Technical Q&A with Lennard Zinn – Generics? Trail, rake and more

Dear Lennard,
My question is regarding CO2 inflation cartridges. Is it okay to use CO2 cartridges that are labeled for use in BB guns and paint ball guns? I have been told that they contain a small amount of oil in them and because certain oils or lubricants can sometimes breakdown rubber that I should not use them. Is this true? These CO2 cartridges are sold at your local Wal-Mart or sporting goods store and are considerably cheaper. A box of 15 or 20 usually runs around 10 dollars as opposed to three or four dollars a cartridge at you local bike shop. Do you know if using these cartridges is okay or have you heard of anyone that has used them? I would appreciate any input that you may have on this subject.
Steve

Dear Steve,
I am not sure that what you heard is the case or not, but I use unthreaded BB gun cartridges all of the time. I do it because, as you say, they are a lot cheaper. I have never had any problems with doing so, either in inner tubes or in tubeless tires.
Lennard

Dear Lennard,
I’ve been following the discussions on bike trail and fork rake with interest. However, I’ve got a question concerning the magnitude of change (in bike handling) likely to be experienced based on changes in trail due to fork replacement. It seems that most bikes (in my size at least) list a trail measure somewhere between 5.2 and 6.0 cm. How much effect in handling would be noticed by a change in fork rake of 1-5 mm? Assume all other variables (like fork length) are constant. Given that most after-market forks have between 4.3 and 5.0 mm of rake I would think the impact is relatively slight. This is particularly important for those riders who buy frames and components separately and can’t test ride the various options.
Dean

Dear Dean,
You are right; the impact is generally relatively slight for a change in rake of 1-5mm. For instance, Alpha Q forks come in 41mm and 44mm rake, and I am willing to bet that there are very few riders who could pick out which one is which in a double-blind test, switching the forks around on their bikes with no idea of which one they are riding.

That said, there are exceptions to this when it comes to the subject of front-end shimmy, which is not a steering or stability issue (although it can result in the rider falling off), but rather a resonance frequency issue. In some cases, the interchanging of a fork with few more millimeters of rake can eliminate the shimmy problem in a bike that had a tendency to shake uncontrollably when riding with no hands on the fork with less rake.
Lennard

Dear Lennard,
Your recent column regarding rake, trail, and offset was extremely educating, especially after downloading the “Frame Geometry and Bike Stability” block from your latest primer. I must say I was thinking completely opposite in regards to fork rake and trail and you have enlightened me. Thank you very much.

I just purchased a new bike that has a 73.5-degree head tube and has a Reynolds Ouzo Pro fork with 43.5mm of rake. It is quick steering or has what I feel is a fidgety front end. I have been thinking about how to make it a little more predictable in terms of steering, especially at high speeds. For reference I am using an 11cm over sized Deda Newton stem with a Deda Newton bar and a Chris King 1-1/8-inch threadless headset.

My question is if I installed a similar fork with a 40mm rake (or other small decrease in rake), as opposed to the current 43.5mm rake, would I be able to tell the difference or is the difference negligible? How much of a difference in rake would I need to go before I would notice a difference. I ask because I have limited funds to purchase different forks to experiment with.
John

Dear John,
As you can see from my previous answer, a change of a few millimeters in rake is not enough usually to elicit notice from the rider, but I have found that this is often not the case on a bike with which the rider has difficulty. In this case, small differences in rake, as well as a change in stiffness or alignment of the fork brought on by a change in model or brand of fork, could make a profound difference in your feeling of handling confidence on this bike. A 73.5-degree head angle with a 43.5mm rake is steeper than I generally use with a fork like that (I usually couple a 44mm fork rake with a 72-degree head angle to get more fork trail and hence more stability), it is not out of the ordinary at all; there are plenty of bikes out there with fork trail similar to yours with which their riders are quite happy. I suspect that stiffness and alignment issues in the frame and fork may also be playing a role in your bike’s fidgetiness, and a fork change may be just what the doctor ordered.

Indeed, changing to a fork with 40mm of rake will increase the trail, slow down the steering, and increase the stability of this bike. I suspect that it is just enough on the edge of your sense of comfort, that this change will be to your liking. I would say that it is worth the investment to give it a try.
Lennard

Dear Lennard,
I enjoyed your column on rake, trail, and offset, and it all makes perfect sense to me now. The question I now have is, do (or should) time trial bikes have a little more trail than road bikes, since the additional trail might help the time trialist ride a straighter line? According to Trek, their 54cm road bike has a 4.5cm offset and a 5.6cm trail, whereas the corresponding time trial bike has a 4.0cm offset and a 6.0cm trail. Coincidence?

