Shim-Sham : How I Busted My First Bike Frame Using A Thudbuster Shim

The Thudbuster LT (review here) is the quintessential essential for any fat ebike. At the last count, I own a total of 5 Thudbusters that move around from one bike to another. One of the problems that I’ve dealt with in the past is that my fat bikes have all different widths of seat tubes meaning that if I want to fit my 27.2mm Thudbuster in a bigger hole that I need to buy a shim for it. Since I am too cheap to spend $10 for a Cane Creek Thudbuster Shim I would just buy the cheapest one I could find on eBay which was usually $3-5 shipped. My incredible cheapness finally ended up causing problems as the cheap eBay shim on my Boris X9 finally cracked the frame where the seat tube meets the top tube. This article is about why you shouldn’t use Thudbuster seat post shims on an aluminum bike frame and what the best alternative is (buy another Thudbuster that is the right diameter for your bike, you cheapskate).

My first broken frame, I've never been so proud of myself as I am right now

My first broken frame, I’ve never been so proud of myself as I am right now. So much mud, so little time.

The Thudbuster LT seat post comes in a variety of different diameters, 25.4, 26.8, 27.0, 27.2, 27.2xl, 30.9, 31.6 & 33.9 of which the 27.2 is by far the most common seat tube diameter. The 27.2XL is a seat post that is 450mm long rather than 400mm long and is really designed for bikes like folders that have a very low seat tube/top tube intersection. Since the Thudbuster LT system puts the seat 3 inches higher than the top of the seat tube 400mm is more than enough seat tube for almost every application. The idea behind the seat tube shims is that if you have a Thudbuster that is a smaller diameter than your seat tube then you can buy and install a shim so that the skinny Thudbuster works properly in a fatter seat tube and the seat post clamp will still work properly. I have about 15 electric fat bikes and I pretty liberally move my Thudbusters from bike to bike on a daily basis. This system worked pretty well until I broke my Boris X9 frame.

Here is the seatpost shim that will allow you to use a 27.2mm Thudbuster on a Sondors ebike. Since the Sondors is a steel frame you should be fine.

Here is the seat post shim that will allow you to use a 27.2mm Thudbuster on a Sondors ebike which runs about $9. Since the Sondors is a steel framed bike, you should be fine.

I contacted Bikes Direct looking for a replacement frame (I was happy to pay). They didn’t have any frame available that would work with the X9 components. I also learned that although all the Bikes Direct frames have a 10-year warranty, if you put any kind of motor on it the warranty is voided. I didn’t try to get a free frame out of them and I’ve always been very honest with them about how hard I’m thrashing on their equipment, but I have to admit I was a little sad to hear them say this. It brings up an interesting point though, pretty much any bike manufacturer is not going to want to warranty a frame if you’re running a high-powered electric motor on it, so just be aware of that from the get-go.

I decided to get the frame fixed and Accudab in Ithaca (a local fabrication business) did a great job of Tig welding the frame for me. Aluminum frames, once they are broken, will never again have the same strength that they originally had so I spent another $137.90 and bought my fifth Thudbuster that had the right 31.6mm seat post diameter from The Bikesmiths on fleabay. Although that is way more than the original $3.50 I paid for the seat post shim, it will be the best way to ensure that the frame does not break again at that spot.

What happened?

I spent a bit of time thinking about how my frame broke and the answer is really pretty simple. I was using a 27.0mm Thudbuster seat post with a 31.6mm shim designed for a 27.2mm seat post. The post was a little on the loose side so I had to crank down the seat clamp extra tight. Since the seat post on the X9 goes up over an inch over the top tube and the shim is only about 2.5″ long that meant that there was a huge amount of stress on the seat tube every time I went over a bump. When the Thudbuster gets hit your seat can move several inches to the back and down which puts even more leverage action on the area that the seat post shim is in. Over a years worth of abuse, the seat post shim caused massive fatigue in the top of the seat post until it finally broke. The funny part is that I didn’t even realize it was broken until I realized I could see light inside of the seat tube and looked closely after wiping away the mud and realized that the seat tube was completely broken about 90% of the way around. Ignorance is bliss.


