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Tech Talk: Reports
Technical Q&A with Lennard Zinn - Rake, trail, offset
By Lennard Zinn
VeloNews technical writer
This report filed December 21, 2004

Dear readers,
I have gotten so many questions on fork rake and front-end geometry from my Nov. 23 posting that I realized the need to fully address this subject again. I have written about this before, both here and in the magazine, but there obviously remains a bit of confusion out there about the subject. Consequently, I have posted a number of the recent questions I have received about it as well as the "block" I wrote on the subject from my most recent book, Zinn's Cycling Primer.

I hope it clarifies some handling questions for a lot of you out there.
Merry Christmas,
Lennard

Dear Lennard,
I'm looking upgrade to a carbon fork with 43mm rake from an aluminum fork with 40mm rake. How will the change in rake affect the bike's handling? Will increasing the wheelbase make it more stable?
Eric

Dear Eric,
No, it will make it faster turning and less stable, albeit with a larger turning radius. The attached "block" from my book should help clarify it.
Lennard

Note: The following letter is a composite of a number of back-and-forth emails between Benjamin and me.

Dear Lennard,
I was a bit disappointed in your answer to Jeremiah (Nov 23 Technical Q&A) regarding rake and trail. First, rake is an angle measurement; offset and trail are a distance measurement. Next, you simply cannot discuss rake and trail without including offset in the discussion. All three are completely interrelated in steering geometry. The inclusion of offset in your answer (and a link to a diagram) could have gone a long way to helping readers understand the relationship. Your answer confused me and others who read it, and that certainly won't sell any more books!

My apologies for mixing terminology...that's what I get for being an outsider! In a different arena, rake is what bicyclists know as the head tube angle. Four-wheeled racers call it caster. Either way, I believe my premise is still legitimate. There are several forks on the market that vary from straight to curved, thereby changing the trail and indirectly the offset (as I know it.) For example, you could get the same trail figure from two frames using different head tube angles (rake) by changing the offset.

I assume the rake angle/head tube angle isn't taken into consideration in biking because most frames seem to be pretty firm in the design? For a given biking style, the head tube and seat tube angles don't seem to vary much. So, this begs the question, if "rake angle" to me is trail to you, and you say trail and offset are the same, then what exactly is the biking terminology for what I know as offset (see illustration)?! I appreciate your time to reply! As I said, I always learn something from you...

From your book:

In order that we share a common language to describe and understand bike design and stability, we must define a few terms. You may find it helpful to refer to the Glossary for definition of bike-part terms. Referring to Fig. 40.1, note that fork rake, "R," is the perpendicular distance (offset) of the center of the front hub from the steering axis. Fork trail, "T," is the horizontal distance between the center of the tire contact patch on the ground and the intersection of the steering axis with the ground. Head angle, "Ø," is the acute angle between the steering axis and the horizontal. The wheel radius is "r."
In that paragraph, Trail=Trail; Rake=Offset; Head Angle=Caster or what I was calling "rake angle." My point was that you can have two bikes with the same exact trail, but totally different handling because of different combinations of Head Angle and Trail (Offset.) Do you buy this?
Benjamin

Dear Benjamin,
I think you mean: different combinations of Head Angle and RAKE (Offset.)

I buy it only in the sense that the wheelbase will be different and hence the minimum turning radius, and the vibration characteristics will be different (the steeper bike will be more rigid). Also, the leverage of the handlebar over the front wheel will be different, because the position of the hub under the bar will have moved fore or aft. But the turning forces generated by the front wheel in a lean will be the same on two bikes with the same trail. That is what trail is all about.

The attached "block" from my book should help clarify it.
Lennard

Dear Lennard,
For those cyclists who doubt that a decrease in fork rake leads to an increase in a bicycle's stability, let them try a demonstration that I saw in an old VHS tape on the history of bicycles. The demonstration was performed by none other than Mike Burrows, the inventor of Chris Boardman's famous Lotus bike and the person most responsible for popularizing the compact frame road bike. Here is what the cycling skeptic should do:

1) Take a bike and turn the fork around backwards, i.e. the dropouts will be facing toward the rear of the bike.
2) Find a flat, smooth, straight road or parking lot.
3) Get a good running start, give the bike a strong push and then let it go.

Amazingly, the bike will roll in a nearly perfect straight line for as long as it remains upright.

I might also note that the specialized track bikes used in a "derny race"- the weird but wonderful European velodrome event in which the cyclists draft a few millimeters behind fat old men riding dorky looking motorbikes- have forks that look like they are turn around backwards. The high speed attained by the daring derny-drafters necessitates a bike with the utmost stability, hence the funny looking, but highly
functional fork with a very small rake.
Robbie


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.