Technical FAQ
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Technical FAQ: Why MTB and road handlebars have different grip diameters

Why don't road and MTB handlebars have the same grip diameter? In this week's Technical FAQ, Lennard explains the logical reason behind this fact. Also, a reader wonders how ultrasound is used to check carbon for damage.

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Dear Lennard,
Right now I am running a SRAM NX Eagle MTB drive train on Salsa Cowchipper drop bars. I mounted the MTB trigger shifter to my drop bars via a Paul Components adapter.

Using a trigger shifter on drop bars is awesome! It’s way more practical and user friendly than bar end shifters.

My question: Why do MTB and road bars have different clamp diameters? If they were the same, we could mount MTB components on drop bar bikes all day long with ease, even given the different bottom bracket widths.

As a gravely/touring cyclist, I like low gear inches for loads and hills. Road drive trains typically are geared too high. If I could easily throw an MTB thumb shifter on drop bars, things would be easy. The two systems don’t need different clamp diameters. Why do handlebar makers conform to this nonsense?
— Robert

Dear Robert,
After we finally got uniformity between stem-clamp diameters of handlebars, we are still left with grip disconformity, and I think it’s here to stay. Unlike the diameter difference of the center clamp section, I think there is a logical reason why road and MTB bars have different grip diameters.

In the early 1990s, I used to rail against the disconformity in diameter of the center section of the bar between road drop bars and MTB bars. Because of our focus on building tall bikes for heavy riders, we (Zinn Cycles) were among the first to go to a 1-1/8” fork steering-tube diameter from the old 1” standard. Problem was, the only stems that fit on them then were MTB stems, which had a 1” (25.4mm) handlebar clamp, while road stems had 26.0mm clamp diameters; we had to make our own stems back then if we didn’t want to use super-cheap drop bars with 25.4mm center sections.

I think the reason the grip diameters differ has to do with the fact that users of MTB, BMX, and flat-bar road bikes like thick grips, while road bar users simply wrap one layer of tape around them. The fatter grip section of road bars is a comfortable diameter for most people when taped, roughly akin to the diameter of MTB grips. My first Schwinn drop-bar bike, back in the late 1960s, had steel drop bars with a 7/8” (22.2mm) grip diameter, and I remember those feeling really skinny in my hands.

My first job in the bike industry was working for Tom Ritchey, in 1981, and most of what I worked on there were his triangulated “bull-moose” handlebars. Those were made by brazing together a bunch of pieces of 7/8” (22.2mm) tubing—the bar, the struts, and the quill were all the same diameter. Steel BMX bars and the bars used on the “clunker” bikes that led to Ritchey producing one of the first production mountain bikes were also 7/8” diameter throughout.

From the 1970s until 1998, when Cinelli changed its size, standard aluminum road drop bar clamp diameter was 26.0mm, with cheap bars having 25.4mm and Cinelli bars having 26.4mm. All of these bars, however, had the same 15/16” (23.8mm) diameter throughout the grip sections. Mountain bike bars generally had 1” (25.4mm) clamp diameter and 7/8” (22.2mm) grip sections. Those two grip diameters remain unchanged, even though the clamp diameter is pretty universal now at 1-1/4” (31.8mm) between road and MTB bars. I don’t see that changing unless handlebar tape becomes as thick as MTB grips (highly unlikely).
― Lennard

Dear Lennard,
I have a 17-year-old steel frame that I’m doing a total rebuild on; the frame has been refurbished and the group set is to be replaced (it is worn out).

It previously had 2003 Campagnolo Centaur throughout, save for the brake calipers, which were Shimano R650 (Campagnolo didn’t do a long reach caliper), which worked well. I’m now intending to install Ultegra R8000 throughout. However, I believe the cable pull of the current Shimano levers has changed, so that the modulation and power of the R650 brakes will be sub-optimal (according to the Shimano Compatibility chart).

Shimano hasn’t updated their long reach brakes for years (and I think were originally designed for a different cable pull).

