Velonews: Technical Faq: A Drivetrain Hack, Tire Pressure, And Road Discs


Jan 3, 2005
<figure ><img title="CrossChained 50-40" src=""/><p>And with the longer rear derailleur cage, it can handle extreme cross-chained combinations, like a 50-40t, shown here.</p></figure><p><strong>Dear Lennard,</strong><br />Seems like there’s a lot of interest in 10- vs. 11-speed as well as road vs. mountain group compatibility. I just completed a mix/match combination I think your readers would like to hear about. I have a <a href="" target="_blank">Giant AnyRoad CoMax</a> with Shimano 105 11-speed group. The bike has a compact 50-34 crank and 11-32 cassette with 32mm tires from the factory. After taking a group trail ride with mixed cross and mountain bikes, I soon discovered the climbing/traction limitations of the factory tires/gearing in a mountain bike environment. I wanted to be able to swap between wheelsets with 32mm cross tires to as big a tire as I could fit in the frame with even lower gearing. The 11-32 cassette is the lowest geared road 11-speed cassette available, and I didn’t want to change to a mountain bike crankset. The solution, it seemed, was to use the XTR 11-40 11-speed cassette. I purchased a used Giant mountain bike 10-speed wheelset and mounted a Panaracer Firecross 45mm tire on the front and a Bruce Gordon Rock-n-Road 43mm tire on the rear.</p>
<p>Since the XTR 11-speed cassette has the same hub width as a 10-speed cassette, it mounted to this new wheelset without modification. With a 50t chain ring and 40t cog, the stock 105 medium-cage rear derailleur wasn’t up to the task of taking up that much chain, and the long-cage mountain-bike derailleurs have a different pull ratio and are therefore incompatible. Also, there isn’t a “B” screw long enough to keep the upper jockey wheel from rubbing that 40t cog.</p>
<p>What I needed was a long-cage 11-speed road derailleur. After examining the exploded views of the Shimano road and mountain-bike rear derailleurs I found, it appeared, the long cage from a direct mount Deore LX 10-speed RD-T670-SGS mountain-bike derailleur would interchange with the 105 medium-cage. I purchased a used Deore RD-T670-SGS, removed the cage/P-tension spring, removed the 105 cage/P-tension spring, and, sure enough, the mounting/pivot pins are identical. I installed the Deore long cage/P-tension spring on the 105 and now have a long cage 105 11-speed rear derailleur.</p>
<p>Prior to my rear derailleur “hack” I would switch chains when I would switch between wheelsets. Also, I reversed the “B” screw, had it adjusted all the way out, and had to install a spacer between the screw and derailleur hanger to prevent jockey wheel rub.</p>
<p>The Long Cage 105 derailleur “hack” solved two problems:<br />1.) I no longer have to switch chains when switching wheelsets; the long cage easily handles the longer chain.<br />2.) The upper jockey wheel on the Deore cage is offset slightly away from the cassette compared to the 105 cage. This allows an almost normal “B” screw adjustment even with that big 40t cog.</p>
<p>The time it takes to swap cages/P-tension springs is about 30 seconds (not including chain) and for me was under $30. I haven’t priced just the cage from Shimano but these Deore derailleurs can be found used for cheap. I can cross chain 50-40 or 34-11 with no issues and it shifts flawlessly. Best of all, since it’s factory Shimano parts, it doesn’t look like a hack; it looks factory.<br /><em>— Tom</em></p>
<p><strong>Dear Tom,</strong><br />That’s great! Thanks for sharing with us your elegant solution to getting a wider gear range on a bike with road shifters.<br /><em>― Lennard</em></p>
<p><strong>Dear Lennard,</strong><br />I live in a year-round warm, humid climate (Florida). As a practice, I pull the seatpost periodically to drain any accumulated water out of my (carbon) frame. Generally no water comes out, but the inside of the frame has a really musty odor of mold/mildew. What suggestions do you have to de-funk/sanitize the inside of a frame?<br /><em>— Carl</em></p>
<p><strong>Dear Carl,</strong><br />Wow. That’s a new one on me. I’ll bet bleach would do the trick. Maybe put it in a spray bottle (you could even dilute it some with water) so you can spray down all sides of the seat tube without tearing your bike apart. Spraying rubbing alcohol inside could also be worth a try and would evaporate faster but might not sit long enough on the mold to kill it the way Clorox would.</p>