Assuming it’s not already set up like this, how much would a time trialist benefit, say, over a 40k time trial, by simply switching their 4.5cm offset fork for one with a 4.0cm offset, thereby increasing trail 0.5cm?
Tom

Dear Tom,
The answer is yes, they often do.

I certainly build my time trial bikes with less rake and more trail for exactly that reason.

In fact, I made a fork with negative rake on which John Stenner (a great rider and friend who tragically died in a bike/car accident more than a decade ago) won the U.S. time trial national championship in the early 1990s. This was before the UCI had mandated that the front and rear wheels had to be of the same size, so Stenner wanted a smaller (26-inch) front wheel for reduced aerodynamic drag, yet he did not want the front end of the bike to drop (and its head angle to steepen and hence trail and stability to decrease) as they would have by simply switching to a 26-inch fork and wheel. He also wanted a lot more stability so he could just concentrate on powering the bike. The solution I came up with was a unicrown fork with negative rake made out of teardrop-shaped steel strut tubing made to separate the wings of biplanes.

As you can see from the formula and discussion in the previous column or in Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists, decreasing the wheel radius would have decreased the fork trail and hence the bike’s stability. Couple that with a steeper head angle by dropping the front end due to the smaller wheel radius and shorter fork, and you lose a lot more trail and stability. However, by making the fork a long unicrown bend with this aerodynamic tubing which could not be bent sharply, made the fork longer, and turning it around backwards so that the rake was negative lifted the front end yet more. John ultimately decided that he wanted to be a bit lower and wanted to drop the bottom bracket height a bit as well, both to get down out of the wind more. So, we ended up with 0.5 degrees less head angle, 70mm (!) less rake, and around 22mm less wheel radius, all resulting in a net large increase in fork trail.

We worked on the countersteering skills that it takes to put a super-stable bike into a lean and to rapidly turn it (also covered in the primer.). Once he mastered the necessary countersteering, he realized that this bike was also giving him a big advantage at the turnaround. The 70mm reduction in rake reduced his wheelbase, and hence his turning radius, by almost 70mm as well. After winning Nationals, he excitedly described how he was able to just stomp the pedals and not be concerned at all with steering; the bike just went straight without him, and he zipped around the turnaround so fast that he could hear the spectators exclaiming about it! It’s a shame he is no longer around to invent things with.
Lennard

Dear Lennard,
I’ve recently upgraded bicycles from a 2000 Klein Quantum to a 2003 K2 Mod 5.0 frame, but it’s much less stable than my Klein. In fact, the difference is so dramatic that it saps all of my confidence on the road!

While on my other bicycle, I can ride and steer with no hands in comfort. On this bicycle, riding with just one hand on the bars takes much concentration. It feels like I’m riding on a sheet of ice all the time.

The bike takes a conscious effort to ride in a straight line. It’s quick to steer and eager to wobble.

Help! I fear I’ll not be able to race on this bike this year as one can’t race without confidence in their ride.
Rob

Dear Rob,
It sounds like a combination of insufficient fork trail combined with front-end shimmy. I suspect that there is also an alignment problem in the frame or fork. I would try a fork with less rake in hopes of eliminating all three problems in one fell swoop. Good luck!
Lennard

And, just to throw a little gasoline on an argument near and dear to many a bike geek:

Dear Lennard,
Googling around I came across a column of yours in which a correspondent made the statement about needing lighter wheels – within the same overall weight when climbing.

This just isn’t so.

Rotating weight is important only when you need to increase your speed dramatically. And even then it is a tiny issue.

One flick of a finger can bring a wheel up to climbing speed – an irrelevant amount of energy.

Also – having added that energy to the wheel it takes no additional energy to keep it spinning at that speed (excluding the tiny friction and aero issues).

For climbing overall weight is the critical parameter – just look at the various components of the energy – and the potential energy increase from lifting the mass against gravity is what matters – aero issues and rolling resistance being constant.My only competitive cycling is racing hill climbs so I think (and calculate) on these issues a lot.
Peter

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Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides “ Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.