I have gotten so many questions on fork rake and front-end geometryfrom my Nov. 23 posting that I realized the need to fully address thissubject 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 receivedabout it as well as the “block” I wrote on the subject from my most recentbook, Zinn’sCycling Primer.
I hope it clarifies some handling questions for a lot of you out there.
I’m looking upgrade to a carbon fork with 43mm rake from an aluminumfork with 40mm rake. How will the change in rake affect the bike’s handling?Will increasing the wheelbase make it more stable?
No, it will make it faster turning and less stable, albeit with a largerturning radius. The attached“block” from my book should help clarify it.
Note: The following letter is a composite of a number of back-and-forthemails between Benjamin and me.
I was a bit disappointed in your answer to Jeremiah (Nov23 Technical Q&A) regarding rake and trail. First, rakeis an angle measurement; offset and trail are a distance measurement.Next, you simply cannot discuss rake and trail without including offsetin the discussion. All three are completely interrelated in steeringgeometry. The inclusion of offset in your answer (and a link to adiagram) 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’tsell any more books!
My apologies for mixing terminology…that’s what I get for being anoutsider! In a different arena, rake is what bicyclists know as thehead tube angle. Four-wheeled racers call it caster. Eitherway, I believe my premise is still legitimate. There are severalforks on the market that vary from straight to curved, thereby changingthe trail and indirectly the offset (as I know it.) For example,you could get the same trail figure from two frames using different headtube angles (rake) by changing the offset.
I assume the rake angle/head tube angle isn’t taken into considerationin 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 seemto vary much. So, this begs the question, if “rake angle” to me istrail to you, and you say trail and offset are the same, then what exactlyis the biking terminology for what I know as offset (see illustration)?!I appreciate your time to reply! As I said, I always learn somethingfrom you…
From your book:
In order that we share a common language to describe andunderstand bike design and stability, we must define a few terms. You mayfind 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 perpendiculardistance (offset) of the center of the front hub from the steering axis.Fork trail, “T,” is the horizontal distance between the center of the tirecontact patch on the ground and the intersection of the steering axis withthe ground. Head angle, “Ø,” is the acute angle between the steeringaxis and the horizontal. The wheel radius is “r.”
In that paragraph, Trail=Trail; Rake=Offset; Head Angle=Caster or whatI was calling “rake angle.” My point was that you can have two bikeswith the same exact trail, but totally different handling because of differentcombinations of Head Angle and Trail (Offset.) Do you buy this?
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 andhence the minimum turning radius, and the vibration characteristics willbe different (the steeper bike will be more rigid). Also, the leverageof the handlebar over the front wheel will be different, because the positionof the hub under the bar will have moved fore or aft. But the turning forcesgenerated by the front wheel in a lean will be the same on two bikes withthe same trail. That is what trail is all about.
The attached“block” from my book should help clarify it.
For those cyclists who doubt that a decrease in fork rake leads toan increase in a bicycle’s stability, let them try a demonstrationthat I saw in an old VHS tape on the history of bicycles. The demonstrationwas performed by none other than Mike Burrows, the inventor of Chris Boardman’sfamous Lotus bike and the person most responsible for popularizing thecompact frame road bike. Here is what the cycling skeptic shoulddo:
1) Take a bike and turn the fork around backwards, i.e. the dropoutswill 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 andthen let it go.
Amazingly, the bike will roll in a nearly perfect straight line foras long as it remains upright.
I might also note that the specialized track bikes used in a “dernyrace”- the weird but wonderful European velodrome event in which the cyclistsdraft a few millimeters behind fat old men riding dorky looking motorbikes-have forks that look like they are turn around backwards. The highspeed attained by the daring derny-drafters necessitates a bike with theutmost stability, hence the funny looking, but highly
functional fork with a very small rake.
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.
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