/0.4/0.6mm range) sizes from 1″ and up, I can select the tubing to best suit the rider, the application and their expectations/requirements.

Hopefully that helps understand the ‘lateral stiffness’ component of a frame. The other much hyped quality is ‘vertical compliance’. I have rarely seen any manufacturer actually back a claim to this quality with data – the reality is that by definition a triangle is a rigid structure, so there really can’t be much in the way of perceptible movement in the rear triangle. The way that I like to build in true comfort at the saddle is by using a long, flexible seatpost – this cantilevered strut can provide passive suspension.

 

 

3 Comments

  1. Good information Rob. I see you mentioned tubing shape (ovalising) briefly but by-and-large the references to tubing seems to be cylindrical and diameter. Is there anything to add on when the tubing gets aero – somewhat oval but also wider or flatter at one face?

    • Hi Dave, I haven’t done any static deflection tests to see if there is a measurable difference between a oval tube and a tear drop shape. I generally assume similar properties based on the depth and width.

  2. Happy New Year, Rob!!! all the best!!! 😉

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