Once you weld the frame it will never be as strong as it once was before it was welded even if you heat-treat it

Metal fatigue is something that bike manufacturers don’t ever discuss, but it is a very real issue. All metal frames have a usable life which is shortened the harder they are ridden. With an electric motor on my Boris X9 build, named The Reaper, I have to admit I’ve ridden this bike very, very hard. I really like having the hydroformed frame in deep powder because when I have to hop off the bike my nuts don’t end up getting smashed on the top tube as my foot sinks down a foot in the soft powder.  I’ve also tested the Ludicrous controller at 60 Amps with the BBSHD on The Reaper pretty extensively to which I can say the biggest drawback is the skinny 4-inch rear tire limitation (running the Nate 3.8). The more I ride in deep powder the more I realize that having the biggest rear tire you can get is critical for maximizing your fun factor, and I’m loving the Snowshoe 2XL. Unfortudently the only fram I have that will fix a 2XL in the rear is the FB 5 2.0 on my Phat Phuk build which is a tight enough squeeze that it rubs a little on the chainstays.

The Thudbuster LT is a must-have for fixed rear suspension trail ebikes of all kinds

The Thudbuster LT is a must-have for fixed rear suspension trail ebikes of all kinds

The Thudbuster LT is an amazing tool that will improve your electric ebike trail riding experience more than any other accessory you will ever think of buying. That being said, don’t ever use a shim with the Thudbuster on an aluminum framed bike (steel framed is OK). Over the years I’ve busted 3 of the long seat clamp bolts on the Thudbusters which I have replaced with Grade 8 bolts from McMaster Carr that will probably never break. I have found that Cane Creek warranty support is second to none and when my bottom clamp on my most recently purchased Thudbuster broke they sent me a new top and bottom clamp without any charge, even for shipping. You can order parts for Thudbusters out of warranty here, but be aware the shipping charges are quite high.

Over the years my chronic hemorrhoids have thanked me for investing in those 5 Thudbusters, and your butt will thank you too.

Ride On.

14 thoughts on “Shim-Sham : How I Busted My First Bike Frame Using A Thudbuster Shim

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  2. It’s too bad no one is making a combination “Dropper Seat Post” and “Suspension Seat Post” all in one. I have a real nice suspension seat post but because I would never ride bike trails with out my first choice the “dropper post” my suspension post sits in the spare parts pile. I guess it depends on where you ride and how you ride, but for the network of trails I ride I like the ability to be able to change saddle height on the fly depending on if you are climbing or descending. I have a really early version of a linkage style suspension seat post that was produced from a cottage industry builder around 20 years ago but it would only get used if I knew I was going to be riding fairly flat gentle trails. For the road I could see using my suspension seat post more but I just get too lazy to switch them out.

    Good point on the shims Karl, I never thought about this before and I can now see why your frame failed on you…….wayne


  3. I have 3 Thudbusters with 2 of them using shims on BikesDirect aluminum frames. I have had no problems so far, and I’m sorry to admit that I weigh more than you. My shims are 4 inches long however.


  4. The wisdom of buying a cheap, steel, Mongoose fat bike is confirmed again. I found I’d bent my seat-tube when I tried to fit my new Thudbuster LT. I just lubed up the old seat-post with grease, and hammered it in to straighten it back out. I believe all serious off-road fat bikes should have chome-moly frames, and massive triple-crown, inverted, downhill forks as well. I want the suspension of a downhill bike, with the tires of a fat bike.


  5. Karl, I think you are the exception because of your godly height and all the leverage forces caused by jacking up the seat post real high. If you are <6ft & <200lbs I doubt you will have any problems using shims.


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  8. I tried the Thudbuster and it absolutely smoked my lower lumbar. Like horrible back pain. I tried 2 more times before I went back to my regular post. Something about the flex action that does not sit well with me.


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  12. I like how you buy a cheap shim that’s too short. Then use it with a seatpost size it wasn’t intended for. Then blame and swear off the use of seat tube shims because of your own incompetence.

    Swollen hemorrhoids are caused by your blood clotting down there. You have thick blood. Way too much sodium in your diet is usually the culprit.


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