<p>Once you’ve removed the bottom bracket, you can also pour bleach inside the frame and slosh it around inside all of the tubes.<br /><em>― Lennard</em></p>
<p><strong>Dear Lennard,</strong><br />I just read your response to the gentleman with the <a href=""target="_blank">“slice tire” problem</a>. A tire pressure chart from <em>Bicycling Quarterly</em> you’ve posted in the past recommended extremely high pressure (130psi) for me in the back and milder on the front (90psi). According to the Vittoria tire pressure guide, I should be running 130 in the front and 135 in the rear (I weigh 225 running Vittoria Corsa 25’s on wide Bontrager RXL TLR rims) I did have several “explosive slices” and even blew a couple of sidewalls using that combination. Since then, I’ve switched to Pave 25’s and lowered my pressure to 110/115 (f/r) and haven’t had any problems. I’ve experimented running lower, but that seems to be the sweet spot, as far a still rolling quickly. However you said you never run above 75 or so … In your opinion am I still running too much pressure?<br /><em>— Brian</em></p>
<p><strong>Dear Brian,</strong><br />What you are doing sounds reasonable with your weight. I weigh over 50 pounds less than you, and that’s what I can get away with.</p>
<p>I should note that the <em>Bicycle Quarterly</em> tire pressure chart you reference did not take wide rims into account.<br /><em>― Lennard</em></p>
<p><strong>Dear Lennard,</strong><br />I recently had the misfortune, due to inattention, to hit a traffic island with my 2013 Specialized Pro Tarmac at what I estimate to be about 10-15 MPH. Amazingly, I kept upright, up on to the island. Obvious damage was a bent front wheel rim (HED Ardennes) and, I believe, an unplugged front derailleur wire at the shifter. I rode the bike an additional three miles homes with no noticeable performance/riding issue.</p>
<p>What else should I look for in terms of possible damage? I’ve checked the frame for cracks and have found none but have not ridden the bike since for fear of some hidden break. The sound made when I hit the traffic island sounded like someone had hit my bike with a baseball bat—very disconcerting. I intend to bring the bike to a bike shop to take a look but would greatly value your input and advice.<br /><em>— Rufino</em></p>
<p><strong>Dear Rufino,</strong><br />It sounds like you are doing the right things. Keep inspecting it and listen for new creaks and clicks when riding it.</p>
<p>Tapping on the frame along the tubes and joints with a coin to listen for abrupt changes in the sound and vibration quality of the clacking sound of the coin on the carbon is also a good idea.<br /><em>― Lennard</em></p>
<p><strong>Dear Readers,</strong><br />In <a href=""target="_blank">my response to Eric last week</a>, I neglected to mention that the higher speeds of road bikes also work to cool the brakes faster as well. Of course, it is incumbent upon the rider to let off on the brakes intermittently to allow them to cool, rather than holding them on steadily so they continue to build up heat. And on extremely hot days like those we discussed in Oman, this would be even more critical.<br /><em>― Lennard</em></p>
<p><strong>Dear Lennard,</strong><br />I believe the main reason rotors on road bikes are generally designed to be smaller than their MTB counterparts is the available tire contact patch. I think the additional braking power on tap with hydraulic calipers can quickly overpower the traction available and cause lock-ups and more skidding with hydraulic disc brakes. The more leverage the caliper has, the more exacerbated the problem becomes. As it is, it’s not hard to overpower the traction usually available with modern, half-decent rim brakes.</p>
<p>In essence, I think that the brakes are designed to be weaker for safety, as losing traction or going over the bars is far more dangerous than longer stopping distances. Boiling hydraulic fluid is akin to over-cooking your rim pads or carbon brake surfaces, so the risk is about the same there.<br /><em>— Ante</em></p>
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