I’m considering buying a set of the TRP RG957 long reach calipers, but I would like to know if the leverage of these calipers is matched to current Shimano lever cable pull. (Because if it isn’t, there’s little point in me changing!!!)
I also don’t know whether I’m worrying unnecessarily and the change in feel or power is minimal and not worth me getting uptight about!
— Louis

Dear Louis,
Yes, the TRP RG957 long-reach calipers are designed to work perfectly with R8000 and other modern Shimano levers.

Officially from Shimano, you should only use those R650 brake calipers with either ST-A070 2X7  or ST-A073 3X7 levers. Similar to what I said here, the R8000 levers  do not have enough leverage to pull the R650 calipers the way they were meant to be pulled; they are designed for higher-leverage brake calipers than the R650 calipers. The brakes will still work, but your hands will be doing more of that work. It’s been awhile since I have tried using newer Shimano levers with Shimano calipers designed for the old, higher-leverage Shimano levers; I recall being able to stop okay.
― Lennard

Dear Lennard,
I was reading the last item in your recent column from Shawn Small about receiving his ASNT Level 2 Certification in “ultrasound techniques.” I am not an expert on nondestructive inspection techniques (full disclosure). However, I remember attending a tour of a military facility that performed inspections/repairs of military aircraft. They mentioned that they had been using ultrasonic excitation of composite materials combined with thermal imaging. As I understood it, the ultrasonic “vibration” produced small amplitude movements where there were discontinuities in the composite structure, and these discontinuities produced heat (friction between the moving surfaces) that showed up in the thermal imaging of the component. Having this equipment would not be an inexpensive proposition and there are probably more simple inspection methods available.
— Danny Brinkley, PE
Staff Engineer – Accident Reconstruction Analysis, PLLC

Dear Danny,
Here’s the explanation from Ruckus Composites:
“What Danny is describing is some version of Thermography or Shearography. Both operate under a similar principle of an energy excitation source and a visual capture method.

For Thermography, the excitation method is a pulse of heat from a halogen bulb (similar to a flash for cameras). The heat penetrates the composites and the Thermal Camera records the Delta T (change in temperature) of each pixel. The software then performs some good old-fashioned calculus to derive the Heat Flux and turns this into a layered image.

For Shearography, the excitation can come from a few different sources. Sometimes it’s a Laser, as we have experimented with, or it can be physical excitation, acoustic, or thermal. The Shearography setup can then record the differences in surface energy and creates a layered image.

Ultrasound images showing damaged and undamaged carbon frames. Photo: Shawn Small

We have experimented with both processes out here at Ruckus Composites and have found both Thermography and Shearography are not well suited for the geometric complexities of bicycle frames and the wide variance of manufacturing techniques and quality. Also, both equipment setups are in the $100k+ range and require significant training and build-out to perform.

Ultrasound works on a Pulse-Echo method. We couple our Ultrasound machine to a very specific Piezoelectric Transducer that creates an Ultrasonic pulse through the bike that echoes off the back wall of the tube and bounces back to the machine. The difficulty with Ultrasound is the training, analysis and characterization by the technician. At Ruckus Composite we have two Level 2 American Society of Nondestructive Ultrasound technicians on staff and are developing our own “Written Practice” this winter on the process and characterization of Ultrasound data regarding composites bicycles.

We take the Science and Engineering of composites to a new level for bikes!

I attached some images that might help.
— Shawn Small
Owner || Engineer
Ruckus Composites”
― Lennard

Feedback on staying warm on descents

Dear Lennard,
Good advice. I’ll add the advice given me by a bike shop employee in Vaison-la-Romaine, as I was on the way to my maiden ascent of Mt. Ventoux: wool.

Wool will keep you cooler in the hot valleys than Lycra will keep you warm on the summits. An ice sock on top of a wool jersey is just as refreshing on hot days. But under a jacket (especially with a wicking under-jersey), wool offers far more insulation for cold descents.
— Andrew

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder ( and purveyor of non-custom huge bikes (, a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn